Band 90, Heft 10

Banded iron-formations (BIFs) occur in the Precambrian geologic record over a wide time span. Beginning at 3.8 Ga (Isua, West Greenland), they are part of Archean cratons and range in age from about 3.5 until 2.5 Ga. Their overall volume reaches a maximum at about 2.5 Ga (iron-formations in the Hamersley Basin of Western Australia) and they disappear from the geologic record at about 1.8 Ga, only to reappear between 0.8 and 0.6 Ga. The stratigraphic sequences in which BIFs occur are highly variable. Most Archean iron-formations are part of greenstone belts that have been deformed, metamorphosed, and dismembered. This makes reconstruction of the basinal setting of such BIFs very difficult. The general lack of metamorphism and deformation of extensive BIFs of the Hamersley Range of Western Australia and the Transvaal Supergroup of South Africa allow for much better evaluations of original basinal settings. Most Archean iron-formations show fine laminations and/or microbanding. Such microbanding is especially well developed in the Brockman Iron Formation of Western Australia, where it has been interpreted as chemical varves, or annual layers of sedimentation. BIFs ranging in age from 2.2 Ga to about 1.8 Ga (e.g., those of the Lake Superior region, U.S.A., Labrador Trough, Canada, and the Nabberu Basin of Western Australia) commonly exhibit granular textures and lack microbanding. The mineralogy of the least metamorphosed BIFs consists of combinations of the following minerals: chert, magnetite, hematite, carbonates (most commonly siderite and members of the dolomite-ankerite series), greenalite, stilpnomelane, and riebeckite, and locally pyrite. Minnesotaite is a common, very low-grade metamorphic reaction product. The Eh-pH stability fields of the above minerals (and/or their precursors) indicate anoxic conditions for the original depositional environment. The average bulk chemistry of BIFs, from 3.8 through 1.8 Ga in age, is very similar. They are rich in total Fe (ranging from about 20 to 40 wt%) and SiO 2 (ranging from 43 to 56 wt%). CaO and MgO contents range from 1.75 to 9.0 and from 1.20 to 6.7 wt%, respectively. Al 2 O 3 contents are very low, ranging from 0.09 to 1.8 wt%. These chemical values show that they are clean chemical sediments devoid of detrital input. Only the Neoproterozoic iron-formations (of 0.8 to 0.6 Ga in age) have very different mineralogical and chemical make-ups. They consist mainly of chert and hematite, with minor carbonates. The rare-earth element profiles of almost all BIFs,with generally pronounced positive Eu anomalies, indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The prograde metamorphism of iron-formations produces sequentially Fe-amphiboles, then Fe-pyroxenes, and finally (at highest grade) Fe-olivine-containing assemblages. Such metamorphic reactions are isochemical except for decarbonation and dehydration. The common fine lamination (and/or microbanding) as well as the lack of detrital components in most BIFs suggest that such are the result of deposition, below wave base, in the deeper parts of ocean basins. Those with granular textures are regarded as the result of deposition in shallow water, platformal areas. Carbon isotope data suggest that for a long period of time (from Archean to Early Proterozoic) the ocean basins were stratified with respect to δ 13 C (in carbonates) as well as organic carbon content. In Middle Proterozoic time (when granular BIFs appear) this stratification diminishes and is lost. The Neoproterozoic BIFs occur in stratigraphic sequences with glaciomarine deposits. These BIFs are the result of anoxic conditions that resulted from the stagnation in the oceans beneath a near-global ice cover, referred to as “Snowball Earth”. All of the most “primary” mineral assemblages appear to be the result of chemical precipitation under anoxic conditions. There are, as yet, no data to support that BIF precipitation was linked directly to microbial activity. The relative abundance of BIF throughout the Precambrian record is correlated with a possible curve for the evolution of the O 2 content in the Precambrian atmosphere.

Disorder in stoichiometric magnesioferrite, MgFe 2 O 4 , was determined from in situ synchrotron powder X-ray diffraction data [λ = 0.3738(4) Å] at 6, 5, and 3 GPa and temperatures up to 1430 K. The a unit-cell parameter increases linearly on heating at the three different pressures. Higher pressures cause a smaller cell volume, as expected. Cation order was analyzed in terms of the inversion parameter, x, { iv [Mg 1-x Fex] vi [Mg x/2 Fe 1-x/2 ] 2 O 4 } and the order parameter Q = 1 - (3/2)x. As pressure increases, the inversion parameter increases in inverse MgFe 2 O 4 spinel. O'Neill and Navrotsky (1983) and Landau models were used to describe the equilibrium non-convergent ordering process in MgFe 2 O 4 , and they both fit the data well.

Raman spectra were collected for crystalline albite from 25 °C to above the 1118 °C melting temperature, where vibrational assignments for the crystal spectra were determined by lattice dynamics (LD). The Raman spectra and associated vibrational assignments are reported for triclinic albite (NaAlSi 3 O 8 ) at 25 °C and monoclinic albite at 1060 °C. The 25 °C calculations determined that localized T-O stretch and O-T-O bend modes are above 900 cm -1 (where T = Si,Al), while motions from the aluminosilicate tetrahedral cage mixed with Na displacements occur in modes as high as 814 cm -1 . Vibrational modes for the most prominent peaks in the spectrum, between 350 and 550 cm -1 , are dominated by four-membered tetrahedral ring deformations. For completeness, calculated infrared mode frequencies and their atomic displacements are reported for the 25 °C structure and compared with normal mode calculation results and observed infrared mode frequencies presented by von Stengel (1977). At higher temperatures, modes above 550 cm -1 broaden and shift to lower frequencies by 15 to 27 cm -1 ; modes below 550 cm -1 broaden, but experience little, if any frequency shifts. Albite melted sluggishly, was completely liquid at 1320 °C, and remained amorphous upon cooling to room temperature. At frequencies above 550 cm -1 , the crystalline albite peaks, and possibly their vibrational assignments, can be correlated to Raman bands for albite glass. Spectral differences below 550 cm -1 between crystal and glass correspond to changes of average tetrahedral ring type upon melting, as shown by Taylor and Brown (1979).

A new occurrence of the rare efflorescent sulfate mineral xitieshanite [Fe 3+ (SO 4 )Cl⋅6H 2 O] has been documented from acid mine drainage (AMD) flows at Green Valley, Vigo County, Indiana, U.S.A. This represents only the second documented occurrence of the mineral, and is a new environment of deposition. Previously, xitieshanite had been documented only within the oxidation zone at the Xitieshan Mine, Quinghai Province, China. The presence of xitieshanite at Green Valley was confirmed through X-ray powder diffraction and chemical analyses. The mineral forms botryoidal semi-gelatinous lumps, as well as radiating clusters of light-green bladed and rod-like acicular crystals up to 1 mm long, in some cases pseudomorphically replacing melanterite. This is the only documented occurrence of the mineral as distinct observable crystals that indicate pseudomorphism. The xitieshanite forms following large summer storms that provide oxygenated waters in the AMD seepways, and a warm, humid environment of deposition. The chlorine in the mineral may originate from coal waste or anthropogenic sources related to de-icing operations at the mine plant.

We present a structural model for (K,Na)MgF 3 perovskite using results from high-resolution synchrotron X-ray powder diffraction and nuclear magnetic resonance (NMR) spectroscopy. (K,Na)MgF 3 perovskite is found to transition from orthorhombic (Pbnm) to tetragonal (P4/mbm) to cubic (Pm3̅m) as potassium concentration is increased. These phase transitions are not accompanied by a discontinuity in pseudo-cubic unit-cell volume and occur close to compositions (K 0.37 Na 0.63 )MgF 3 and (K 0.47 Na 0.53 )MgF 3 , respectively. 19 F NMR spectra indicate that the Na + and K + cations do not occupy the A cation site at random and end-member local environments are favored for all compositions. Based on results from both X-ray diffraction and NMR, we propose that diffuse diffraction is the result of strain between coexisting regions of different octahedra (MgF 6 ) tilts brought about by the ionic radius mismatch of Na + and K + cations. We suggest A-site cations group with like cations as neighbors to reduce excess volume and total strain.

The crystal structure of NaMgF 3 perovskite (neighborite) has been studied at 4 GPa and temperatures up to 1000 °C using the Rietveld structure-refinement method. In situ synchrotron X-ray powder diffraction data was collected using monochromatic radiation. The orthorhombic (Pbnm) to cubic (Pm3̅ m) transition was observed when the temperature increased from 900 to 1000 °C. Structure refinements show that the ratio of polyhedral volumes of the A and B sites (VA/VB) of the orthorhombic phase increases with temperature, approaching the ideal value (5) for the cubic structure. However, this ratio becomes smaller at 4 GPa compared to the result from previous studies at the same temperature but ambient pressure, indicating that pressure makes it more difficult to transform from the orthorhombic phase to the cubic phase in this kind of perovskite.

In this first systematic, theoretical study of the complete electric field gradient (EFG) tensor of 57 Fe Mössbauer spectroscopy as a function of chemistry and local structural distortion using electronic structure calculations, local EFG tensors were calculated at the central Fe 2+ sites in clusters of seven octahedra, where the neighbor octahedra were populated with a mixture of Mg 2+ and Al 3+ cations. The independent parameters of the EFG tensor.particularly the principal value VZZ, the asymmetry parameter η, and the direction of the principal axis with regards to the octahedral sheet.were examined in detail as functions of octahedral flattening for each of the thirteen possible configurations of cations. We demonstrate that the argument that the EFG tensor at a particular site is largely determined by the point group symmetry of the corresponding crystallographic site is not correct. We find that in clusters with monoclinic or triclinic local point group symmetry, the value of VZZ changes discontinuously from positive to negative as flattening increases. The principal axis changes from being in the plane of the octahedral sheet to being normal to it at this discontinuity, and the value of η reaches a maximum and begins to decline. This discontinuous behavior is caused by the continuous change of the EFG eigenvalues as the Fe(3d) character of the highest occupied spin-down orbital evolves with flattening. Taking EFG tensor results from all clusters, and using probabilities for the occurrence of each possible cation configuration in a chemically disordered octahedral sheet of a given bulk composition, we simulate averages and distributions of η, principal axis angles, and quadrupole splittings. While the value of η is broadly distributed from zero to one at all flattening angles and bulk compositions, there are two distinct populations of principal axes, normal to and in-plane with the octahedral sheet, as has been observed experimentally. We find these in-plane and normal populations of principal axes correspond exactly to populations with positive and negative values of VZZ, respectively. Histograms representing quadrupole splitting distributions (QSDs) show features found in experimental QSDs, such as a constant high edge, a variable low edge, and a QSD width that changes dramatically with flattening. These results represent the first predictions relating average structural parameters, as would be obtained by X-ray diffraction, to characteristics of the Fe 2+ QSD, as obtained by 57 Fe Mössbauer spectroscopy, in a chemically disordered material.

Bazhenovite was originally reported as a rare, thiosulfate-containing mineral, with the chemical formula CaS 5 ⋅CaS 2 O 3 ⋅6Ca(OH) 2 ⋅20H 2 O. The structure was predicted to be layered, but no structural details were given. A crystal of “bazhenovite” from the pyritized siderite fragments occurring in the melt products of burning dumps at the type locality in the Chelyabinsk coal basin, South Urals, Russia has been examined by single-crystal X-ray diffraction and vibrational spectroscopy (FTIR and Raman). The structure was solved in space group P2 1 /c and refined assuming twinning on {100} to a final Robs = 5.03% (731 reflections) and Rall = 6.98% (966). Although the unit-cell dimensions of the examined crystal [a = 8.391(2), b = 17.346(6), and c = 8.221(4) Å, β = 119.33(5)°, V = 1043.2(8) Å 3 ] match those of the original bazhenovite description [a = 8.45(1), b = 17.47(1), and c = 8.24(1) Å, β = 119.5°, V = 1059 Å 3 ], the thiosulfate group was not detected by either structural analysis or spectroscopic investigations. The structure is an alternating sequence of two kinds of layers, labeled A and B respectively, stacked along [010]. The A layer is the ordered part of the structure and consists of a linkage of Ca(OH) 2 (H 2 O) 6 antiprisms and Ca(OH) 4 (H 2 O) 2 octahedra. Taking into account both structural and spectroscopic results, the B layer is inferred to consist of a disordered assemblage of S 3 2- , and to a lesser extent, S 4 2- groups, with the possible presence of additional H 2 O and H 2 S. The possibility that bazhenovite and the mineral here examined could represent two distinct phases differing slightly from each other with respect to the thiosulfate content is discussed.

Light-induced degradation in realgar (arsenic sulfide) has been studied by means of four-circle single-crystal X-ray diffraction and X-ray photoelectron spectroscopy. Because of the alteration of realgar exposed to light, the a lattice parameter and c sinβ value increase linearly from 9.327 to 9.385 Å and fro|m 6.320 to 6.364 Å, respectively. In contrast, the b lattice parameter remains substantially constant. Anisotropic variations of the lattice parameters engender a continuous increase of the unit-cell volume from 799.5 to 810.4 Å 3 . Nevertheless, no correlation exists between the continuous increase of the unit-cell volume and the bond distance variations in As 4 S 4 molecules because the As 4 S 4 molecule in the unit cell expands very little during light exposure. The most pronounced change was in the distance between centroids UiAs 4 S 4 cages. The spread of As 4 S 4 intermolecular distances increases continuously from 5.642 to 5.665 Å, which directly affects the unit-cell volume expansion of realgar. In addition, the O1s peak increases rapidly after light exposure. The result substantiates the following reaction proposed by Biudi et al. (2003): 5As 4 S 4 + 3O 2 -> 4As 4 S 5 + 2As 2 O 5 . That is, realgar is transformed into pararealgar if oxygen exists and produces the As 4 S 5 molecule. The additional S atom contributes to anisotropic expansion for the a and c axes because the direction of the additional S atom points toward [41̅4] in the unit cell. Furthermore, an S atom in the As 4 S 5 molecule is released from one of the equivalent As-S-As linkages in As 4 S 5 which becomes the As 4 S 4 molecular of pararealgar. After the As 4 S 5 molecule is divided into an S atom (radical) and the As 4 S 4 (pararealgar type) molecule, the free S atom is re-attached to another As 4 S 4 (realgar type) molecule, and reproduces an As 4 S 5 molecule. The reproduced As 4 S 5 molecule is again divided into an S atom (radical) and an As 4 S 4 (pararealgar type) molecule. This cycle whereby realgar is indirectly transformed into pararealgar via the As 4 S 5 molecule is promoted by light and repeated during light exposure.

Many natural surface-environment samples, such as soils and marine and lacustrine sediments, are complex mixtures involving several mineral phases (both petrogenic and authigenic), several amorphous and nanoparticulate inorganic phases (usually authigenic or pedogenic), and a heterogeneous amorphous component of “organic matter” (OM). The main inorganic amorphous and crystalline components are often operationally separated and quantified by various selective or sequential chemical extraction methods that are subject to various artifacts and that can be significantly affected by the presence of OM. Here we develop a general method of absolute quantification based on powder X-ray diffraction (pXRD) measurements that are obtained using standard θ-θ or θ-2θ type diffractometers. The method does not require calibration or the use of standards and does not require instrument parameter determinations but relies instead on exact normalization conditions that we prove. In particular, we develop a .general integrated intensity formula. (GIIF) for X-ray diffraction. All relevant sampleradiation interaction phenomena are considered, including polarization, mass absorption, Compton scattering, and resonance absorption re-emission. We show that the mole fraction of any given phase (crystalline, amorphous, quasicrystalline, or nanophase) is exactly given by the collection sphere integrated intensity of the resolved phase-specific contribution to the Compton corrected and electron unit normalized diffraction pattern, T(θ), divided by the wavevector (q) integrated average squared atomic form factor of the phase. Electron unit calibration is achieved by a global normalization that directly gives the product A⊥Io of the effective cross sectional area (A⊥) of the incident (and outgoing) beam and the effective incident beam intensity (Io), including counter efficiency, beam path losses, etc. The problem of incomplete collection sphere integration (including the q = 0 region) is resolved by showing that all the results hold for a given Bragg angle range of measurement for a sufficiently large range. We evaluate the accuracy of the method by application to synthetic binary amorphous-crystalline mixtures of (1) a rock standard, that provides an assembly of crystalline phases, (2) a certified OM standard, and (3) a synthetic inorganic amorphous phase (silica gel). We expect that the method will be particularly useful in surface sediment and environmental geochemistry applications.

Illite crystals in siliciclastic sediments are heterogeneous assemblages of detrital material coming from various source rocks and, at paleotemperatures >70 °C, of superimposed diagenetic modification in the parent sediment. We distinguished the relative proportions of 2M 1 detrital illite and possible diagenetic 1M d + 1M illite by a combined analysis of crystal-size distribution and illite polytype quantification. We found that the proportions of 1M d + 1M and 2M 1 illite could be determined from crystallite thickness measurements (BWA method, using the MudMaster program) by unmixing measured crystallite thickness distributions using theoretical and calculated log-normal and/or asymptotic distributions. The end-member components that we used to unmix the measured distributions were three asymptotic-shaped distributions (assumed to be the diagenetic component of the mixture, the 1M d + 1M polytypes) calculated using the Galoper program (Phase A was simulated using 500 crystals per cycle of nucleation and growth, Phase B = 333/cycle, and Phase C = 250/cycle), and one theoretical log-normal distribution (Phase D, assumed to approximate the detrital 2M 1 component of the mixture). In addition, quantitative polytype analysis was carried out using the RockJock software for comparison. The two techniques gave comparable results (r 2 = 0.93), which indicates that the unmixing method permits one to calculate the proportion of illite polytypes and, therefore, the proportion of 2M 1 detrital illite, from crystallite thickness measurements. The overall illite crystallite thicknesses in the samples were found to be a function of the relative proportions of thick 2M 1 and thin 1M d + 1M illite. The percentage of illite layers in I-S mixed layers correlates with the mean crystallite thickness of the 1M d + 1M polytypes, indicating that these polytypes, rather than the 2M 1 polytype, participate in I-S mixed layering.

This work presents the first systematic determination of the partial molar volumes of lanthanide sesquioxides in silicate melts. For this purpose, the densities of various lanthanide-bearing Na-silicate melts distributed along various binary joins, where the end-members are Na-disilicate and one of the lanthanide sesquioxides (i.e., Ce 2 O 3 , Pr 2 O 3 , Nd 2 O 3 , Sm 2 O 3 , Eu 2 O 3 , Gd 2 O 3 , Tb 2 O 3 , Dy 2 O 3 , Ho 2 O 3 , Er 2 O 3 , Tm 2 O 3 , and Yb 2 O 3 ), have been measured using the double-bob Archimedean method. The present results show that the addition of any lanthanide to Na-disilicate leads to an increase in the melt density and that the melt density increases with increasing atomic number of the lanthanide. From the present density data set, the molar volumes of these melts have been calculated and the partial molar volumes of each lanthanide sesquioxide in these melts have been determined using a linear regression through each binary join (i.e., Na-disilicate-lanthanide sesquioxide). This study indicates an ideal behavior with respect to the molar volume (i.e., a linear variation of the molar volume along each binary join) for Na-silicate melts containing up to 6 mol% of lanthanide oxide. Comparison between the partial molar volumes of lanthanide sesquioxides obtained in this study and the molar volumes of molten lanthanide sesquioxides given in the literature raises the possibility, however, that this ideality is not maintained along the entire Na-disilicate-lanthanide sesquioxide binary joins. Excess volumes of mixing appear to be required to describe the combined volumetric data set.

The XCO 2 recorded by mineral-fluid equilibria in contact metamorphosed siliceous carbonates commonly defines two groups of rocks in the same aureole. One group records relatively low XCO 2 that results from infiltration of chemically reactive H 2 O-rich fluid. The other records relatively high XCO 2 , up to 0.99, that results from decarbonation reactions with little or no infiltration. A complementary dichotomy in apatite compositions exists in five contact aureoles in Italy, Scotland, and the U.S.A. Apatite in the low-XCO 2 group is close to an F-OH solid solution. Apatite in the high-XCO 2 group is a relatively Cl-rich Cl-F-OH solution. The halogen content of fluid coexisting with analyzed apatite was characterized in two aureoles to determine the origin and significance of the dichotomy in apatite composition. Calculated αHF/aH 2 O, αHF/αHCl, αHF, and mF T (the total F molality of fluid) are systematically higher in fluid coexisting with the low-XCO 2 group. In contrast, αHCl/αH 2 O in the high-XCO 2 group may be higher than or overlap with αHCl/αH 2 O in the low-XCO 2 group. Calculated αHCl and mCl T in the high-XCO 2 group are lower than or overlap with aHCl and mCl T in the low-XCO 2 group. The Cl-rich apatites in the high-XCO 2 group are explained by crystallization at relatively low αH 2 O, αHF, and mF T rather than at high αHCl or mCl T . In comparison, the F-OH apatites in the low-XCO 2 group formed by infiltration of rock by and equilibration with relatively H 2 O-rich, high mF T /mCl T fluid, reflecting the same metasomatic process responsible for F-rich humite-group minerals and skarns in many contact aureoles. Calculated halogen contents indicate that the non-CO 2 fraction of fluid in equilibrium with both groups had modest, seawater-like salinity, and that the reactive H 2 O - and F-rich fluid that infiltrated the low-XCO 2 group had a plutonic source.

P-T pseudosections, constructed in MnNCKFMASH and adjusted for chemical compositional changes resulting from zoned garnet growth (chemical fractionation) in a pelitic rock, show negligible changes in the position of the peak metamorphic mineral assemblage field (garnet + biotite + plagioclase + sillimanite + quartz) compared to the position of this field calculated with the bulk-rock composition. Pelitic rock samples with less than 5% modal garnet were modeled using bulk-rock chemical compositions (unfractionated), and compositions adjusted for 1, 2, and 5% garnet growth, in order to model the effects of changes in effective composition on pseudosection topology. Differences in the location of mineral mode zero lines along the garnet growth P-T path and in the peak mineral assemblage field are generally less than 10 °C and/or less than 0.3 kbar. However, at some P-T conditions, significant changes in topology are observed. For example, at pressures above 9 kbar, large temperature shifts in the zoisite mode zero line change the pseudosection topology so that biotite+zoisite stability in the pseudosection with 5% garnet fractionation has a larger temperature range (>120 °C) than in the unfractionated pseudosection (<50 °C). The effects of porphyroblast growth-induced fractionation of bulk-rock chemistry can be determined from mineral chemistry and mineral modes; pseudosections can be constructed with the adjusted chemical compositions to resolve whether fractionation affects the pseudosection topology in the P-T range of interest. In the case of the North Cascades samples discussed here, garnet fractionation is estimated to have minimal effects on P-T paths determined from pseudosections. Therefore, pseudosection modeling based on bulk-rock chemistry can be used to estimate peak metamorphic P-T conditions and constrain parts of metamorphic P-T-t paths once the effects of fractionating minerals are understood.

Kosmochloric diopside with high K content up to 0.56 wt% (0.026 K atom per formula unit) was discovered from the Osayama serpentinite mélange in the Chugoku Mountains, SW Japan. K-bearing clinopyroxene fills microcracks (5.150 µm in wide) together with uvarovite within albite vein of a tremolite rock. Compositions of analyzed clinopyroxene consist mainly of kosmochlor + augite (92.98 mol%; Ko 19-38 Aug 56-76 ) components and minor amounts of jadeite (0.6 mol%), aegirine (0.5 mol%), Ca-Tschermak (0.3 mol%), and K-kosmochlor (0.2 mol%). Although the K content in clinopyroxene is also variable and heterogeneous even in a single vein, clinopyroxene with higher K content occurs in Ko-rich part. Higher magnification secondary electron images confirmed that exsolution and inclusion are essentially absent in the analyzed clinopyroxenes. The good negative correlation between Cr+ Na+ K and Ca+ Mg+ Fe 2+ indicates the Cr incorporation into the octahedral site. Furthermore, K correlates with Na and Cr, indicating a simultaneous enrichment of K for Na and Cr during pyroxene growth. Textual relations, and parageneses and compositions of minerals suggest that the K-Cpx precipitated together with uvarovite in brittle microcracks directly from a Ca- and Cr-rich hydrothermal fluid at approximately P < 0.3 GPa and T < 400 °C. Although it has been experimentally concluded that only ultrahigh-P (>4 GPa) environment permits to host relatively large K + cation into the clinopyroxene structure, our finding indicates that the incorporation of K into the kosmochlor-diopside series solid solution with at least 0.2 Cr cation p.f.u. is possible even at low P conditions.

The dehydration behavior of a natural stilbite sample from Poona (India) was investigated by in situ FTIR. The thermal induced variations of the water molecule bending (ν2) mode around 1653 cm -1 , the stretching (ν3 and ν1) modes around 3587 and 3426 cm -1 , and the corresponding second-order modes in the wavenumber region 4000.8000 cm -1 were followed as indicative of the dehydration process. The observed spectral variations indicate that stilbite undergoes a transformation at about 448 K due to the loss of half of the original content of water molecules. The rehydration of stilbite is partial in samples heated up to 630 K. Concerning both the dehydration and rehydration behaviors of stilbite, our results are in concert with those proposed in the literature. In addition, the growth of a new mode around 4550 cm -1 is observed in the temperature range 430-650 K and may indicate the presence of hydroxyl groups created by the breaking of the T-O-T linkages.

To study the temperature-dependent structural changes accompanying the phase transformation leadhillite ↔ susannite and to verify the close structural relationships between heated leadhillite and susannite, a leadhillite crystal has been investigated by X-ray single-crystal diffraction methods within the temperature range 25-100 °C. The values of the unit-cell parameters were determined at 25, 32, 35, 37, 40, 42, 45, 48, 50, 53, 56, 59, 62, 65, 68, 71, 75, 79, 82, 85, 90, 95, and 100 °C. After the heating experiment the crystal was cooled over the same temperature intervals and the unit-cell dimensions were determined again. The values measured with both increasing and decreasing temperature are in excellent agreement, indicating that no hysteresis occurs within the temperature range examined and that the phase transformation is completely reversible in character. Analysis of the components of the spontaneous strain shows only normal thermal expansion up to 50 °C and that the structural distortions leading to the topology of the heated leadhillite take place in the temperature range 50-82 °C. Our study confirms that the crystal structure of heated leadhillite is topologically identical to that of susannite and that the slight structural changes occurring during the phase transformation leadhillite ↔ susannite are mainly restricted to the sulfate sheet. Changes in the orientation of sulfate tetrahedra and in the Pb-O coordination polyhedra occur in a continuous way within the temperature range investigated as indicated by the second-order character of the phase transition. In this way, the leadhillite structure gradually goes toward that of susannite without abrupt structural changes.

Extinction data sets for four centimeter-sized anisotropic crystals were collected in air with standard spindle-stage methods and submitted to a new Windows-based version of EXCALIBR, termed EXCALIBRW. EXCALIBRW solved these data sets with varying degrees of accuracy related to the external shape of the crystal: the more rounded the crystal, the more precise the results. For an olivine crystal ground into a sphere, the results were similar to those obtained for a crystal immersed in an index-matching fluid. However, even for samples bounded with growth or cleavage faces, the program determined the orientation of the optical indicatrix and 2V with an error of only 1−2°. Thus, this logical extension of spindle-stage methods is helpful: (1) to orient centimeter-size single crystals for various types of mineralogical measurements (e.g., spectroscopy or diffusion studies in which it might be undesirable to place the sample in a liquid); (2) as a non-destructive means of identifying gemstones based upon a determination of their optical class (i.e., isotropic vs. uniaxial vs. biaxial); and (3) for optical characterization by determination of 2V. In addition, the newest version of EXCALIBR is easier to use, mathematically more robust in its solution algorithms, and provides solutions for crystals in less favorable orientations than the earlier versions of EXCALIBR.

Zoltaiite, ideal formula BaV 2 4+ V 12 3+ Si 2 O 27 , space group P3̅, a = 7.601(1), c = 9.219(1) Å, V = 461.34(1) Å 3 , Z = 1, is a new mineral found on the eastern edge of the Shuswap metamorphic complex of British Columbia, Canada. It is a metamorphic mineral formed under greenschist-facies P-T conditions as part of an assemblage that includes quartz, celsian, apatite, sphalerite, pyrrhotite, galena, and pyrite. Zoltaiite has a Mohs hardness of 6.7, no cleavage, an anhedral to semi-prismatic habit, and a calculated density of 4.83 g/cm 3 . It is opaque with reflectance and color similar to those of sphalerite. The strongest eight lines of the X-ray powder diffraction pattern [d in Å (I) (hkl)] are 3.103(78)(021), 2.934(89)(21̅2), 2.785(67)(013), 2.679(48)(022), 2.403(50)(211), 2.190(100)(212), 1.934(53)(213), and 1.438(63)(140). The empirical formula, derived from electron-microprobe analysis and the crystal structure, is Ba 1.05 (Ti 1.31 V 4+ 0.69 ) Σ2.00 (V 3+ 11.06 Fe 3+ 0.49 Cr 0.34 ) Σ11.89 Si 2.06 O 27 based on O = 27. The crystal structure was solved by direct methods and refined on the basis of F 0 2 using all unique reflections measured with MoKα Xradiation on a CCD-equipped diffractometer. The final R factor was 3.2%, calculated using 659 unique observed reflections. The unit cell contains four layers of two types parallel to (001): X, an octahedral and tetrahedral sheet, and Y, an octahedral plus barium sheet; both layers are doubled through inversion centers resulting in the sequence XXYY... Two consecutive equivalent layers are interconnected through shared octahedral edges, whereas consecutive non-equivalent layers are linked through shared corners. The high calculated density is consistent with the dense packing of the structure.

The crystal chemistry of seven crystal fragments taken from differing regions of the same colorless to yellow-greenish tourmaline macro-crystal from pegmatite pockets in aplite veins (island of Elba, Italy) was studied with a multi-disciplinary (SREF, XRDT) and multi-analytical approach (EMPA, SIMS). EMPA and XRDT studies showed relationships between color and chemical zoning and crystal- growth evolution, indicating which fragments could be considered representative of the chemical evolution of the genetic micro-environment in which the crystal developed. Results showed that the colorless fragment is an elbaite while the yellow-greenish crystal fragments are Mn 2+ -rich (up to 1.34 apfu) and belong to the alkali group and fluor subgroup. They are characterized by dehydroxylation and alkali-defect type substitutions that cooperate in reducing Li and increasing Mn contents. The Y site is populated by Al, Li, and Mn 2+ , and the Z site by Al and Mn 2+ (up to 0.10 apfu). In contrast with data in the literature, Mn 2+ populates both octahedral sites according to the order-disorder reaction: Y Mn + Z Al ↔ Y Al + Z Mn. As Mn 2+ content increases, progressive disorder takes place. This disorder is quantitatively lower than that of the ZMg in dravite, due to the low structural tolerance of the small Z cavity in the incorporation of larger cations by the Z R 2+ → Z Al substitution. Relationships of direct proportionality between lattice parameters and both <Y-O> and <Z-O> are observed. The expansion of both octahedra, as well as of lattice parameters, increases linearly as a function of Y Mn 2+ and Z Mn 2+ . The latter has greater weight in dictating unit-cell variations, due to the degree of size mismatch between Z Mn 2+ → Z Al and Y Mn 2+ → Y Li substitutions, and the way in which the Z octahedra are articulated in the structure.

Infrared spectra of magnesite, MgCO 3 , have been measured in the diamond anvil cell from 0 to 60 GPa at 300 K. Mode shifts were determined for the in-plane bend, the asymmetric-stretch, and the out-of-plane bend of the carbonate group, as well as a magnesium ion translational mode. The in-plane bend shows a small positive shift and ultimately merges with the out-of-plane bend, exhibiting an accidental degeneracy by 60 GPa. The asymmetric stretch exhibits highly nonlinear behavior, shifting monotonically to pressures of ~30 GPa, at which point its shift markedly decreases to ~50 GPa. At 50 GPa, it again shifts strongly with pressure. We calculate force constants for the carbonate group under pressure from our data, and correlate these with the C-O bond length. Our data are consistent with a slight lengthening (~0.01 Å) of the C-O bond between 30 and 50 GPa to a maximum bond distance of ~1.27 Å. Although our finding of bond lengthening is in general agreement with previously published Rietveld refinements of X-ray diffraction data for magnesite under pressure, our data indicate that the lengthening is likely substantially smaller than previously proposed. The expansion of the C-O bond in magnesite can be produced by rotations of the MgO 6 octahedra, and such non-ionic crystalchemical effects may be a key contributor to the remarkable stability of R3̅c-structured carbonates to very high pressures.

Studies of melt inclusions trapped in magmatic phenocrysts can provide a new perspective on several key outstanding problems in the understanding of the genesis of orthomagmatic ore deposits, particularly with respect to the concentration of metals in parental magmas. The published data shows a mismatch between low and high abundances of Cu (and Ag) in unheated and remelted melt inclusions, respectively. This experimental study investigates the possibility that quartz-hosted rhyolitic melt inclusions may change their composition during laboratory heating under different conditions. Exceptional volatility of Cu and Ag and inert behavior of other metals (Zn, Pb, Mo, Sn, W) and lithophile trace elements at high temperature (850 °C) is demonstrated. Heating experiments with melt inclusions require specific conditions that should take the high volatility of Cu and Ag into account. The open system behavior of Cu and Ag can also affect the composition of melt inclusions within the time frame between trapping and eruption

We document the presence of a high pressure phase transition in metastable calcite-III using infrared spectroscopy. The post-calcite-III transition initiates at a pressure of 15.5 (±2) GPa, and is completed between 25 and 30 GPa. The transition is particularly apparent in the ν 4 -in-plane bending vibration of the carbonate group, in which two new peaks progressively supplant the doublet associated with calcite-III. Additionally, both the ν 3- asymmetric and ν 1 -symmetric stretches of the carbonate group in the high-pressure phase occur at substantially lower frequencies than the extrapolated positions of the corresponding calcite-III peaks. The geometry of the carbonate unit within the high-pressure phase is likely closer to trigonal symmetry than in the calcite-III structure, and the C-O bond is probably longer than in the lower pressure calcite-III phase.

Pyrochlore-type (A 2 B 2 O 6 O’) ceramics are considered for the immobilization of highly radioactive waste. Understanding the alteration process of such potential nuclear waste form materials in aqueous media is critical for the prediction of their long-term stability in a nuclear repository. Current models on pyrochlore alteration are based on a diffusion-controlled hydration and ion exchange process. However, we present results of a hydrothermal experiment at 200 °C with a natural, polycrystalline pyrochlore and 18 O-enriched aqueous solution, which are not compatible with a process based on solid-state diffusion. TOF-SIMS and confocal μ-Raman mapping of the run product revealed the occurrence of 18 O-enriched alteration zones with sharp chemical gradients to relict unreacted areas when compared to the extent of the alteration zones. The data are consistent with a pseudomorphic reaction that involves the dissolution of the pyrochlore parent accompanied by the simultaneous reprecipitation of a defect pyrochlore at a moving dissolution-reprecipitation front.

Garnet from an aposkarn achtarandite-bearing rodingite-like rock in Sakha-Yakutia, Russia, has a Sc content close to 6 wt% Sc 2 O 3 (~0.45 apfu). The scandian garnet is a relict mineral from a hightemperature, shallow-level melilite skarn. Structural and electron microprobe data for a crystal of the scandian garnet with cell parameter a = 12.331(1) Å, Ia3̅d allows refinement of the structural formula (Ca 2.97 Mg0.02Y0.01) Σ3 (Fe 3+ 0.663 Zr 0.584 Ti 4+ 0.294 Sc 0.153 Cr 0.152 Mg 0.094 Fe 2+ 0.04 Hf 0.008 V 0.003 ) Σ2 (Si 1.898 Al 0.420 Ti 4+ 0.359 Fe 3+ 0.323 ) Σ3 O 12 . Investigation of the composition of many of the scandian garnets reveals the existence of a solid-solution between kimzeyite-schorlomite Ca 3 (Zr,Ti) 2 (Al,Fe) 2 SiO 12 and the scandium analog of andradite Ca 3 Sc 2 Si 3 O 12 . This is the first report of a natural scandian garnet.