Volume 88, Issue 11-12

A critical question in radiometric dating of metamorphic rocks is whether minerals used to define internal isochrons have achieved isotopic equilibrium during a given metamorphic event, followed by preservation of the equilibrium afterward. A garnet peridotite at Zhimafang in the Sulu terrane of eastern China shows incongruent U-Pb, Rb-Sr, and Sm-Nd ages. Zircon SHRIMP U-Pb dating yielded an isochron age of 224 ± 8 Ma, pointing to zircon growth during Triassic UHP metamorphism. Two mineral assemblages gave consistent Rb-Sr internal isochron ages of 201 ± 4 and 205 ± 4 Ma, but much older Sm-Nd internal isochron ages of 376 ± 16 and 378 ± 24 Ma. Apparently, the Sm-Nd radiometric system fails to reequilibrate among the dated minerals during the Triassic UHP metamorphic event for the garnet peridotite. This result is confirmed by the state of oxygen isotope equilibrium (or disequilibrium) among the constituent minerals in the garnet peridotite. In particular, garnet is not in oxygen isotope equilibrium with any other of the analyzed minerals. The degree of oxygen isotope disequilibria among the other minerals varies from pair to pair. Oxygen isotope equilibrium is observed only between orthopyroxene and olivine for both samples and between phlogopite and clinopyroxene for one sample. It appears that the U-Pb, Rb-Sr, Sm-Nd, and O isotope systems in minerals of the garnet peridotite are in partial equilibrium during UHP metamorphism, i.e., equilibrium with respect to some isotopes and minerals, but not all. Based on available experimental diffusion data, our study of the combined U-Pb, Rb-Sr, Sm-Nd, and O isotope systems in the garnet peridotite demonstrates that the time scale for the UHP metamorphism and subsequent HP eclogite-facies recrystallization is possibly in the range of about 12 to 26 Ma, which was not long enough for the reequilibration of the Sm-Nd and O isotope systems, but just long enough for the reequilibration of the Rb-Sr isotope systems. Therefore, the rate of Sr diffusion in phlogopite (thus Sr isotope reequilibration between the mafic minerals) is constrained to be faster than rates of Nd and O diffusion in garnet (thus Sm-Nd and O isotope reequilibration between the same minerals) under conditions of subduction zone metamorphism.

The solubility of Au metal in rhyolite melt and coexisting magnetite has been determined at 800 °C, 140 MPa, and ƒ O2 ≈ NNO in a Au-metal saturated, sulfur-free, vapor-brine-silicate melt system. Whole crystals of magnetite suspended within quenched rhyolite glass were analyzed by LA-ICPMS. These data yield a solubility of Au in magnetite on the order of 2 mg/g. The solubility of Au metal in rhyolite melt hosting the magnetite crystals is on the order of 500 μg/g. These data indicate a partition coefficient for Au between magnetite and melt, D Au Mt/melt ≈ 4. Using reasonable estimates of the mass fraction of magnetite that crystallizes in crustal magmatic systems, we modeled the proportion of Au sequestered by magnetite during fractional crystallization. We considered fractionation in two steps: the idealized derivation of a rhyolite by fractionation of basalt at depth, and the closedsystem crystallization of the rhyolite in a magma chamber below the site of ore deposition. Magnetite sequesters 14-54% Au from basaltic parent melt that produces a rhyolite melt via crystal fractionation, at modal abundances of 1-5%. Less then 4% Au is sequestered from a crystallizing rhyolite melt because of low modal abundances of magnetite (<~2%). Our experimental and model results suggest that early crystallization of magnetite can play a mitigating role in the ability of a calc-alkaline magmatic system to yield a Au-rich ore fluid.

We characterized the surface oxidation layer (~50 Å) of chalcocite reacted with air-saturated aqueous solutions at pH = 2-11 and with the ambient atmosphere. Copper L-edge and O K-edge X-ray absorption spectroscopy probes the unoccupied electronic states in the conduction band of the surface alteration phase that, in turn, enables us to identify the mineralogy of the surface layer. At low pH, the surface alteration phase is primarily cuprite (Cu 2 O). At higher pH, tenorite (CuO) is the dominant phase. No evidence for sulfate or carbonate phases are present; however, chalcocite reacted only in air does develop a CuSO 4 surface phase in addition to cuprite and minor tenorite.

Molar absorptivity of the infrared (IR) fundamental OH-stretching vibration band at 3550 cm -1 was determined for rhyolitic glasses. Five obsidian samples, unheated and heated at 500-700 °C using an internally heated pressure vessel, were used to evaluate the dependence of the molar absorptivity and final quenched H 2 O speciation on H 2 O contents and temperature. Water contents of the obsidians were measured by Karl-Fischer titration first, then the amount of unextracted H 2 O was calibrated by IR spectroscopy and a conventional vacuum extraction method. Total H 2 O contents of the obsidians were determined to be 0.24-1.25 wt%. IR spectra of the unheated and heated obsidian samples were obtained using an FT-IR microspectrometer. We determined the molar absorptivity for the 3550 cm -1 band to be 75 ± 4 L/mol/cm without significant dependence on the H 2 O contents and heating temperature. This value can be used to determine precise H 2 O contents up to 1.25 wt% in rhyolitic volcanic glasses.

Periodic density functional theory (DFT) was used to calculate the structure of zircon (ZrSiO 4 ) and reidite (scheelite polymorph of ZrSiO 4 ) at pressures from 0-41 GPa. Subsequently, the 29 Si nuclear magnetic resonance (NMR) chemical shifts at each pressure were determined using the GIPAW method. The results show that in both zircon and reidite the chemical shifts become more negative with increasing pressure. The chemical shift of reidite is predicted by calculation to be -91.8 ppm and measured as -91.1 ppm. The results are used to interpret the previously observed systematic decrease in 29 Si NMR chemical shifts (increasingly negative) in the crystalline fraction of radiation-damaged zircon with increasing α-dose. The original hypothesis that strain induced by the compression of the crystal by internal regions of damaged, amorphized material at high pressure was the origin of the shifts was disproved. An alternative source of internal pressure in the radiationdamaged crystals linked to “‘stuffing” of the lattice with interstitial O or He atoms is proposed.

The structural behavior of Al 3+ in peralkaline glasses and melts along the Na 2 Si 3 O 7 -Na 2 (NaAl) 3 O 7 join has been examined to 1200 °C at ambient pressure with 29 Si MAS NMR and Raman spectroscopy. The distribution of Al 3+ among coexisting Q 4 , Q 3 , and Q 2 structural units in the glasses and melts was determined as a function of bulk Al/(Al + Si) and temperature. The Al 3+ resides principally in Q 4 structural units, which contain more than 70% of the total amount of Al 3+ . The Q 2 units contain the smallest amount of Al 3+ among the Q 4 , Q 3 , and Q 2 structural units. There is no evidence for temperature-dependent distribution of Al 3+ among the coexisting structural units at least to 1100- 1200 °C. The equilibrium constant for the speciation reaction, 2Q 3 ↔ Q 4 + Q 2 , passes through a maximum with increasing bulk melt Al/(Al + Si) at constant temperature. Its temperature-dependence also depends on bulk melt Al/(Al + Si) so that the enthalpy of the speciation reaction, △H, passes through a maximum with increasing bulk Al/(Al + Si). This △H ranges between 13 and 23 kJ/mol for bulk melt Al/(Al + Si) between 0 and 0.33. These composition-dependent maxima in the equilibrium constant (at constant temperature) and △H of the speciation reaction reflect the changes in Al/(Al + Si) of Q 2 , Q 3 , and Q 4 structural units with Al/(Al + Si) of the melt.

The hydrothermal transformation of 2-line ferrihydrite into hematite proceeds slowly if a sufficient quantity of some strongly adsorbing ligand, such as phosphate or citrate, is sorbed on the starting product. In this work, we studied such transformation at temperatures ranging from 125 to 200 °C, a molar P/Fe ratio of 0-6%, and a molar citrate/Fe ratio of 3%. The products were characterized by X-ray diffraction (XRD), infrared spectroscopy, Mössbauer spectroscopy (MS) at various temperatures and in an applied field of 60 kOe, magnetic and thermal analysis, and transmission electron microscopy (TEM). At 150 °C, pure 2-line ferrihydrite transformed rapidly into hematite. The products of transformation of 2-line ferrihydrite with P/Fe = 2.75% or citrate/Fe = 3% had a magnetic susceptibility of >240 × 10 -6 m 3 /kg and were, according to XRD and MS data, mixtures of hematite with structural P, 6-line ferrihydrite, and a magnetic phase. This phase exhibited most of the characteristic reflections and MS features of maghemite, and occurred as 7-30 nm subrounded particles with lattice fringes corresponding to the maghemite (310) and (220) planes. It was designated “hydromaghemite” because it lost >3% water between 110 and ~350 °C. At 150 °C, complete transformation into hematite occurred in <120 days.

A 2 cm long and 0.8 cm thick, single graphite crystal embedded in quartz, feldspar, and orthopyroxene of a granulite-facies metamorphic rock from southern India exhibits unique homogeneity in δ 13 C values along layers parallel to the (0001) surface and smooth unidirectional variation of about 1‰ along the c-crystallographic axis. The fluid-precipitated graphite shows isotope homogeneity indicative of crystal growth along the (0001) plane. The C-isotope data along with the textural features suggest that the formation of graphite initiates in a layer perpendicular to the c-axis, possibly by a spiral growth mechanism. The initial layer is followed layer after layer, until the final stages of graphite growth. A second stage of graphite growth unrestricted by surrounding minerals is inferred from the un-deformed idiomorphic hexagonal form of the overgrown crystals. The C-isotope values in the overgrown rim of the crystal are lowered by about 2‰, suggesting a multistage precipitation. Morphologic and stable isotope studies thus confirm uniform crystal growth of graphite recording an isotope evolution pattern relating to fractionation from a fluid in an infinite reservoir during the main crystal formation. Spiral growth parallel to the (0001) face can explain the perfect uniformity of stable isotope composition in a single layer, whereas the variation across the (0001) surface gives insights into the time-integrated fluid evolution attending the crystal growth.

The mineral woodruffite, Zn 2+ x/2 (Mn 4+ 1-x Mn 3+ x )O 2 ·yH 2 O, x ~ 0.4 and y ~ 0.7, is the first known example of a new type of Mn oxide characterized by large tunnels that measure 3 and 4 octahedra (6.9 × 9.2 Å) on a side. These tunnels are rectangular in cross-section and are the largest of any yet reported in natural or synthetic Mn oxides. The thermal stability of woodruffite is comparable to that of todorokite and other large-tunnel Mn oxide phases, breaking down at ~300 °C and eventually transforming to a spinel-type structure. The woodruffite structure may serve as a model for a new class of octahedral molecular sieves with enhanced capabilities as catalysts and selective cationexchange agents.

Calderonite, ideally Pb 2 Fe 3+ (VO 4 ) 2 (OH), a new member of the brackebuschite group, has been found in the upper oxidation zone of two Pb-Zn hydrothermal deposits located at Santa Marta and Azuaga, Badajoz province, Spain. Brackebuschite and calderonite probably form a complete solid solution, locally with important substitution of Cu and Zn for Fe and Mn. The monoclinic cell parameters derived from powder X-ray diffraction (XRD) data are: a = 7.647(5) Å, b = 6.094(1) Å, c = 8.900(2) Å, β = 112.0(2)° and V = 384.5(4) Å 3 , and the six strongest lines, d-spacing (Å),(I),(hkl), are: 4.893(4)(011), 4.166(3)(002), 3.242(10) (2̄̄11), 3.058(3)(020), 2.980(5) (1̄03) and 2.746(5)(003). Electron microprobe analyses (EMPA) show a certain degree of compositional variation not only between the Santa Marta and Azuaga samples but also among grains from the same locality. A representative formula of Santa Marta calderonite, determined from EMPA, based on 9 O atoms: is (Pb 1.950 Ca 0.004 Ba 0.015 ) 1.969 (Fe 3+ 0.892 Cu 0.059 Zn 0.008 Al 0.015 ) 0.974 (V 1.847 As 0.008 Si 0.039 P 0.057 ) 1.951 O 7.507 (OH) 1.493. Fe 3+ is principally substituted by Cu 2+ , and V 5+ by Si 4+ . ρ c = 6.05 g/cm 3 . The thermogravimetric analysis yields a weight loss of 1.91%, which corresponds to the 1.493 H needed to maintain the charge balance. Differential thermal analysis shows endothermic effects at 279 and 663 ­°C due to dehydroxylation. A single-crystal XRD refinement was carried out on a selected crystal with cationic content determined by EMPA and starting atomic positions from brackebuschite. Final R-value of 5.81% based on 952 reflections with I > 2σ I , assuming P2 1 /m symmetry, the cell dimensions are a = 7.649 Å, b = 6.101 Å, c = 8.904 Å, b = 112.23°. Calderonite is red orange to red brown, semitransparent to translucent with vitreous luster and red streak and powder. The fracture is splintery. Optically, it is biaxial positive, with a 2V x = 86° and strong dispersion. In plane-polarized light, it is strongly pleochroic (X = light greenish brown, Y = brown, Z = reddish brown).

The Middle Paleozoic Maksyutov Complex is an important component of the Eurasian collisional orogeny. It consists of dominant mica-rich garnet schist and mica-poor quartzofeldspathic gneiss enclosing minor mafic eclogite boudins (unit no. 1). Employing Raman spectroscopy, we identified three cuboidal microdiamond inclusions (~2-3 micrometers in diameter) in garnet hosts from two different mica-poor gneissic samples. Broad spectral bands and high magnification SEM images suggest that the cuboids are fine-grained nanocrystalline diamond aggregates characterized by limited long-range ordering. Their poor crystallinity is compatible with relatively low-temperature, solid-state growth in the absence of both melt and a C-O-H-N fluid. Poor crystallinity, and small grain size suggest that such aggregates may represent the lowest temperature microdiamonds yet identified in nature. Their formation required ultrahigh-pressures (UHP) at a minimum of 3.2 GPa, and a metamorphic temperature of ~650 °C. Blocky graphite up to 10+ mm across in the matrix of mica-rich carbonaceous garnet schist may represent pseudomorphs after much larger neoblastic diamonds. Thermobarometric calculations for analyzed coexisting garnet + omphacite + phengite from six Maksyutov unit no. 1 mafic eclogites indicate retrograde physical conditions of 610-680 °C, 1.7-2.6 GPa, slightly lower-pressure conditions than the coesite stability field. Complete conversion of diamond to blocky graphite in the mica-rich schists, and recrystallization of coesite to quartz in the schists, quartzofeldspathic gneisses, and eclogite pods reflect relatively slow exhumation from ~110 km depth to upper crustal levels over 60-90 m.y. Phengite inclusions in zircon and garnet hint at modest activity of H 2 O during prograde UHP metamorphism of the eclogites and mica-poor gneisses. The latter have retained rare, tiny microdiamond inclusions in garnet on decompression. Abundant white mica in the carbonaceous garnet schists probably reflects a C-O-H-N fluid-mediated, kinetically enhanced prograde production of diamond, and efficient obliteration of this phase accompanying leisurely ascent of the subduction complex. In contrast, associated micapoor gneisses and eclogites were relatively dry during exhumation, so retained rare nanocrystalline microdiamond inclusions in garnet.

Protoanthophyllite, (Mg, Fe) 7 Si 8 O 22 (OH) 2 , is a newly discovered amphibole species from the Takase ultramafic complex in Japan. It occurs as prismatic crystals up to 5 mm in length in a thermally altered serpentinite that experienced contact metamorphism. The protoanthophyllite is associated with forsterite, talc, serpentine minerals, chlorite, chromian spinel, magnetite, pentlandite, and calcite. Some protoanthophyllite crystals contain minute lamellae of anthophyllite, other pyriboles, or both. Protoanthophyllite is biaxial negative, with refractive indices n α = 1.593(2), n β (calc.) = 1.609, n γ = 1.615(2), and 2Vx = 64(5)°. Electron microprobe analyses give an empirical formula of (Mg 6.31 Fe 0.61 Na 0.06 Mn 0.01 Ni 0.01 ) Σ7.00 (Si 7.90 Al 0.14 ) Σ8.04 O 22 (OH) 2 . It is orthorhombic with space group Pnmn. The unit-cell dimensions are a = 9.3553(8), b = 17.9308(15), and c = 5.3117(4) Å: with V = 891.0(3) Å 3 and Z = 2. A single-crystal X-ray structure determination shows that, following the convention of Thompson (1981), protoanthophyllite has an (X) configuration. The topology of the silicate tetrahedral chains is similar to that of the anthophyllite A-chains. Silicate tetrahedral chains are O-rotated in protoanthophyllite, whereas those in protoferro-anthophyllite are S-rotated. Iron atoms are concentrated in the 4-coordinated M4 sites. The unit-cell volume is ~1.5% larger than the equivalent volume of anthophyllite with Mg/(Mg + Fe) = 0.885, suggesting a high-temperature or low-pressure stability relative to anthophyllite, assuming that protoanthophyllite is not metastable.

Aluminum orthoposphate (AlPO 4 ) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to α-quartz. We report berlinite-quartz solid solutions to crystallize from a synthetic P-rich peraluminous granitic melt, similar in composition to the most volatile-rich silicate melt inclusions found in pegmatites. The crystallization took place in experiments performed in cold-seal pressure vessels at 450-700 °C and 0.1-0.2 GPa H 2 O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5-7 mol% SiO 2 on the phosphate side of the solvus and to the maximum of 1 mol% AlPO 4 on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinitequartz solid solutions are strongly zoned and show complex intergrowths between the P-rich and silica-rich phases. They were studied by electron microprobe, transmission electron microscopy, and Raman spectroscopy. In the light of our new experimental results, the extreme P enrichment reported earlier for some natural quartz-hosted melt inclusions may be explained as a post-entrapment contamination by the berlinite-bearing host.

Osmium-rich Pt group element (PGE) alloys occur worldwide in association with chromite in ultramafic (peridotite) complexes. It has been suggested that these Os-rich alloys formed under extreme P-T conditions in the lowermost mantle, before the metallic core of the Earth formed, or later, in the outer core, and have been transported to the upper mantle as xenoliths in deep-rooted mantle plumes. Our investigation of syn- and pregenetic inclusions (including silicate and chromite) found in Os-rich alloys from peridotites in northern California and southwest Oregon yield no evidence that these alloys formed under extreme P-T conditions. Instead these inclusions point to a hydrous magmatic origin in the shallow upper mantle, most likely in an arc-environment. Indeed, the common occurrence of Os-rich PGE alloys as primary inclusions in massive (commonly podiform), chromite deposits and, conversely, the occurrence of chromite, olivine, pyroxene, laurite, and siliceous (boninitic) melt inclusions in Os-rich PGE alloys suggest a common origin for all these minerals. Integrating our observations with recent experimental work and with observed field relations, we find support for a model in which massive chromite deposits, olivine, laurite, and Os-rich PGE alloys form in a single magmatic process. In an arc-environment, H 2 O-rich fluids and siliceous melts (e.g., boninites) are produced in the mantle wedge above the descending and dehydrating plate. Large differences in interfacial energy between the precipitated chromite and PGE alloys, and the hydrous fluid(s) and siliceous melt(s), cause a strong concentration of chromite and PGE alloys in the hydrous fluid(s). This general scenario is capable of simultaneously explaining all key observations, including: (1) the formation of massive chromite deposits; (2) nodular chromite textures; (3) Os-rich PGE alloys, laurite, olivine, and pyroxene as common inclusions in massive chromite; (4) inclusions of chromite, olivine, pyroxene, and hydrated siliceous inclusions (the current study) in the Os-rich PGE alloys; and (5) a similar range of variation in 187 Os/ 188 Os ratios among Os-rich PGE alloys and massive chromite deposits from individual ultramafic bodies world-wide.

Viscosity of hydrous silicate melts is a critical property for understanding magma transport, bubble growth, volcanic eruption, and magma fragmentation. We report new inferred viscosity for hydrous rhyolitic melt in the viscosity range of 10 9 to 10 15 Pa·s based on the kinetics of hydrous species reactions in the melt upon cooling (i.e., based on the equivalence between the glass-transition temperature and the apparent equilibrium temperature), as well as data from bubble-growth experiments. By combining viscosity data of rhyolitic melts containing from 6 ppm to about 8.0 wt% total H 2 O (both our own data and literature data), we propose the following relation for the dependence of viscosity on total H 2 O content at a given temperature and pressure: where η is viscosity and 1/η is fluidity, η 1 is the viscosity of the dry melt, x is the mole fraction of total dissolved H 2 O, n and η 2 are two fitting parameters, and η 2 can be interpreted to be the viscosity of the hypothetical melt consisting of pure H 2 O. The above simple equation appears to work well in modeling viscosity of hydrous rhyolitic melts. Using the above functional form and a weighted nonlinear regression, experimental data covering 570 to 1920 K and 0.0006 to 8.2 wt% total H 2 O are used to obtain the following non-Arrhenian model for the viscosity of Mono Crater rhyolitic melts: logη = -log{exp(18.5611 - 49584/T) + exp[1.47517 - (1795.5/T) 1.9448 ]x 1+ (1812.2/T)2 } where T is in K, and x is the mole fraction of total dissolved H 2 O on a single oxygen basis. The 2σ uncertainty in the above 6-parameter formula is 0.36 in terms of logη (for experimental data with errors smaller than 1 log unit), much less than uncertainties in all previous models for hydrous rhyolitic melt. We hence recommend its use for predicting the viscosity of high-SiO 2 and calc-alkaline rhyolitic melts. Our model shows that a minute H 2 O content can still affect the melt viscosity significantly, especially at low temperatures (i.e., high viscosity). For example, at 973 K, the viscosity of rhyolitic melt increases by 1.2 orders of magnitude from a melt with 0.1 wt% total H 2 O to a melt with less than 100 ppm total H 2 O. Because a nominally “dry” melt may still contain some H 2 O, such as several hundred ppm, it is critical for experimentalists to report its total H 2 O content for viscosity measurements. Some inconsistency in literature viscosity data might be attributable to small variations in H 2 O content in the samples.

A region of metamorphosed supracrustal rocks (pelite, quartzite, marble, and graywacke) and coeval intrusive igneous rocks crop out in a 250 km long orogenic belt in northern Madagascar. The NW-SE trending belt is situated between a juvenile Neoproterozoic magmatic arc terrane (to the north) and an Archean craton, strongly reworked in early Neoproterozoic times (~800-670 Ma), to the south. Pelitic schist and granulite exposed along a ~70 km long transect from Andapa to Sambava contain assemblages ranging from sillimanite-garnet-biotite-orthoclase-cordierite to sillimanite-garnet- biotite-orthoclase and sillimanite-garnet-biotite-muscovite. These assemblages crop out over much of the area in which migmatites and hornblende + augite ± hypersthene ± biotite + perthite granites are common. Partial melting, biotite dehydration reactions, and granite emplacement are interpreted to have been nearly synchronous on the basis of field, structural, and petrographic observations. Pressure and temperature estimates from garnetiferious metapelitic granulite and pelitic migmatite are generally in the range of 6.5-8.5 kbar and 800-900 °C using conventional thermobarometric methods. The occurrence of cordierite moats and discordant, but synmetamorphic, leucosomes in the granulites suggest an isothermal decompression-type path. Similarly, a core-rim P-T trajectory indicates ca. 2-3 kbar decompression at high temperature in the metapelitic granulite. U-Pb geochronology of sphene, monazite, and zircon in various metamorphic and igneous rocks from the same region yields a Cambrian age of 510-520 Ma for the time of gneiss formation, granulite metamorphism, and igneous activity. Calculated minimum cooling rates range from 6 to 18 °C/ Ma. Such cooling rates are more rapid than those associated with normal isostatic processes, and suggest that the terrane was tectonically exhumed at high temperature.

We theoretically investigated the local structure of the aperiodic states of zircon (ZrSiO 4 ) using ab initio quantum mechanical calculations. The low and high density liquid and solid glassy phases were obtained by constant volume Car-Parrinello molecular dynamics simulations, using the molar volume of metamict and crystalline zircon, respectively. As in naturally metamict zircons, the polymerization of Si units, the segregation of Zr atoms, and an overall decrease of the Zr coordination were observed. However, the local ordering of our theoretical glasses differs from that of the natural amorphous samples. In the theoretical glasses, the Zr-O distances in the first coordination polyhedra are significantly more distributed and five- and six-coordinated Si species were observed. The relaxation of the glass structure on a time scale exceeding the possibility of molecular modeling is a possible explanation for these discrepancies. The dielectric and 29 Si NMR responses of the glassy phases were computed, providing new constraints for the analysis of experimental data recorded from metamict zircons. The calculated NMR spectra are in good agreement with the experimental NMR spectra in the four-coordinated Si region. Our results show that the usual regular systematic decrease of the 29 Si chemical shift as a function of the polymerization of Si units (Q n species) cannot be used to interpret the 29 Si NMR spectrum of amorphous zircon. In particular, whereas the empirical scale would indicate an average polymerization of Q 3 , the average polymerization of Q 1.5 observed in the low density zircon glass can account for the experimental data. In a non-uniform model of the structure of metamict zircon, this lower average polymerization is consistent with a clustering of cations limited to few coordination polyhedra.

Twenty impact diamond samples from the Popigai, Ries, and Lappajärvi astroblemes were analysed using cathodoluminescence (CL) at room temperature (RT). Five of the samples were further investigated at liquid nitrogen temperature (LNT). Cathodoluminescence images allowed for the discrimination of diamond from graphite, thus contributing to a better understanding of the reciprocal relationships between these carbon polymorphs and their overall textural features. Cathodoluminescence spectral measurements of the diamonds revealed emission bands and peaks located at 1.8 eV (688 nm), 2.23 eV (556 nm), 2.32 eV (534 nm), 2.39 eV (519 nm), 2.49 eV (498 nm), and 2.8-2.9 eV (443-427 nm). The bands at 2.8-2.9 eV and at 1.8 eV, observed at RT, were related respectively to vibronic levels (involved in electronic transitions), located at dislocation defects and to dislocations. Regarding the other lines, which were only visible at LNT, there may be a relationship between the peaks at 2.32 eV, 2.23 eV, and 2.39 eV, and the content of amorphous carbon phases. Some spectral features may be considered a possible signature of impact diamonds. In particular, the band at 1.8 eV, which is uncommon in terrestrial natural diamonds, and the peaks at 2.23 eV and 2.32 eV, are present in all the samples studied.

A critical question in radiometric dating of metamorphic rocks is whether minerals used to define internal isochrons have achieved isotopic equilibrium during a given metamorphic event, followed by preservation of the equilibrium afterward. A garnet peridotite at Zhimafang in the Sulu terrane of eastern China shows incongruent U-Pb, Rb-Sr, and Sm-Nd ages. Zircon SHRIMP U-Pb dating yielded an isochron age of 224 ± 8 Ma, pointing to zircon growth during Triassic UHP metamorphism. Two mineral assemblages gave consistent Rb-Sr internal isochron ages of 201 ± 4 and 205 ± 4 Ma, but much older Sm-Nd internal isochron ages of 376 ± 16 and 378 ± 24 Ma. Apparently, the Sm-Nd radiometric system fails to reequilibrate among the dated minerals during the Triassic UHP metamorphic event for the garnet peridotite. This result is confirmed by the state of oxygen isotope equilibrium (or disequilibrium) among the constituent minerals in the garnet peridotite. In particular, garnet is not in oxygen isotope equilibrium with any other of the analyzed minerals. The degree of oxygen isotope disequilibria among the other minerals varies from pair to pair. Oxygen isotope equilibrium is observed only between orthopyroxene and olivine for both samples and between phlogopite and clinopyroxene for one sample. It appears that the U-Pb, Rb-Sr, Sm-Nd, and O isotope systems in minerals of the garnet peridotite are in partial equilibrium during UHP metamorphism, i.e., equilibrium with respect to some isotopes and minerals, but not all. Based on available experimental diffusion data, our study of the combined U-Pb, Rb-Sr, Sm-Nd, and O isotope systems in the garnet peridotite demonstrates that the time scale for the UHP metamorphism and subsequent HP eclogite-facies recrystallization is possibly in the range of about 12 to 26 Ma, which was not long enough for the reequilibration of the Sm-Nd and O isotope systems, but just long enough for the reequilibration of the Rb-Sr isotope systems. Therefore, the rate of Sr diffusion in phlogopite (thus Sr isotope reequilibration between the mafic minerals) is constrained to be faster than rates of Nd and O diffusion in garnet (thus Sm-Nd and O isotope reequilibration between the same minerals) under conditions of subduction zone metamorphism.

Alacranite from the type locality (Uzon caldera, Kamchatka, Russian Federation) was submitted for structural analysis. A single crystal was selected and the following lattice parameters were determined: a = 9.942(4), b = 9.601(2), c = 9.178(3) Å, β = 101.94(3)°, V = 857.1 (5) Å 3 . The crystal structure was solved in the P2/c space group using direct methods and refined to R = 6.79% for 472 observed reflections. The structure of alacranite consists of an ordered sequence of As 4 S 4 and As4S5 cage-like molecules, with a molecular packing closely resembling that found in the β-As 4 S 4 phase. Both As-As and As-S intramolecular distances are in the range usually observed for covalent bonds. The structural model confirms the chemical formula As 8 S 9 for alacranite, and accounts for differences in the unit-cell parameters of alacranite compared to those of the natural analogue of β-As 4 S 4 . This latter mineral, therefore, should receive a new name.

To understand the behavior of octahedral hydroxyl-stretching modes (ν OH ) as a function of increasing tetrahedral layer charge, infrared and Raman spectroscopic studies were carried out on a series of synthetic saponites. The nature of the monovalent interlayer cation (Na or K) and the hydration of the samples were shown to strongly influence spectral features. In the case of hydrated Nasamples, the ν OH position shifts from 3678 to 3686 cm -1 with increasing layer charge. For hydrated K-saponites, two components are observed around 3720 and 3680 cm -1 , assigned to OH perturbed by compensating cation and free OH groups, respectively. In the case of dehydrated samples both K and Na-saponites exhibit two components at 3677 and 3720 cm -1 . A linear relationship between the relative area of these two ν OH components and the layer charge was then evidenced. To assign the features obtained for hydrated Na-samples in the OH-stretching modes domain, the Raman spectra were decomposed using four ν OH components around 3675, 3680, 3685, and 3690 cm -1 . The evolution with layer charge of the relative proportions of these four components can be described by taking into account the location of hydrated cations, related to the distribution of tetrahedral substitutions. Such an assumption was confirmed using a simple statistical model that yields four populations of OH groups in the octahedral layer of hydrated Na-saponites.

The structure and chemistry of {111} twin boundaries and stacking faults in Fe-rich sphalerite crystals from the Trepča mines in Kosovo were studied using electron microscopy. The {111} twin boundaries were found to be deficient in S and enriched in O, Mn, Fe, and Cu. The deficiency in S is compensated with O, which is responsible for stabilizing the hexagonal stacking of the fault structures and the formation of {111} twin boundaries in sphalerite. Comparing the intensities of Bijvoet-related reflections we show that there is no inversion of the polar axis across the twin boundaries. In addition to twin boundaries we found two types of stacking faults with R SF1 = 1/3·[1̄12] and R SF2 = 2/3·[1̄12]. The excretion of isostructural copper from the sphalerite crystals peaks at the twin boundaries until it precipitates in the form of small chalcopyrite grains, aligned along the {111} twin boundaries.

We report the first occurrence of moissanite (SiC) as a rock-forming mineral (8.4 vol%) in one unique specimen of a terrestrial rock. The sample has a homogeneous, porphyritic texture, and was found as a beach pebble thought to be derived from a Tertiary volcanic province of the Aegean Sea region. The matrix is bluish-colored and consists of very fine-grained brucite, calcite, and magnesite, in which macrocrysts of quartz (25.3 vol%) and moissanite are found. Other accessory phases are phlogopite-3T, magnesiochromite, an Fe-rich phase, Cl-bearing brucite, Al-rich orthopyroxene, and unidentified MgFe-silicates (4 vol%). The bulk-rock composition shows a “kimberlitic” chemistry (55.8 wt% SiO 2 , 28.5 wt% MgO, 1.4 wt% CaO, 18.1 wt% LOI). Colorless gemmy, and blue or black moissanite crystals are subhedral and display characteristic hexagonal symmetry (6H polytype). Most moissanite grains contain metallic Si and Fe-silicide (Fe3Si7) inclusions, and more rarely, other Fe-silicides with varying amounts of Al (≤24.5 wt%), Ca (≤8.0 wt%), Mn (≤6.8 wt%), Ti (≤16.3 wt%), and Ni (≤2.6 wt%). The δ 13 C value of the moissanite is -28.1‰. According to available data, the f O2 stability field of SiC is five to six log units below the iron-wüstite (IW) buffer curve. Therefore, the observed Fe-bearing silicates cannot have been equilibrated with SiC under ambient pressure. Instead, our finding indicates that the rock most likely formed at the ultrahigh-pressure conditions of the upper mantle or transition zone.

Water solubility in Ca-Na-Mg carbonate melt was measured at 900 °C from 0.25 to 2.25 kbar using a modified double-capsule technique and rapid-quench cold-seal vessels. The data can be described by the empirical relationship W = 8.705 P 0.635 , where W is water solubility in weight percent and P is pressure in kbar. The relative error in water solubility calculated by this equation is estimated to be about 15%. Already at 1 kbar, water solubility reaches nearly 10 wt%, which is two to three times the value observed for most silicate melts under similar conditions. The exceptionally high water solubility in carbonatite magmas together with the preferential partitioning of alkalis into a coexisting fluid phase may explain the massive aqueous metasomatism (fenitisation) usually observed around intrusive carbonatite bodies.

Magmas erupted at convergent margins consist of components from both mantle wedge and subducted slab. In an effort to quantify the relative contributions of these sources, we have determined 19 trace element partition coefficients (D values) for orthopyroxene and clinopyroxene, in equilibrium with spinel and hydrous high-MgO melt under conditions appropriate to melting in the mantle wedge, i.e., 1.3 GPa, 1245 °C, and f O2 of NNO + 1. All trace elements are more incompatible in clinopyroxene during hydrous melting than during anhydrous melting of fertile and depleted peridotite. Orthopyroxene D values are relatively insensitive to pressure, temperature, and phase composition. The new D values are used to calculate the trace-element composition of the mantle wedge, which produced primitive South Sandwich Islands and St. Vincent (Lesser Antilles) arc basalts. Both sources correspond to previously depleted mantle that has been enriched in LILEs and LREEs by slab-derived fluids. In the case of the South Sandwich Islands, the calculated source is in very close agreement with dredged fore-arc lherzolites. Our partitioning data confirm that hydrous melting of wedge peridotite itself cannot produce the characteristic enrichments of LILEs over REEs and HFSEs. Our estimates of the slab component in South Sandwich Islands and St. Vincent are consistent with estimates from other arcs, derived by alternative methods.

The mode of occurrence and chemical composition of five types of micas with Rb- or Cs-dominant populations of interlayer cations, collected from the Red Cross Lake rare-element pegmatites in north-central Manitoba, are described here. All five micas are candidates for new mineral species but crystal-structural data and Li contents could not be determined to date because of extremely small particle size, restricted to the margins of strongly zoned microcrystals. Based on electron-microprobe analyses, on Li contents estimated from Li/F (at.) = 1.0, and on bulk analysis of ferromagnesian micas for FeO and Fe 2 O 3 , the micas correspond to Rb- and Cs-dominant polylithionite (with representative interlayer populations of Rb 0.82 K 0.12 Cs 0.07 and Cs 0.74 Rb 0.12 K 0.08 apfu, respectively), Rb and Cs-dominant magnesian annite (Rb 45 K 0.37 Cs 0.20 and Cs 0.67 Rb 0.20 K 0.12 apfu, respectively), and Cs-dominant ferroan phlogopite (Cs 0.92 Rb 0.04 K 0.02 apfu).

Bedrock samples were examined from a region in south-central New Hampshire in order to understand the factors responsible for elevated As-concentrations (up to 180 ppb) in the groundwater. Although the As is predominantly from natural sources, the precise mineralogy of the As-bearing phases was unknown. As-bearing samples were examined in detail using advanced electron microscopy techniques, including high-angle annular dark field scanning electron microscopy (HAADFSTEM), STEM elemental mapping, and high resolution TEM. Numerous As-bearing minerals were observed, predominantly arsenopyrite (FeAsS) with some westerveldite (FeAs). The arsenopyrite was partially altered to nanocrystalline (~20 nm) magnetite and westerveldite, most likely during hydrothermal circulation of fluids following the emplacement of a nearby granitoid pluton. A reaction rim surrounds the pristine cores of arsenopyrite, with compositionally variable amorphous zones (containing Fe, As, K, and O), a Cu-sulfide, and a uranium-bearing phase, suggesting at least three distinct fluid compositions during alteration. Because the K-Fe-As-oxide is amorphous and the nanocrystalline FeAs has a high surface area per unit mass, the dissolution rate of As-bearing phases during recent low-temperature weathering is increased. The rapid dissolution of the reduced As phase (FeAs), which is unstable under oxidizing conditions, may be an important factor in the elevated As-concentrations in the groundwaters of this region.

Fly ash from a coal-fired power plant was investigated to obtain detailed information on its physical and chemical properties, and to gain an understanding of potential environmental and health impacts associated with its disposal in landfills. The studied material was produced through combustion of Illinois Basin coal and trapped within the power plant by an electrostatic precipitator. It is a finegrained, low-Ca fly ash containing primarily SiO 2 , Al 2 O 3 , and Fe 2 O 3 , and is enriched in many toxic elements (e.g., Be, Zn, As, Cd, Tl, Pb, and U) by a factor of up to 30 relative to coal. The ash consists of mainly hematite, magnetite, mullite, quartz, and amorphous material. These constituents occur mostly as spherical particles with diameters of less than 13 μm. We examined the physical, chemical, and structural characteristics of individual fly ash particles by scanning and transmission electron microscopy and electron probe microanalysis. The results demonstrate that, with the exception of complex plerospheres, individual particles are chemically fairly homogeneous, but a pronounced compositional variation exists among particles with similar physical and structural attributes. Electron microprobe data document that several trace elements, including U, are partitioned into the Ferich particles. Transmission electron microscopy revealed that various types of small (<1 μm) crystalline Ca-rich phases, including lime, are attached to the glass spheres, particularly the nonmagnetic glass. These crystals may contain substantial amounts of S. Even though only a few of these crystals were analyzed quantitatively, our data indicate that the Ca-rich and S-rich phases may be important hosts for trace elements such as V and Zn. The observed element partitioning and the existence of surface-attached crystals enriched in certain trace elements suggest that fly ash from coal-fired power plants might have a more deleterious environmental impact than is inferred from bulk analytical data.

Teeth and bones consist of an apatite-type structure and such biogenic apatites usually occur in nano-crystalline form. Because of the small particle size in biological tissues, local structural details of biogenic apatite have still not been resolved in detail. Comparison of the phonon spectra of enamel apatite with those of inorganically formed apatite was carried out to improve our understanding of the vibrational behavior of biogenic apatite. In situ mid-infrared absorption spectra of dental enamel and geologic fluorapatite were recorded from 300 K to ca. 750 K. Lattice vibration modes were studied at low temperature in the infrared region of 150 to 650 cm -1 . The IR excitations indicate that geologic apatite undergoes heavier thermal changes than enamel apatite at temperatures between 60 K and 300 K. In situ high-temperature IR spectra confirm the different thermal evolution of dental enamel and geologic fluorapatite. The P-O overtones or combinational vibrations and hydrous species of enamel apatite show two different thermal regions below and above 600 K. The thermal behavior in the region below 600 K corresponds to the loss of adsorbed and part of the lattice water, combined with an increase of structural OH groups. In the second thermal region (above 600 K), the similarity of thermal response of dental enamel to that of the geologic apatite from 300 K suggests the existence of a highly ordered system. This result may be explained by the dehydration and atomic rearrangements in the channels of enamel apatite structure below 600 K.

Speleothems in the karst caves of South Korea, which receive many visitors, are losing their aesthetic appeal due to black coloring. Mineralogical, textural, and chemical analyses were conducted on the speleothems to discover the cause of the discoloration. An abrupt color change from the natural color seen in the inner zones to the black color of the outer zones suggests that pollution commenced just after the opening of caves to visitors, and has continued since then. The main mineral compositions of both the outer black and the inner layers are the same, but the concentration of non-carbonate carbon is much higher in the black layers than in the inner layers. Electron microscopy showed that chain-like agglomerates (ca. 0.2-1.1 μm diameter) of sub-micrometer carbon spheres (ca. 0.02-0.05 μm diameter) are absent from the inner layer but present in the black layer, as well as in the cave aerosol. On the basis of their sub-micrometer size, agglomeration pattern, and composition, the carbon spheres and their agglomerates are considered to originate mostly from automobile exhaust. They are presumed to have been carried into the caves by visitors from urban environments and then deposited on the surface of growing speleothems. Protection of speleothems from discoloration requires control of these fine anthropogenic aerosols.

The saguaro, Carnegiea gigantea (Englemann), is a columnar cactus that grows to 15 m tall and weighs up to several tons, of which 85 to 90% of the mass is water. Roughly 18% of the dry mass consists of the biomineral weddellite (CaC 2 O 4 ·2H 2 O). The C in the weddellite derives from atmospheric CO 2 via photosynthesis. A mature saguaro can contain on the order of 1 × 105 g of weddellite. The weddellite crystals occur as aggregates up to 1 mm wide. After the death of the saguaro, a series of minerals crystallize in the rotting flesh. These minerals form from elements released from the decay of the cactus by microorganisms and thus is a type of biologically induced mineralization. During the initial stages of decay, authigenic Mg- and Ca-bearing minerals crystallize from elements released by the putrefying flesh and include lansfordite (MgCO 3 ·5H 2 O), nesquehonite (MgCO 3 ·3H 2 O), several polymorphs of MgC 2 O 4 ·2H 2 O including glushinskite, monohydrocalcite (CaCO 3 ·H 2 O), calcite, vaterite, and several unidentified Mg-bearing phases. As the saguaro decays, the soft, water-rich pith shrinks, but the ribs and skin remain intact, producing warm, moist pockets within the dead saguaro. Abundant, glassy lansfordite crystals to 1 mm in diameter grow in these pockets during the cooler winter months. Further decay leaves a dried hollow shell covered by the saguaro skin, inside of which nesquehonite and monohydrocalcite crystallize. Lansfordite and nesquehonite are unstable in the desert and rapidly amorphize after exposure to the atmosphere. Magnesium oxalates are locally abundant in the decayed flesh and occur as crystals up to 1.5 mm in length. The common occurrence of fungal hyphae on the glushinskite suggests that it forms as a result of the reaction between oxalic acid released by fungi and the Mg-rich solutions of the rotting saguaro. During the final stages of decay, the pith consists of a pale-brown to tan-colored sand of weddellite and its transformation product monohydrocalcite. This sand lithifies to porous spongelike masses during the final stages of saguaro decay. This monohydrocalcite further alters to calcite. The δ 13 C VPDB of the monohydrocalcite and calcite after weddellite range from -1.65 to + 0.76‰. The calcite is subsequently solubilized and remobilized, precipitating as caliche in the desert soil, or redistributed by wind. In arid environments, the desert fauna metabolize the atmospheric C bound in the organic matter to CO 2 . In contrast, decay of the saguaro adds atmospheric C to the soil as inorganic C via the transformation of the biomineral weddellite to calcite. In areas with high saguaro density, it is estimated that up to 2.4 g/m 2 /yr of calcite can be added to the desert from the decayed cacti. This inorganic C has geologically long soil residence times, thus effectively sequestering the atmospheric C.

Yucca Mountain, Nevada, has been approved by the U.S. Congress as the site of the first high-level radioactive waste repository in the U.S., paving the way for the Department of Energy to submit a license application to the Nuclear Regulatory Commission. The U.S. concept of a radioactive waste repository involves both engineered (man-made) barriers and geologic barriers to minimize the longterm migration of waste from the repository. The minerals at Yucca Mountain are an integral part of the geologic barrier to the migration of radionuclides, and understanding the distribution of reactive and potentially sorptive minerals is crucial to modeling the long-term performance of the repository. We have determined the three-dimensional distribution of minerals at Yucca Mountain by analyzing a number of drill cores using quantitative X-ray diffraction methods. Clinoptilolite and mordenite are locally abundant in the rhyolitic tuffs in both the unsaturated and saturated zones. They formed by the alteration of glassy rhyolitic tuffs, and their occurrence is stratified, often following the distribution of nonwelded, initially vitric tuffs near and below the water table. Because of their large cation-exchange capacities, these minerals have traditionally been considered to be important in retardation of the migration of cationic radionuclides such as 137 Cs and 90 Sr. However, analysis of radionuclides that may be contained at Yucca Mountain reveals that, for the most part, the radioactive species of concern are not those that would be readily sorbed by natural zeolites. Although clinoptilolite and mordenite are highly selective for Cs and Sr, the half-lives of these radionuclides (~30 years) are sufficiently short that the man-made containment at any radioactive waste repository is expected to outlive these radionuclides. The most important radionuclides are thus anionic species and long-lived actinides that often form large, complex aqueous species, both of which are not strongly sorbed by cation exchangers such as zeolites. However, zeolites play an important role in a repository at Yucca Mountain for several other reasons. Emplacement of radioactive waste will change the temperature and P H₂O conditions in large volumes of rock, and zeolites will be important sources and sinks of water and thermal energy during heating and cooling in the vadose zone. In addition, dehydration/hydration reactions are accompanied by volumetric changes that can affect macroscopic rock properties. Longerterm reactions, e.g., of clinoptilolite to analcime, would give rise to larger volume reductions and production of water and silica. These studies show that the importance of zeolites extends far beyond cation exchange by zeolites to phenomena affecting the entire thermohydrologic system.

Mineralogical transformations of 2-line ferrihydrite were studied under oxic and Fe 3+ -reducing conditions to establish the role, if any, of 6-line ferrihydrite (“well” organized ferrihydrite) in the reaction pathway and as a final product. In oxic experiments, concentrated suspensions (0.42 mol/L Fe 3+ in 0.1 mol/L NaClO 4 ) of freshly synthesized 2-line ferrihydrite, with and without 3% Ni 2+ , were aged at an initial pH = 7.2 (unbuffered and unadjusted) and 25 °C for more than three years. X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy measurements were performed on the solids after different aging periods. The primary mineralogical products observed were 6-line ferrihydrite and goethite, with minor hematite. Aggregation and crystallization of the 2- line ferrihydrite liberated protons and depressed suspension pH, but coprecipitated Ni 2+ retarded this process. The joint, interrelated effects of Ni and pH influenced both the extent of conversion of 2- line ferrihydrite and the identity of the major transformation products. Six-line ferrihydrite dominated in the Ni ferrihydrite suspension, whereas goethite dominated in the absence of Ni. Aggregation-induced crystallization of 2-line ferrihydrite particles seemed responsible for 6-line ferrihydrite formation. Mineralogical changes to Ni ferrihydrite under anaerobic conditions were investigated at circumneutral pH using the Fe 3+ -reducing bacterium Shewanella putrefaciens. Residual 6-line ferrihydrite dominated bioreduced samples that also contained goethite and magnetite. The conversion of 2-line ferrihydrite to 6-line ferrihydrite was considerably more rapid under anaerobic conditions. The sorption of biogenic Fe 2+ apparently induced intra-aggregate transformation of 2-line ferrihydrite to 6-line ferrihydrite. Collectively, abiotic and biotic studies indicated that 6-line ferrihydrite can be a transformation product of 2-line ferrihydrite, especially when 2-line ferrihydrite is undergoing transformation to more stable hematite or magnetite.

Ochreous precipitates deposited by waters draining a sulfide-rich lignite seam exposed in an abandoned mine in the Czech Republic show marked variations in color and mineralogy as a function of effluent pH. When initially formed under acidic (pH 3.7) conditions, the precipitates are orange in color and their mineralogy is dominated by schwertmannite. Following confluence with alkaline (pH 8.3) waters that have not been affected by mine drainage, the effluent pH rises to 7.3, the color changes abruptly to reddish-brown, and the principal precipitate mineral is a two-line ferrihydrite. This change in mineralogy and the associated variation in color can thus serve as direct indicators of the genetic environments under which the minerals were formed.

This study of a pyrrhotite-dominated massive sulfide deposit in the Blue Ridge province in southwestern Virginia shows that sulfate minerals formed by the oxidation of the pyrrhotite transform from one to another by a combination of oxidation, dehydration, and neutralization reactions. Significant quantities of sulfate minerals occur in the underground adits (Area I) and under overhangs along the high sidewall of the adjoining open pit (Area II). Samples from this site were analyzed to determine mineralogy, equilibrium relative humidity, chemical composition, and acid generation potential. In Area I, pyrrhotite oxidizes to marcasite + melanterite, which eventually oxidizes to melanterite + sulfuric acid. Melanterite is extruded from the rocks as a result of the volume change associated with this reaction. It accumulates in piles where halotrichite, copiapite, and fibroferrite form. In Area II, FeSO 4 solutions produced by pyrrhotite oxidation migrate to the exposed pit face, where they evaporate to form melanterite. The melanterite rapidly dehydrates to form rozenite, which falls into a pile at the base of the wall, where melanterite, copiapite, and halotrichite are present. The observed paragenesis can be understood using a log α O₂ - log α H₂O diagram that we developed from published thermodynamic data and observations of coexisting phases. Dissolution experiments showed that fibroferrite-rich samples had the highest acid producing potential, followed by copiapite-rich samples and then halotrichite-rich samples. The most abundant metals in solutions produced by dissolving impure bulk samples of the salts were Mg, Al, Zn, Cu, Ca, and Pb. The molar concentrations of the metals varied with mineralogy. However, all of the sulfate minerals release metals and acid when they dissolve and therefore represent a potentially significant environmental risk.

The secondary mineralogy and microtextures of weathered waste-rock dumps derived from the mining of galena-sphalerite ore in quartz veins containing manganoan carbonates were examined using back-scattered electron imaging, X-ray diffraction, and chemical analysis. Sphalerite, pyrrhotite, and arsenopyrite were coated or replaced by iron oxyhydroxides in the earliest stage of the weathering, and were then replaced by sulfur. Galena shows a thin alteration rim of anglesite. Oxidation of pyrite has resulted in porous boxworks of Fe oxyhydroxides. The relative resistance to oxidation, from most resistant to least resistant, was observed to be pyrite ≈ galena > arsenopyrite ≈ sphalerite > pyrrhotite. Rhodochrosite dissolved to form hydrohetaerolite pseudomorphs, and manganoan calcite has an outer alteration rim of hydrohetaerolite and an inner zone of smithsonite. Rock and mineral fragments were cemented by fine aggregates of plumbojarosite, Fe oxyhydroxides/sulfates, and manganates. Microchemical analysis and sequential extractions showed a close association of As with Fe oxyhydroxides/sulfates, of Pb, Cu, Zn, and As with plumbojarosite, and of Pb and Zn with manganates. Despite their lower acid-neutralizing capacity, manganoan carbonates played an important role in the fixation of Pb and Zn by the formation of manganates.

Metal-sulfate salts in mine drainage environments commonly occur as solid solutions containing Fe, Cu, Mg, Zn, Al, Mn, Ni, Co, Cd, and other elements. Thermodynamic data for some of the endmember salts containing Fe, Cu, Zn, and Mg have been collected and evaluated previously, and the present study extends to the system containing Ni. Morenosite (NiSO 4 ·7H 2 O)-retgersite (NiSO 4 ·6H 2 O) equilibria were determined along five humidity buffer curves at 0.1 MPa and between 5 and 22 °C. Reversals along these humidity-buffer curves yield ln K = 17.58-6303.35/T, where K is the equilibrium constant, and T is temperature in K. The derived standard Gibbs free energy of reaction is 8.84 kJ/mol, which agrees very well with the values of 8.90, 8.83, and 8.85 kJ/mol based on the vapor pressure measurements of Schumb (1923), Bonnell and Burridge (1935), and Stout et al. (1966), respectively. This value also agrees reasonably well with the values of 8.65 and 9.56 kJ/mol calculated from the data compiled by Wagman et al. (1982) and DeKock (1982), respectively. The temperature- humidity relationships defined by this study for dehydration equilibria between morenosite and retgersite explain the more common occurrence of retgersite relative to morenosite in nature.

Minerals of the jarosite group can be a significant environmental sink for hexavalent Cr by substitution of chromate for sulfate. The thermochemistry of the synthetic solid solution between jarosite KFe 3 (SO 4 ) 2 (OH) 6 and its chromate analog KFe 3 (CrO 4 ) 2 (OH) 6 was investigated by high-temperature oxide-melt solution calorimetry. The enthalpies of formation (△H 0 f ) of the latter, as well as of five intermediate compositions in the series KFe 3 (S 1-z CrzO 4 ) 2 (OH) 6 , were determined, where z corresponds to the Cr content. The variation of △H 0 f with Cr content deviates from ideality, and negative enthalpies of mixing between jarosite and its chromate analog are observed, suggesting some ordering of the sulfate/chromate groups in the solid solution. The measured enthalpy of formation from the elements of the end-member KFe 3 (CrO 4 ) 2 (OH) 6 is △H 0 f = -3762.5 ± 8.0 kJ/mol. In view of this work, and considering literature data, △G 0f = -3305.5 ± 3.4 kJ/mol, △S 0 f = -1533.6 ± 29.2 J/(mol·K), and S0 = 487.7 ± 29.2 J/(mol·K) are recommended for KFe 3 (CrO 4 ) 2 (OH) 6 .

Thirty samples of amphibole-rich rock from the largest mined vermiculite deposit in the world in the Rainy Creek alkaline-ultramafic complex near Libby, Montana, were collected and analyzed. The amphibole-rich rock is the suspected cause of an abnormally high number of asbestos-related diseases reported in the residents of Libby, and in former mine and mill workers. The amphibole-rich samples were analyzed to determine composition and morphology of both fibrous and non-fibrous amphiboles. Sampling was carried out across the accessible portions of the deposit to obtain as complete a representation of the distribution of amphibole types as possible. The range of amphibole compositions, determined from electron probe microanalysis and X-ray diffraction analysis, indicates the presence of winchite, richterite, tremolite, and magnesioriebeckite. The amphiboles from Vermiculite Mountain show nearly complete solid solution between these end-member compositions. Magnesio-arfvedsonite and edenite may also be present in low abundance. An evaluation of the textural characteristics of the amphiboles shows the material to include a complete range of morphologies from prismatic crystals to asbestiform fibers. The morphology of the majority of the material is intermediate between these two varieties. All of the amphiboles, with the possible exception of magnesioriebeckite, can occur in fibrous or asbestiform habit. The Vermiculite Mountain amphiboles, even when originally present as massive material, can produce abundant, extremely fine fibers by gentle abrasion or crushing.

Compositional data and Fe 3+ /ΣFe ratios obtained by electron microprobe and Mössbauer analyses are given for a suite of three amphibole and amphibole-asbestos samples collected from the former vermiculite mine near Libby, Montana. A crystal structure analysis, compositional data, and Fe 3+ /ΣFe values for two samples from a previous study are also reported. The results confirm the conclusion drawn in the previous study that these amphiboles are dominantly compositions ranging from winchite to richterite. Mössbauer spectroscopy yielded Fe 3+ /ΣFe ratios from 58% to 72% for the five samples. The crystal structure was determined for a single crystal selected from a bulk sample. Its formula (as determined by electron microprobe analysis and Mössbauer spectroscopy) is (K 0.19 Na 0.32 ) A (Na 0.85 Ca 1.12 Mn 0.03 ) B (Mn 0.01 Mg 4.43 Fe 3+ 0.34 Fe 2+ 0.19 Ti 0.01 Al 0.02 )(Al 0.03 Si 7.97 O 22 )(OH 1.63 F 0.37 ). The refinement was carried out based on space group C2/m, with a = 9.879(2), b = 18.024(3), c = 5.288(1) Å, b = 104.377(3)° and using data collected at room temperature. Mg is partitioned among the M1, M2, and M3 sites. All of the Fe 3+ occupies M2, while Fe 2+ is split between M2 and M3; Ca and Na fill the M4 site, while Na and K occupy the partially filled A site. The A-site occupancy is calculated as 0.51 based on chemical data, but only 0.48 based on X-ray diffraction results. Minerals with the former values would be classified as richterite and those with the latter as winchite.

By using dispersion staining methods and pre-constructed conversion tables, it is possible to quickly and accurately determine two principal refractive indices (RI) of the six regulated asbestos minerals, chrysotile, grunerite (amosite), riebeckite (crocidolite), tremolite, actinolite, and anthophyllite, in a single immersion oil mount. This procedure is especially suitable for commercial environmental laboratories specializing in the analysis of asbestos components in bulk building materials. The effectiveness of this practical procedure has been proven through rigorous testing and extensive usage over the last decade by the majority of environmental laboratories in the U.S. The principle of this procedure is also readily applicable to RI determination in other applications: mineralogy, forensics, pharmaceutical research, particle identification, etc.

A green-colored clay mineral occurring on the walls of amygdaloidal cavities and along fractures in the Deccan flood basalts at Killari, Maharashtra, India has been identified as iron-rich saponite, with chemical composition (Na 0.60 K 0.04 Ca 0.47 )(Mg 2.05 Fe 2+ 3.95 )(Si 6.45 Al 1.55 )O 20 (OH) 4 . The XRD data of the Killari sample with d (001) = 1.70 nm and b 0 = 0.9275(2) nm conform to that of ferrous saponite. The FTIR spectra show vibrational bands typical of trioctahedral smectites. Differential thermal and thermogravimetric analyses show strong endothermic peaks due to dehydroxylation at 390-420 and 1070-1130 K accompanied by weight losses of 17 and 2.3 wt%. A weak exothermic peak at about 1220 K is also observed. These peaks are characteristic of smectites. 29 Si and 27 Al MAS NMR studies show that silicon and most of the aluminum in the clay mineral are in fourfold coordination. We have demonstrated for the first time the usefulness of ferrous-saponite in reduction of hexavalent chromium. X-ray photon electron spectroscopic (XPS) studies of the fine clay sample treated with dichromate solution show that this ferrous saponite has the capability of adsorbing and reducing hexavalent chromium to the trivalent state.

Dry and in situ (fluid-cell) Atomic Force Microscopy (AFM) and Low-Pressure Gas Adsorption experiments were used to investigate the surfaces of pure Na-smectite particles. These two techniques permit the identification of different surfaces of the platelets (lateral, basal, and interlayer surfaces) and to quantify their surface area. Calculation of the surface area was done for AFM, by measuring directly the dimensions of the clay particles on AFM images, and for gas adsorption experiments, by applying the Derivative Isotherm Summation (DIS) procedure designed by Villiéras et al. (Villiéras et al. 1992, 1997a, 1997b). In the present study, we find a discrepancy between measurements of the basal and interlayer surface area. This difference is due to the stacking of platelets in dry conditions compared to their dispersion in aqueous suspension. A particle is estimated to be formed of nearly 20 stacked layers in the dehydrated state used in the gas adsorption experiment, whereas it is estimated to be composed of only 1 or 2 layers in aqueous suspension, on the basis of AFM measurements. However, the two techniques give similar results for the lateral surface area of the platelets (i.e., about 8 m 2 /g) and the perimeter to area ratio value of the particles because the stacking of platelets does not alter these values. This correlation confirms the effectiveness of the interpretation of the gas adsorption experiments lowest pressure domains as the adsorption on lateral surfaces. The lateral surface area has important implications in the calculation of specific sorption site density on clay material. The relevance of the lateral surface area value (8 m2/g) was tested subsequently with sorption data found in the literature. Based on those results, we show that one essential parameter for the calculation of particle edge-site density is the mean perimeter to area ratio value. This parameter can be obtained by microscopic techniques but the measurement is tedious. The good correlation between the AFM results and the DIS-method results confirms that the latter procedure offers a quick and reliable alternative method for the measurement of the lateral surface area. AFM experiments can be further conducted to constrain the dispersion around the DIS value and the anisotropy of suspended particles.

Medieval stained glass (13 th -15 th centuries) from three restoration works (Santa Maria del Mar and the Pedralbes Monastery church in Barcelona, and the Tarragona Cathedral in the northwestern Mediterranean area have been studied to characterize glass decay. Electron microprobe analysis gave evidence of two types of glass: an Na-rich type (stable, Mediterranean, of Roman-like tradition), and a K-Ca-rich type, similar to coeval Central European medieval stained glasses. The latter shows glass decay in the form of destructive (micro- and mesopitting) and constructive (patina and microcrust) glass surfaces. Nevertheless, this type of decay in the K-Ca-rich group of glasses is in terms of thickness reduction in flat glass an order of magnitude less than that commonly found in Central European countries with a continental climate. Macroscopic and microstructural studies (SEM-EDS, XRD) allowed us to identify biodeterioration decay with chemical effects similar to that for pure chemical hydration leaching and corrosion, associated with biomineralization with unspecific sulfate (gypsum, syngenite) and calcite mineralization as well as bioinduced (weddellite, whewellite, etc.) mineralization. Medieval-stained glass biodeterioration decay in the Mediterranean area shows patterns and products comparable to those developed on stone in the same historic buildings.

Synthetic Fe 2+ monosulfide, FeS am , displays a disordered tetragonal mackinawite structure. It is nanocrystalline, with an average primary particle size equivalent to a crystallite size of 4 nm and a corresponding specific surface area of 350 m 2 /g. It can be described in terms of a mixture of two end-member phases with different long-range ordering, which we refer to as MkA and MkB. MkA has an average primary particle size of 2.2 × 1.7 nm and lattice parameters a = b = 4.0 Å, c = 6.6 ± 0.1 Å. MkB has an average primary particle size of 7.4 × 2.9 nm and lattice parameters a = b = 3.7 Å, c = 5.5 ± 0.2 Å. A typical disordered mackinawite precipitate consist of 30% MkA and 70% MkB and the proportion of MkA decreases with age. Lattice expansions relative to crystalline mackinawite (a = b = 3.7 Å, c = 5.0 Å) may be explained by intercalation of water molecules between the tetrahedral sheets and by lattice relaxation due to small crystallite size. The formation of two phases of FeS am is consistent with competing pathways involved in its formation from aqueous solution. MkA may be equivalent to sheet-like precipitated aqueous FeS clusters. The reactivity of FeS am is dependent on the proportion of the two end-member phases. These in turn are dependent on the conditions of formation, especially pH, and the age of the precipitate. These observations partly explain the reported differences in FeS am reactivity in experimentation and in the environment. The structural model has implications for the behavior of natural acid volatile sulfides in scavenging elements from solution in natural environments.

The interactions between 10 -4 M Cr 3+ aq and seven different Mn-oxides (six natural samples, including hausmannite, manganite, romanechite, cryptomelane, lithiophorite, and pyrolusite; and one synthetic sample, birnessite) have been observed in aqueous solution at pH 4.4 (pH 5.2 in the case of hausmannite) and room temperature. For each mineral-solution interaction, the aqueous chemical concentrations of [Mn aq ], [Cr] T aq , and [Cr 6+ aq ] were measured with time. Reacted samples were examined by XPS to determine if and to what extent the surface chemical states of Cr, Mn, and O had changed. Microscopic observations of the reacted surfaces were obtained using AFM and highresolution, low-voltage FESEM. Cr-uptake onto/into the Mn-oxides is observed to occur in Cr-hydroxide/ hydrate undersaturated solutions as a result of adsorption, absorption, and surface precipitation. In addition, the rate and extent of reductive dissolution and Cr 3+ aq oxidation varied considerably between the seven Mn-oxides. The measured ratios of [Mn] aq :[Cr 6+ ] aq were in agreement with the values expected from the proposed stoichiometric reactions where they could be easily written. The chromium as detected by XPS on the surface of the reacted Mn-oxides was predominately Cr 3+ , however pyrolusite contained both Cr 3+ and Cr 6+ . Several previous studies have implicated a chromium surface precipitate to be responsible for the cessation of the Cr 3+ aq oxidation reaction. Our surface sensitive FESEM and AFM observations tend to suggest that Cr uptake is by isolated site binding, and very small (<30 nm) surface clusters. On two of the seven Mn-oxides tested (hausmannite and birnessite), Cr uptake was followed by slow Cr release after a substantial portion of the total aqueous Cr had been converted to Cr 6+ aq . The Mn-oxides that exhibited the greatest and longest lasting Cr 3+ -oxidizing ability were the Mnoxides containing Mn 3+ , and in particular those containing Mn 3+ and Mn 2+ . It is believed that the combined presence of a reducible Mn ion (e.g., Mn 3+ ) and a highly soluble Mn 2+ ion facilitates a sustained Cr 3+ -oxidation reaction because fresh Mn 3+ surface is exposed during the dissolution reaction. Finally, the Cr 3+ aq reacted Mn-oxides tested showed a partial rejuvenation of their Cr 3+ -oxidizing ability after a period of drying and storage. An assessment of this process will require additional experimentation.

The sorption of Sr 2+ and Pb 2+ on zeolitized tuffs from the Nevada Test Site (NTS) was investigated using macroscopic batch sorption experiments and X-ray absorption spectroscopy (XAS) as a function of geochemical parameters, including pH, ionic strength, and type of background electrolyte. The sorption of Sr 2+ is dependent on the ionic strength of the medium and independent of pH, suggesting that Sr 2+ sorption is controlled by ion exchange at permanent charge sites. At higher ionic strengths, background electrolyte cations compete effectively with Sr 2+ for cation-exchange sites and Sr 2+ sorption is suppressed. At the two lower ionic strengths (0.01 and 0.1 M), Pb 2+ sorption is also consistent with adsorption by cation exchange. At the highest ionic strength (1.0 M), however, exclusion of Pb 2+ from cation-exchange sites resulted in pH dependent adsorption, consistent with sorption on amphoteric surface hydroxyl sites or formation of surface precipitates. XAS was used to test these hypotheses. Based on XAS data, Sr 2+ formed hydrated surface complexes coordinated with approximately eight O atoms at an average distance of 2.60 (±0.02) Å, regardless of conditions, consistent with the formation of mononuclear, outer-sphere surface complexes at the Ca2 site in the B channel of clinoptilolite. The coordination environment of sorbed Pb 2+ was more complex and a function of pH and ionic strength. The first shell consisted of two to three O atoms at an average distance of 2.20 (±0.02) Å. At low pH and ionic strength, XAS data were consistent with Pb 2+ adsorption at the Na1 and Ca2 cation exchange sites in channels A and B of clinoptilolite, respectively. At the highest ionic strength (1.0 M) and low pH, XAS provides evidence for formation of Pb 2+ monodentate, corner-sharing inner-sphere complexes, whereas at higher pH, XAS analysis is consistent with formation of edge-sharing bidentate inner-sphere complexes. As surface coverage increased, appearance of a second Pb 2+ peak suggests the formation of polynuclear, inner-sphere surface complexes. These results have significant implications for the transport of radionuclides and other contaminants at the NTS and other nuclear test sites and for the modeling of these processes.

Biomineralization produces crystals of elaborate shapes, never seen in inorganic mineralogy, with tightly regulated compositions and axis orientations. The calcite coccoliths produced by unicellular marine algae provide an example of such control at very tiny scales. Atomic force microscopy (AFM) of two species provided nanoscale images allowing us to define crystallographic orientation in the crystal elements and to establish the relationship between crystallographic orientation and coccolith morphology. Both species adopt the inorganically stable calcite rhomb, but differences in crystal orientation enable them to construct distinct architectures with properties tailored to suit the requirements of their ecological niche.