Volume 96, Issue 1

The mineralogical composition of mining wastes deposited in voluminous tailing impoundments around the world is the key factor that controls retention and release of pollutants. Here we report a detailed mineralogical, geochemical, and microbiological investigation of two tailing impoundments near the town of Pezinok, Slovakia. The primary objective of this study was the mineralogy that formed in the impoundment after the deposition of the tailings (so-called tertiary minerals). Tertiary minerals include oxyhydroxides of Fe, Sb, As, Ca and are present as grains and as rims on primary ore minerals. X-ray microdiffraction data show that the iron oxyhydroxides with abundant As are X-ray amorphous. The limiting (lowest) Fe/As (wt/wt%) ratio in this material is 1.5; beyond this ratio, the hydrous ferric oxide does not retain arsenic. The grains with less As and little to moderate amounts of Sb are goethite; the grains where Sb dominates over Fe are poorly crystalline tripuhyite (FeSbO 4 ). Even the most heavily contaminated samples (up to 29 wt% As 2 O 5 ) are populated with diverse communities of microorganisms including typical arsenic-resistant heterotrophic species as well as iron reducers and sulfur oxidizers. Several recovered clones cluster within phylogenetic groups that are solely based on environmental sequences and do not contain a single cultivated species, thus calling for more work on such extreme environments.

This study explores the effects of metamorphism on luminescence of diamond. Four diamond suites extracted from metamorphic rocks were characterized using optical cathodoluminescence, FTIR spectroscopy, photoluminescence at the liquid nitrogen temperature, and cathodoluminescence spectroscopy. The studied diamonds are from sedimentary conglomerate and lamprophyric breccia metamorphosed to the greenschist facies (Wawa, Canada), and from the ultrahigh-pressure (UHP) terranes of Kokchetav (Kazakhstan) and Erzgebirge (Germany). The majority of studied diamonds that experienced metamorphism have distinct cathodoluminescence and photoluminescence when compared to diamonds from unmetamorphosed rocks, i.e., kimberlites or placers. Diamonds in metamorphic rocks do not show the prevalent blue cathodoluminescence emittance at 415-440 and 480-490 nm typical of common kimberlite and placer diamonds. Ultrahigh-pressure diamonds are mostly non-luminescent, whereas diamonds from greenschists display green, yellow, orange, and red cathodoluminescent colors emitting light at 520 and 576 nm, and between 586 and 664 nm. The shift of cathodoluminescence in octahedrally grown Type Ia diamonds metamorphosed in the greenschist facies results from the presence of optical centers with zero-phonon lines at 575 and 637 nm detected via photoluminescence spectroscopy. The centers may have formed due to irradiation of diamonds in the upper crust followed by annealing at T > 500 °C. This process creates vacancies that diffuse through the diamond lattice and get trapped at N sites, creating NVo and NV- optical centers. The distinct luminescence of metamorphosed diamonds can persist for billions of years in the crust, but is annealed in the mantle.

Through the complementary use of single-crystal X-ray diffraction and X-ray absorption spectroscopy, we present in this paper the first direct results on the site occupancy of thorium in the fluorapatite structure and the structural distortion created by its substitution. Structure refinements based on single-crystal X-ray diffraction data from synthetic Th-doped fluorapatite indicates that Th substitutes almost exclusively in the M2 site. A single-crystal X-ray study of natural fluorapatite from Mineville, New York, also indicated that substituting heavy scatterers (including Th) are concentrated in the apatite M2 site, but definitive site assignments of specific elements were not possible. Extended X-ray absorption fine-structure spectroscopy (EXAFS) was used to probe the local structure of Th in the synthetic fluorapatite (single-crystal form) with a Th concentration of roughly 20 000 ppm, as well as Th in the natural Mineville fluorapatite (powder form) with a Th concentration of ~2000 ppm. The EXAFS fitting results also indicate that Th partitions into the M2 site and yield a ~0.05-0.08 Å decrease of average M2-O bond distances associated with local structure distortions that are not obtainable from single-crystal X-ray diffraction studies.

The crystal chemistry of a pegmatitic Fe-bearing muscovite (with FeO ~5.1 wt%) from Val di Crana, Arvogno (Verbania, Nothern Italy) has been investigated by means of wavelength-dispersive X-ray spectroscopy and constant-wavelength [λ = 0.9462(2) Å] single-crystal neutron diffraction at 295 and 20 K (2θ max = 90°). The structure refinement at 295 K shows no significant deviation from the full occupancy of the K site (using the scattering length of potassium alone), and a disordered Si/Aldistribution in the two independent tetrahedral sites (i.e., T1 and T2) and Fe/Al-distribution in the M2 octahedral site. The difference Fourier map of the nuclear density shows that only one independent H site occurs in the muscovite structure, at both room and low temperature. No evidence of disorder in two sub-sites, as observed in a previous study, was found. The thermal displacement of the H site is here described anisotropically. A trifurcated hydrogen bonding scheme was found: the O6-H bond distance corrected for “riding motion” is ~0.984 Å at 295 K, and three significantly weak hydrogen bonds to the O atoms O2, O4, and O6 occur, with H···O2 = 2.635(5), H···O4 = 2.657(4), and H···O5 = 2.647(4) Å and O6-H···O angles all similar in value (~138°). The structure refinements show that the structure configuration of muscovite is maintained at least down to 20 K: the cation disordering in the tetrahedral and octahedral site and the H-bonding scheme are preserved. The structural evolution induced by decreasing temperature is governed by the contraction along a direction perpendicular to (001), mainly due to the compression of the “inner” K-O bonds, which leads to an increase of the tetrahedral rotation angle (α) of the six-membered ring. A further effect in response to lowering the temperature is a significant reduction of the magnitude of the thermal displacement parameters.

Natural single-crystal specimens of the montebrasite/amblygonite series from Brazil, with general formula LiAlPO 4 (F,OH), were investigated by electron microprobe, Raman spectroscopy, X-ray diffraction, and infrared absorption. Since little is known about impurities and their local symmetries, electron paramagnetic resonance (EPR) was applied. Six different paramagnetic impurities and radiation defects were detected by EPR. Three of them, all substituting for Al 3+ ions, namely, iron (Fe 3+ ), vanadium (V 4+ ), and niobium (Nb 4+ ) impurities were characterized in this work. The Fe 3+ (3d 5 )-related EPR spectra and angular dependencies show occupation of low-symmetry sites that are revealed in the high asymmetry parameter of the electronic fine structure, E/D = 0.27. Vanadium and niobium impurities are identified through their typical strong hyperfine interactions. Both form interesting examples for which the properties of 3d 1 ion (V 4+ ) and 4d 1 ion (Nb 4+ ) in the same host matrix can be compared. It is shown that both ions form complex defects of type VO 2+ (vanadyl) and NbO 2+ (niobyl), showing superhyperfine interaction with two equivalent hydrogen ions and not to fluorine. The EPR rotation patterns are analyzed in detail for three mutually perpendicular crystal planes. Spin Hamiltonian parameters are calculated and discussed.

The reliability of trace element concentrations obtained by EPMA can be significantly improved with the use of high-quality secondary standards. In the case of Au residing in sulfides, such standards are lacking. Natural materials have heterogeneous Au distribution, whereas synthesis is very difficult. The benefits of using ion implants as trace-element reference material for EPMA were assessed by characterizing grains of magnetite, pyrite and galena implanted with 1 × 10 14 to 5 × 10 14 Au atoms/cm 2 at energies from 1 to 3 MeV. The first interesting observation is the excellent lateral micrometer-scale homogeneity of the Au levels across the implants. The ratio of analytical to statistical standard deviations never exceeds 1.7. Additionally, the Au X-ray intensities measured by EPMA show excellent correlation with those predicted for multilayered structures used to model the continuous Au concentration profile for the three implants investigated. Small discrepancies arise only at low accelerating voltage. In these situations, the predicted Au X-ray intensities become sensitive to uncertainties in the determination of the location of the Au concentration profile because of insufficient excitation of the bottom of the Au layer. Fortunately, by varying the implantation energy, optimal implants yielding X-ray intensities that are insensitive to uncertainties on the Au depth profile can be obtained for a wide range of accelerating voltages. These results suggest that ion implants may represent excellent EPMA reference material, especially in cases where natural and synthetic standards are unavailable. Interesting materials presenting specific analytical challenges can be engineered due to the excellent control of the implantation parameters.

We investigated the incorporation of hydrogen into zircon at 1650 and 1550 °C, and pressures of 2.5 and 1.5 GPa under water-saturated conditions in a piston-cylinder apparatus. Concentrations were determined by polarized Fourier transform infrared spectroscopy using the zircon absorption coefficient ε i = 36 241 cm -2 per mol H 2 O/L and range from ~90 to 200 ppm H 2 O by weight. Crystals grown in the presence of Ti 4+ or Th 4+ do not differ significantly in their H 2 O content. We also synthesized zircons with various concentrations of Lu 2 O 3 and Al 2 O 3 to characterize changes in band positions and hydrogen concentrations related to coupled substitutions in zircon. Trivalent cations correlate in a nearly 1:1 molar fashion with hydrogen highlighting a potentially important coupled substitution in high water activity environments. Bands from undoped and doped zircons in the OH stretching region of the infrared spectrum show broad agreement when compared to spectra from natural samples. Heating experiments at 1 atm and 1000 °C produce a decrease in the integrated area; while some bands disappeared entirely, others are particularly stable with little decrease in integrated area after 128 h at 1000 °C. Results presented here help eliminate uncertainties that arose from Fourier transform infrared studies of natural zircons and provide further clarification for the origin of band positions in natural samples. In addition to the water activity of the crystallizing medium, the H 2 O content of natural grains will likely be significantly influenced by trivalent cation concentrations. In crustal zircons especially, trivalent atomic contents generally exceed those of phosphorus, meaning that hydrogen may be particularly important for trivalent cation charge compensation. An unanticipated result of this study was the development of a reasonably effective technique that produces relatively homogenous zircons doped with minor impurities. This technique could potentially be utilized in studies aimed at developing zircon standards, because it yields crystals that appear to be more homogenous than those produced by the flux method, and are generally free of inclusions.

Four manganese ore samples were subjected to sequential extractions with hydrogen peroxide and dithionite-citrate-bicarbonate. The first attack was meant to remove all manganese-bearing phases, whereas the second treatment was done to remove any residual manganese and the iron oxyhydroxides. The ores and the residues of both treatments were characterized by chemical analyses, X-ray diffraction, optical microscopy, and Mössbauer spectroscopy. The combination of a hydrogen peroxide extraction followed by the well-known dithionite-citrate-bicarbonate extraction allowed to establish of how phosphorus is present in these ores. No correlation between phosphorus and the manganese phases was found in any sample. In one sample, phosphorus was found as crandalite, an aluminum phosphate. On the other hand, the association between phosphorus and goethite was clearly observed in the other samples.

The incorporation of the carbonate ion into the crystal structure of hydroxylapatite results in the creation of vacancies, oxygen-loss, and disorder, with consequent changes in physical and chemical properties. High-pressure experimental investigation up to 10 GPa of two synthetic carbonated hydroxylapatite samples with up to 11 wt% CO 3 , using a diamond-anvil cell and synchrotron powder X-ray diffraction, provides the first rigorous assessment of the mechanical behavior of the carbonated hydroxylapatite. The pressure-volume data suggest that the isothermal bulk modulus of these carbonated hydroxylapatites has been significantly decreased by the presence of the carbonate (up to about 15%), which in turn will affect all the carbonated apatite-related reactions in the geosystem. Since hydroxylapatite is one of the major components of the bones and teeth, the incorporation of the carbonate in the hydroxylapatite weakens teeth and bones not only chemically, but also physically.

X-ray diffraction and Raman spectroscopy confirm a rare terrestrial occurrence of monoclinic tridymite in clast-rich impact melt rock from the Eyreville B drill core in the Chesapeake Bay impact structure. The monoclinic tridymite occurs with quartz paramorphs after tridymite and K-feldspar in a microcrystalline groundmass of devitrified glass and Fe-rich smectite. Electron-microprobe analyses revealed that the tridymite and quartz paramorphs after tridymite contain different amounts of chemical impurities. Inspection by SEM showed that the tridymite crystal surfaces are smooth, whereas the quartz paramorphs contain irregular tabular voids. These voids may represent microporosity formed by volume decrease in the presence of fluid during transformation from tridymite to quartz, or skeletal growth in the original tridymite. Cristobalite locally rims spherulites within the same drill core interval. The occurrences of tridymite and cristobalite appear to be restricted to the thickest clast-rich impact melt body in the core at 1402.02-1407.49 m depth. Their formation and preservation in an alkali-rich, high-silica melt rock suggest initially high temperatures followed by rapid cooling.

To determine the phase boundary between the perovskite and post-perovskite structures in MnGeO 3 , in situ X-ray observations were carried out at pressures of 57-68 GPa and temperatures of 1000-1900 K using the Kawai-type high-pressure apparatus equipped with sintered diamond anvils interfaced with synchrotron radiation. The phase boundary was determined to be P (GPa) = 39.2 + 0.013T (K) based on Tsuchiya’s (2003) gold pressure scale. The Clapeyron slope, dP/dT, of 13(+12/-5) MPa/K, determined in the present study is larger that of MgGeO 3 and MgSiO 3 .

Hydrogen is the most abundant element in the solar system, suggesting that hydrogen is one of the plausible light elements in the planetary cores. To investigate the solubility of hydrogen into FeSi and phase relations of the FeSi-H system under high pressure, we performed in situ X-ray diffraction experiments on the FeSi-H and FeSi systems at high pressure and high temperature. Hydrogen starts to dissolve in FeSi (hydrogenation) and form FeSiH x with cubic B20 structure above 10 GPa. Hydrogen content (x), estimated from the volume difference between the FeSi-H and FeSi systems, increases from 0.07 to 0.22 with increasing pressure for P > 10 GPa. Comparing the present results with hydrogenation pressure of Fe, presence of Si in metal increases the minimal pressure for H incorporation. Hydrogen, therefore, can only incorporate into the Fe-Si core at the deeper part (P > 10 GPa) in the planetary interior.

Silicate-oxide equilibria (abbreviated as QUIlF) have proven to be very powerful tools for reconstructing the temperature and oxygen fugacity evolution of magmatic systems containing magnetite and ilmenite with olivine, quartz, or pyroxenes. In this paper, we extend these QUIlF equilibria to include rocks where silica activity is controlled by equilibria between feldspars and feldspathoids. We present data on the orthomagmatic assemblage of titanomagnetite + ilmenite + feldspar + nepheline + compositionally variable olivine, which we call AUNIlF: The AUNIlF reference curve (with unit activities for albite, nepheline, and fayalite) is stable at oxygen fugacities ≥2 log units below the QUIlF surface at temperatures of about 700 to 800 °C, temperatures typical of late-magmatic stages. At temperatures > ~800 °C, the AUNIlF reference assemblage would only be stable at unrealistically low f O₂ conditions more than 5 log units below FMQ (where FMQ is the fayalite-magnetite-quartz buffer), which explains the rarity or absence of orthomagmatic AUNIlF assemblages. We determine the most reduced conditions indicated by displaced AUNIlF assemblage from Mont Saint-Hilaire (Quebec, Canada) to be ΔFMQ = -1.15 at ~800 °C (olivine is Fa 67 and a SiO₂ = 0.41) and conclude that AUNIlF assemblages involving pure fayalite do not stably occur in terrestrial magmatic systems. The stability field of naturally occurring AUNIlF assemblages is a function of albite, nepheline, and olivine compositions and is controlled by the ratio of silica activity to fayalite activity (a SiO₂ / a Fa ). At values higher than ~0.77 for a SiO₂ /aFa, olivine is Fa < ~70 when silica activity is buffered by the nepheline-albite equilibrium. In these situations, AUNIlF is stable at oxygen fugacities ≥ -1.15 (ΔFMQ). At values below a SiO₂ /a Fa ~0.77, the AUNIlF equilibrium is shifted to lower oxygen fugacities and ilmenite becomes unstable relative to ulvøspinel. Analogous to the construction and application of AUNIlF, a QUIlF-type reaction curve for potassic systems (KULIlF) involving leucite and alkali feldspar is presented and applied to naturally occurring assemblages. Potassic rocks invariably crystallize forsteritic olivine in the presence of ilmenite and magnetite, reflecting higher oxygen fugacities during crystallization than their sodic counterparts. As a result of low fayalite component in olivine, the a SiO₂ /a Fa ratio becomes ≥4 in assemblages of potassic systems consisting of alkali feldspar, magnetite, leucite, ilmenite, and olivine.

Volatile degassing is a major process driving volcanic eruptions. Therefore, a full understanding of mechanisms ranging from bubble nucleation, growth, coalescence, to magma fragmentation is required. We have simulated magma degassing during ascent in the volcanic conduit by depressurizing hydrated haplogranite melts in high-pressure and high-temperature optical cells (a hydrothermal diamond-anvil cell and an internally heated pressure vessel fitted with sapphire windows). This allowed the whole process of bubble nucleation, growth, and coalescence to be directly observed in situ through images captured from the recording videos. Bubble nucleation pressures, number densities, growth laws, and characteristics of coalescence were estimated as a function of melt water content, decompression rate, and temperature. Melt/vapor surface tension during bubble nucleation and coalescence was calculated. Our data show good agreement with those previously obtained in classical vessels. Methodological improvements are proposed for the experimental simulation of magma degassing in volcanic conduits.

The full elastic tensors of the sheet silicates muscovite, illite-smectite, kaolinite, dickite, and nacrite have been derived with first-principles calculations based on density functional theory. For muscovite, there is excellent agreement between calculated properties and experimental results. The influence of cation disorder was investigated and found to be minimal. On the other hand, stacking disorder is found to be of some relevance for kaolin minerals. The corresponding single-crystal seismic wave velocities were also derived for each phase. These revealed that kaolin minerals exhibit a distinct type of seismic anisotropy, which we relate to hydrogen bonding. The elastic properties of a shale aggregate was predicted by averaging the calculated properties of the contributing mineral phases over their orientation distributions. Calculated elastic properties display higher stiffness and lower p-wave anisotropy. The difference is likely due to the presence of oriented flattened pores in natural samples that are not taken into account in the averaging.

A series of synthetic apatite crystals along the fluorapatite-chlorapatite Ca 5 (PO 4 ) 3 (F,Cl) join have been synthesized at 1220 to 1375 °C from Ca 3 (PO 4 ) 2 dissolved in a CaF 2 -CaCl 2 melt. The solid solutions have then been characterized both chemically and structurally. Because of well-known difficulties in measuring the chemical composition of apatites containing F and Cl by electron microprobe a new semi-micro wet chemical method has been developed. Apatite is relatively well dissolved in mineral acids including HNO 3 . Nitric acid digestion of apatite for analysis of F and Cl has not been applied in the past to our knowledge. One reason is the potential risk of losing gaseous HF and HCl during acid decomposition. We present an analytical procedure that enables the analyses of F, Cl, Ca, P, and trace elements after digestion of 10 mg apatite with a small amount of 1 N HNO 3 in gas-tight PTFE tubes at 80 °C. Analytical results from three independent closed acid digestions of 20 synthetic fluor-chlorapatites are presented. The reliability of this method was tested on the basis of charge balance, the Ca/P, Ca/(F+Cl), and P/(F+Cl) ratios, and the total cation sum. Independently derived Cl/F ratios by XRD using Rietveld refinement of apatite crystals from the same synthesis experiments are additionally presented. The apatite solid solutions were characterized by IR spectroscopy, X-ray powder diffraction using Rietveld refinements (XRD), and single-crystal diffraction (SCXRD) structure determinations. IR spectra indicate no or only a very minor hydroxylapatite component in the solid solutions. Lattice parameters of the apatite solid solutions vary systematically with composition. The a-lattice parameter varies linearly. In contrast to F - , which occupies a special position at (0,0,1/4) in the unit cell, Cl - occupies split positions ranging from 0.346(4) to 0.4428(2) in the z-coordinate because of the larger ionic Cl - radius. Single-crystal structure determinations reveal that the Ca2 site is also split into Ca2A and Ca2B sites for F - and Cl - as nearest neighbors, respectively. In comparison to the Ca2A-F - arrangements, the larger Cl - ion presses the triangular arrangement of Ca-ions outward to facilitate an energetic more favorable constellation resulting in the formation of Ca2B-Cl - arrangements. The occupancies of the Ca2A and Ca2B sites, as determined by SCXRD, correlate strictly with the F-Cl content. Apatites rich in Cl (x Cl > 0.5) show a deficiency in halogens, which is probably charge balanced by oxygen (Cl - +Cl - ↔ O 2- +□) forming an oxyapatite component. The oxyapatite component prevents the F-absent, Cl-rich apatite from crystallizing with a monoclinic symmetry and stabilizes the observed hexagonal symmetry.

Two pyrophyllite samples, one S037, from the Coromandel region of New Zealand and the other, S076, from Berosovska, Ural, Russia, were studied by thermogravimetric (TG and DTG), infrared (IR), and X-ray diffraction (XRD) methods to investigate structural transformations of these samples at different stages of their partial dehydroxylation. The samples were heated at different temperatures during 45 min and the degree of dehydroxylation was estimated as a ratio of mass loss of each particular heated specimen to the total mass loss of the sample during total dehydroxylation. Sample S076 consists only of trans-vacant (tv) layers because its DTG curve and IR spectrum contain a single dehydroxylation maximum at T max = 723 °C and an OH stretching band 3675 cm -1 , respectively. In sample S037 tv and cis-vacant (cv) layers are interstratified at random and its DTG and IR spectrum contain, respectively, two dehydroxylation maxima at 595 and 760 °C and two stretching bands at 3675 and 3668 cm -1 . The positions and intensities of the reflections in the experimental powder XRD patterns of sample S037, as well as the refined parameters of the unit cell, almost coincide with those determined for 1A pyrophyllite. However, the XRD patterns contain an “additional” peak with d = 4.454(6) Å, which is absent in normal 1A pyrophyllite. This peak can be considered as an indicator of a structure in which tv and cv layers are interstratified. The XRD patterns from the oriented specimens of the studied samples heated above 500 °C show splitting of the basal reflections. Simulation of the XRD patterns from oriented specimens of the S076 sample show that the phase composition of the specimens is the same independent of the heating temperature and represents a physical mixture of a non-treated original pyrophyllite and an almost completely dehydroxylated phase, which contains 5% of non-dehydroxylated layers. The higher the temperature, the higher the content of the dehydroxylated-rich phase in the heated sample. Simulation of the XRD patterns from the heated S037 samples show that the phase composition of each specimen is a physical mixture of low- and high-dehydroxylation phases, referred to as LD and HD phases, respectively. Each of these phases is a mixed-layered structure in which the original non-dehydroxylated (ND) and completely dehydroxylated (CD) layers are interstratified at random. The main difference between these phases is that in the LD phase the content of ND layers prevail, whereas in the HD phase CD layers dominate. The increase in temperature and degree of dehydroxylation change the relative content of the LD and HD phases as well as the relative amount of ND and CD layers in these phases. Interstratification of ND and CD layers in the LD and HD phases is not consistent with the commonly accepted model according to which during dehydroxylation of pyrophyllite and related layer silicates, nondehydroxylated and dehydroxylated domains can coexist within one layer. As a result of this inconsistency, the model explaining the wide temperature interval of pyrophyllite dehydroxylation needs to be reconsidered. A similar inconsistency arises with the model predicting the formation of several intermediate semi-dehydroxylated structures during pyrophyllite dehydroxylation because evidence for such phases was not observed. A new model of pyrophyllite dehydroxylation is presented, which is consistent with the hypothesis, that the reaction is homogeneous and spontaneous nucleation and growth of completely dehydroxylated layers takes place during pyrophyllite dehydroxylation. In terms of this model, the large temperature interval of pyrophyllite dehydroxylation is related to particle size distribution as well as to structural disorder. A mechanism for the formation of the mixed-layered structures of the LD and HD phases is proposed.

A 94-layer long-period mica polytype was studied by transmission electron microscopy (TEM). This is the longest periodicity found up to now in micas (c ≈ 95.9 nm). It was observed in a fragment of a Mg-rich annite (biotite) crystal from dacite rocks of Džep, Serbia. The crystal region containing it extends about 800 nm along 1/c*. One-dimensional lattice fringe images obtained by bright-field (BF) illumination allowed identification of the very long-period polytype. The latter was characterized by selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). It is an inhomogeneous polytype belonging to the subfamily-A, based on the 2M1 structural series. Its 94-layer stacking sequence is: [(22̄) 3 202̄(22̄) 3 2̄2(22̄) 2 2̄2(22̄) 3 2̄2(22̄)202̄(22̄)2̄2(22̄) 3 2̄2(22̄) 2 2̄2(22̄) 4 2̄2(22̄) 2 2̄2(22̄) 3 2̄2(22̄) 4 2̄2(22̄) 2 2̄2]. The fringe contrasts of the BF images were correlated to the stacking sequence determined from HR images. The correlation verified that the same sequence occurred eight consecutive times. Analytical electron microscopy (AEM) revealed that the chemical composition of the 94-layer mica polytype is similar to that previously observed in randomly stacked and faulted areas of the same crystal. No remarkable chemical variation occurs between the 94-layer polytype and its adjacent crystal regions, the latter containing non-periodic stacking faults.

Stichtite, ideally Mg 6 Cr 2 CO 3 (OH) 16 ∙4H 2 O, from Stichtite Hill, Tasmania, Australia, and barbertonite, also ideally Mg 6 Cr 2 CO 3 (OH) 16 ∙4H 2 O, from the Kaapsehoop asbestos mine, South Africa, have been studied by powder X-ray diffraction and their structures have been refined using the Rietveld method. Stichtite from Stichtite Hill crystallizes in the rhombohedral space group R3̄m, with unitcell parameters a = 3.09575(3) and c = 23.5069(6) Å, V = 195.099(6) Å 3 , with Z = 3/8. Barbertonite from the Kaapsehoop asbestos mine crystallizes in the hexagonal space group P6 3 /mmc. The co-type specimens of barbertonite were found to be intergrown mixtures consisting of barbertonite and stichtite. Unit-cell parameters of barbertonite from the co-type specimens were a = 3.09689(6), c = 15.6193(8) Å, and V = 129.731(8) Å 3 and a = 3.09646(6), c = 15.627(1) Å V = 129.76(1) Å 3 , and Z = ¼. Rietveld refinements of both stichtite and barbertonite show that they are polytypes rather than polymorphs and do not represent distinct mineral species. Several possible nomenclature systems are discussed for the naming of hydrotalcite minerals and groups. Raman band assignments are also presented for stichtite from Stichtite Hill. Stichtite and hydrotalcite minerals make up a large proportion of the ore at the Mount Keith nickel mine in Western Australia. Bulk powder diffraction shows the ore contains 6.1 wt% stichtite and 5.6 wt% iowaite. Hydrotalcite group minerals provide an important potential reservoir of CO 2 . At Mount Keith, the amount of CO 2 mined as stichtite could exceed 45 000 metric tons per year, while exchange of Cl for CO 3 could fix in excess of 40 000 metric tons CO 2 per year if end-member iowaite is reacted to form pyaroaurite.

The new mineral vorlanite, (CaU 6+ )O 4 , D calc = 7.29 g/cm 3 , H = 4-5, VHN 10 = 360 kg/mm 2 , was found near the top of Mt. Vorlan in a calcareous skarn xenolith in ignimbrite of the Upper Chegem caldera in the Northern Caucasus, Kabardino-Balkaria, Russia. Vorlanite occurs as aggregates of black platy crystals up to 0.3 mm long with external symmetry 3̄m. The strongest powder diffraction lines are [d(Å)/(hkl)]: 3.107/(111), 2.691/(200), 1.903/(220), 1.623/(311), 1.235/(331), 1.203/(420), 1.098/(422), 0.910/(531). Single-crystal X-ray study gives isometric symmetry, space group Fm3̄m, a = 5.3813(2) Å, V = 155.834(10) Å 3 , and Z = 2. X-ray photoelectron spectroscopy indicate that all U in vorlanite is hexavalent. The mineral is isostructural with fluorite and uraninite (U 4+ O 2 ). In contrast to synthetic rhombohedral CaUO 4 , and most U 6+ minerals, the U 6+ cations in vorlanite are present as disordered uranyl ions. [8] Ca 2+ and [8] U 6+ are disordered over a single site with average M-O = 2.33 Å. Vorlanite is believed to be a pseudomorphic replacement of originally rhombohedral CaUO 4 . We assume that this rhombohedral phase transformed by radiation damage to cubic CaUO 4 (vorlanite). The new mineral is associated with larnite, chegemite, reinhardbraunsite, lakargiite, rondorfite, and wadalite, which are indicative of high-temperature formation (>800 °C) at shallow depth.

The crystal structure of braitschite, Ca 6.15 Na 0.85 RE 2.08 [B 6 O 7 (OH) 3 (O,OH) 3 ] 4 (H 2 O), is reported here in space group P6/m with unit-cell parameters a = 12.1506(6), c = 7.3678(4) Å, and V = 942.03(8) Å 3 . Data were collected from a single crystal using a MoKα source and a CCD detector, solved by direct methods, and refined to an R factor of 2.81%. The mineral structure consists of hexaborate fundamental building blocks that polymerize along [001] and are bound by Ca 2+ and REE 3+ counterions. The framework forms hexagonal channels, which are occupied alternately by Ca 2+ cations and water molecules. In an investigation of its thermal stability, braitschite maintains its crystallinity to a temperature of 400 °C, after which it undergoes decomposition. Using Rietveld refinements against powder X-ray diffraction data, we were able to track the loss of water molecules in channels and hydroxyl groups in the covalent B-O network with increasing temperature.

The oxidation of used nuclear fuel in a geologic repository has important implications for the mobility of radionuclides and fission products in the environment. Hexavalent uranium (uranyl) minerals, including oxyhydroxides and silicates, form as alteration phases on the surface of fuel pellets in laboratory simulations. However, alpha-radiolysis of water forms hydrogen peroxide in solution, which may favor the alteration of these secondary phases to the uranyl peroxide mineral studtite. This study investigates the alteration of dehydrated schoepite, UO 3 (H 2 O), and soddyite, [(UO 2 ) 2 (SiO 4 )] (H 2 O) 2 , in the presence of aqueous solutions containing hydrogen peroxide. Crystalline samples were reacted with various concentrations of hydrogen peroxide and the resulting material was analyzed by powder X-ray diffraction. Both dehydrated schoepite and soddyite readily convert to studtite in the presence of hydrogen peroxide following the reaction stoichiometry. These results indicate that the possible impact of peroxide buildup on the stability of alteration phases in a repository setting should not be overlooked.

The hydrothermal reactivity of the <1 μm fraction, K-saturated SWy-2 Wyoming low-charge montmorillonite was studied in the 250-400 °C temperature range with reactions lasting between 5 and 120 days, with a solid/solution mass ratio of 1:10, and in 1 mol/L KCl solution. From X-ray diffraction (XRD) profile modeling results on K-saturated and ethylene-glycol solvated samples the illitization process appears to occur as a progressive replacement of expandable layers by layers with illitic behavior, in a single illite-smectite phase. However this treatment overestimates the amount of illite layers because of the presence of smectitic non-expandable layers. This was revealed by calcium exchange of the products, which causes re-expansion of the apparent illite layers. The illitization model then obtained consists of four phases with increasing illite content: discrete smectite, a randomly interstratified mixed-layered mineral (MLM) and two ordered MLMs that progressively replace the phases with less illite content. This polyphase model is consistent with results from transmission electron microscopy and chemical microanalysis, which show several types of particle morphology with different interlayer K content and Al-for-Si substitution that seem to correspond to the several phases detected by XRD. Thus, the actual illitization mechanism of smectite in our experiments is not a progressive reaction but a dissolution-precipitation process following the Ostwald step rule in which metastable smectite transforms into illite through a series of metastable illite-smectite phases.

The 13 single-crystal elastic moduli of two samples of diopsidic pyroxenes close to the ideal composition (CaMgSi 2 O 6 ) from different locations have been measured at ambient pressure and room temperature by Brillouin spectroscopy. We obtain (in GPa, ±1σ uncertainty): C 11 = 229.0(4), C 22 = 179.0(4), C 33 = 242.5(4), C 44 = 78.9(3), C 55 = 68.1(2), C 66 = 78.2(3), C 12 = 78.0(7), C 13 = 69.8(6), C 23 = 58.0(7), C 15 = 9.9(3), C 25 = 6.1(5), C 35 = 40.9(3), C 46 = 6.6(2), and C 11 = 226.1(9), C 22 = 179.5(8), C 33 = 239.2(9), C 44 = 78.1(6), C 55 = 69.2(4), C66 = 76.4(8), C 12 = 77.4(10), C 13 = 70.2(13), C 23 = 56.7(16), C 15 = 9.9(7), C 25 = 5.9(12), C 35 = 41.0(7), C 46 = 6.8(4) for the two diopside samples. There are no major differences in the elastic tensor of the nearly pure diopside composition compared with the second sample containing slightly higher Al and Fe contents. Polycrystalline averaging of the C ij for the aggregate bulk and shear moduli yield Ks = 114.6(7) GPa, G = 72.7(4) GPa and K s = 113.7(8) GPa, G = 72.2(5) GPa for the two diopside samples. The shear moduli reported here are 8% larger than those determined from previous measurements on a similar natural sample. We confirm the existence of systematic correlations between the composition of the M2 site and most of the C ij values. No unusual compositional dependence of C ij values or the shear modulus near end-member diopside composition is observed.