Volume 93, Issue 5-6

The crystal structure of the type specimen of vurroite from Vulcano (Aeolian Islands, Italy) has been solved and refined using single-crystal X-ray diffraction data collected at the Elettra synchrotron facility (Basovizza, Trieste). Vurroite has an OD (order-disorder) structure belonging to the category III of OD structures composed of equivalent layers. The OD-groupoid family (λ and σ partial operations) and MDO structures were derived by means of the application of the OD theory. The two theoretically possible polytypes with maximum degree of order (MDO polytypes) have pseudo-orthorhombic metric, with lattice parameters a ≈ 45.6, b ≈ 8.4, c ≈ 54 Å, and a ≈ 22.8, b ≈ 8.4, c ≈ 54 Å, and space group symmetries F2/d11 and A2/d11, respectively. Only the former polytype (MDO1) could be identified in the analyzed crystals. The MDO 1 structure was solved and refined to R = 6.26% for 4968 reflections with Fo > 4σ(Fo). In the standard C2/c setting of the space group the unit-cell parameters of MDO 1 are a = 8.371(2), b = 45.502(9), c = 27.273(6) Å, β = 98.83(3)°, V = 10265(4) Å 3 , Z = 4. Frequent twinning with (001) as the twin plane, together with the occurrence of disordered domains in the structure, was observed. The crystal structure of vurroite contains lozenge-shaped composite rods made of coordination polyhedra of Pb and Sn, interconnected into layers parallel to (010) of the standard monoclinic setting. These layers are separated by ribbons of As and Bi, each in distorted octahedral coordination. The ribbons form wavy, discontinuous double layers of PbS archetype. Lone electron pairs of As and Bi are accommodated in the central portions of the PbS-like layers. The structure of vurroite contains building blocks topologically similar to those found in the zinckenite group and in the structure of kirkiite. It can be considered a box-work structure containing the smallest possible pseudo-hexagonal block in the form of a sole octahedron.

Visible, near-infrared, thermal, and Mössbauer spectroscopic data from the exposed, bright track soil at the “Paso Robles” site within Gusev crater, Mars, indicate the presence of Fe 3+ -sulfates and possibly Fe 3+ -phosphates admixed with the host soil. When the spectroscopic analyses are combined with constraints imposed by chemical data, the determined dominant Fe 3+ -sulfate component is hydrous, and all of the spectroscopic methods suggest that it is probably ferricopiapite or some closely related, structurally similar species, possibly mixed with other Fe 3+ sulfates such as butlerite or parabutlerite, or perhaps (para)coquimbite, fibroferrite, or metahohmanite. Such an assemblage is consistent with formation in a highly oxidized, relatively dehydrated environment with the bulk-sulfate assemblage having OH/(OH + 2SO 4 ) of < ~0.4. Some Fe 3+ is likely to be associated with phosphates in the soil in the form of ferristrunzite or strengite.

Dingdaohengite-(Ce), ideally Ce 4 Fe 2+ Ti 2 Ti 2 (Si 2 O 7 ) 2 O 8 , is a new member of the chevkinite group minerals from the world-famous Bayan Obo REE-Nb-Fe Mine near Baotou city, Inner Mongolian Autonomous Region, North China. It occurs in the magnesian skarn in the excontact of granite within dolomitic marble. Most individual crystals vary from 0.2 to 1.0 cm in length. Associated minerals are diopside, tremolite, richterite, allanite-(Ce), magnetite, ilmenite, spinel, titanite, pyrochlore, F-rich phlogopite, fluorapatite, quartz, and fluorite, etc. Dingdaohengite-(Ce) is probably of bimetasomatic origin among Ca-Mg-carbonate rock and/or carbonatite, and REE-, F-rich postmagmatic hydrothermal solutions. The mineral is black and becoming brown black in thin fragments. It is translucent to opaque with a submetallic-metallic luster, and a brown streak. It is brittle with conchoidal fracture. No cleavage or parting is observed. Its hardness is VHN25g 606.0-717.4 kg/mm2 (Mohs hardness near 5.9). The measured density is 4.83(7) g/cm 3 and the calculated density is 4.88(0) g/cm 3 . Its reflectance values (for λ = 589 nm) are 11.4-12.5%. It is biaxial negative. The strongest six X-ray diffraction lines in the powder pattern [d in Å (I) (hkl)] are 2.7524(100)(121), 2.7263(98)(313), 3.1978(68)(212), 2.5460(54)(304), 2.8702(52)(020), and 3.1622(46)(312). An electron-microprobe analysis on the crystal used to collect X-ray intensity data for crystalstructure refinement gives SiO 2 19.29, TiO 2 18.26, Al 2 O 3 0.04, FeO 8.49, Fe 2 O 3 1.67, ThO 2 0.16, MgO 1.32, CaO 2.17, Nb 2 O 5 0.47, Ta 2 O 5 0.00, La 2 O 3 19.53, Ce 2 O 3 28.08, Nd 2 O 3 n.d., Y 2 O 3 0.00, Na 2 O 0.00, sum 99.48 wt%; the Fe 3 + /Fe 2+ ratio was converted by Mössbauer spectroscopy. The empirical formula is (Ce 2.13 La 1.49 Ca 0.48 Th 0.01 ) Σ4.11 Fe 2+ (Ti 0.88 Fe 2+ 0.47 Mg 0.41 Fe 3+ 0.26 Al 0.01 ) Σ2.03 (Ti 1.96 Nb 0.04 ) Σ2.00 (Si 2 O 7 ) 2 O 8 , based on 22 O atoms with prevalence of Ti in the C1 site of the structure. Dingdaohengite-(Ce) is monoclinic, a = 13.4656(15) Å, b = 5.7356(6) Å, c = 11.0977(12) Å, β = 100.636(2)°, V = 842.39(46) Å 3 , and Z = 2. The crystal structure of dingdaohengite-(Ce) was refined with space groups P2 1 /a and C2/m. Pseudo-extinction was found, i.e., reflections with h + k = 2n are systematically strong, while those with h + k = 2n + 1 are weak, which show that the true space group of dingdaohengite-(Ce) is P2 1 /a (pseudo-C2/m).

Pressure-volume-temperature data have been obtained for CaGeO 3 perovskite up to 9.6 GPa and 1100 K using a cubic anvil, DIA-type high-pressure apparatus in conjunction with synchrotron X-ray diffraction. The data were analyzed using Birch-Murnaghan equation of state and thermal pressure approach with the bulk modulus at ambient pressure, K T0 , and its pressure derivative, K′ T0 , constrained by previous measurements. A fit of the unit-cell volume data to the high-temperature Birch-Murnaghan (HTBM) equation of state gives (∂K T /∂T)P = -0.025 ± 0.015 GPa/K, a = 1.047 ± 0.356 × 10 -5 /K, and b = 3.282 ± 0.735 × 10 -8 /K 2 for the thermal expansion α expressed by a + bT. The thermal pressure approach yields αK T = 4.04 ± 0.37 × 10 -3 GPa/K and (∂ 2 P/∂T 2 )V = 6.17 ± 1.28 × 10 -6 GPa/K 2 . The energy dispersive X-ray diffraction data reveal no indication of a structural phase transition over the P-T range of the current experiment. A systematic relationship, K S0 = 6720/V(molar) - 13.07 GPa, has been established based on these isostructural analogues, which predicts K S0 = 261(15) for MgSiO 3 perovskite and 225(8) for CaSiO 3 perovskite, respectively.

A new approach to the use of spectroscopic absorbance measurements for anisotropic crystals allows results to be extracted using unpolarized light incident on random crystal orientations. The theory of light propagation in anisotropic absorbing crystals is developed from Maxwell’s equations to devise an expression for the transmittance of linearly polarized light traveling in an arbitrary direction in weakly absorbing media. This theory predicts the distribution of transmittance and absorbance as a function of direction and polarization angle of incident light. It is shown how a previously deduced empirical expression, commonly used in infrared spectroscopy, is a good approximation to the full theory under a wide range of conditions. The new theory shows that principal polarized absorbances correspond to the eigenvalues of an absorbance ellipsoid. An expression is derived for the unpolarized absorbance as a function of the angles describing incident light direction, A unpol (ϕ,ψ), and the principal polarized absorbances, A a , A b , A c in an anisotropic crystal A unpol (ϕ,ψ) = ½[A a (cos 2 ϕcos 2 ψ + sin 2 ψ) + A b (cos 2 ϕsin 2 ψ + cos 2 ψ) + Acsin 2 ϕ]. Integration of this expression over all incident angles leads to a simple relationship between total measured unpolarized absorbance and the three principal polarized absorbances. Using this theory, a procedure is proposed for estimating both total (A a + A b + A c ) and principal absorbances from spectroscopic measurements of absorbance using unpolarized light on a set of randomly oriented crystals.

The predictions of the theory of light propagation in weakly absorbing anisotropic minerals are tested against systematic measurements of the infrared absorbance spectra of calcite, olivine, and topaz oriented in both principal and random sections, using both polarized and unpolarized light. We show that if the linear polarized maximum absorbance is smaller than ~0.3, or if the ratio of maximum and minimum absorbance is close to unity, then (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are, to a good approximation, linearly proportional to thickness, regardless of the direction of the incident light; (2) the angular variation of polarized light absorption is indistinguishable from the theoretical predictions within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the polarized maximum and minimum absorbance; and (4) the average unpolarized absorbance of randomly oriented grains is one third of the Total Absorbance (defined as the sum of the three principal absorbances). Therefore, calibrations relating Total Absorbance to absorber concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be applied to measurements with unpolarized light on a population of randomly oriented sections. We show that 10 such measurements are sufficient to achieve a petrologically useful accuracy. The method enables water concentrations in nominally anhydrous minerals to be determined from samples where the preparation of oriented specimens is not feasible, such as high-pressure experimental runs and fine-grained mantle xenoliths. The method may also be used for obtaining quantitative measurements on low-symmetry minerals.

Selenium is an important trace element. Its geochemical cycles involve various oxidation states, including Se 2- , yet thermodynamic data for selenides are sparse. A general method for selenide thermochemistry has been developed by using oxidative drop-solution calorimetry in a molten oxide solvent. The samples are dropped from room temperature into molten sodium molybdate (3Na 2 O·4MoO 3 ) solvent at 975 K, with oxygen bubbling through the melt to ensure rapid and complete conversion of selenide to dissolved selenate species. This method is analogous to that developed for sulfides (Deore and Navrotsky 2006). Complete dissolution of selenides is documented by furnace tests and visual inspection, and supported by consistent results for enthalpy. Enthalpies of formation (Δ f H°, kJ/mol) from the elements were determined for zinc selenide (ZnSe) (-169.94 ± 4.31 kJ/mol), cadmium selenide (CdSe) (-139.04 ± 6.13 kJ/mol), and lead selenide (PbSe) (-99.26 ± 6.06 kJ/mol). These three Δ f H° values agree with previously published data, often obtained by less direct means. This is the first report of the enthalpy of formation of CdSe measured by any calorimetric method. For ZnSe, a more reliable Δ f H° value is proposed since prior data are quite scattered. The results confirm that oxidative drop-solution calorimetry is an available method for selenide thermochemistry. It should be applicable to binary and multinary selenides with large homogeneity ranges, to solid solutions, and to nanophase materials.

Precise dating of K-rich minerals in volcanic rocks via the 40 Ar/ 39 Ar geochronometer has become crucial to resolving many geological problems. In some cases, ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals, necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework. To assess the integrity of biotite and evaluate the accuracy of ages obtained from this often problematic mineral, incremental heating 40 Ar/ 39 Ar experiments were performed on large, millimeter-sized, euhedral biotite crystals from four key Eocene tuffs in the Green River and Wind River Basins and are compared with sanidine 40 Ar/ 39 Ar ages. Unaltered biotite crystals with homogenous K-compositions from two tuff beds yielded concordant plateau ages that are indistinguishable from cogenetic sanidine ages. In contrast, biotite crystals from two other tuffs yielded discordant spectra, with relatively young initial steps followed by older, down-stepping apparent ages. Plateau ages from the discordant experiments are older than the sanidine ages by 1 to 14%. Integrated (total fusion) ages from these experiments exhibit scatter toward both younger and older ages that correlate with the degree of spectral discordance. Electron microprobe transects reveal that biotite crystals yielding discordant age spectra contain 1-10 μm thick K-depleted (<8% K 2 O) alteration zones along internal cleavage planes that are absent in biotite crystals yielding concordant age spectra. We propose that these altered zones promote open-system behavior. Phenomena such as loss of K or 40 Ar*, 39 Ar K recoil into internal K-depleted phases, and 39 Ar K recoil entirely out of internally corroded biotite crystals are all potential mechanisms that can be related to this alteration. Due to the presence of these multiple potential pathways that promote the gain or loss of isotopes, such altered biotite crystals are unsuitable for high-resolution 40 Ar/ 39 Ar dating, comparisons with sanidine ages, or calibration of the geomagnetic polarity timescale.

The validity of the structural model of antigorite based on the occurrence of four- and eightmembered tetrahedral rings is confirmed. The quality of crystals, single-crystal X-ray and synchrotron diffraction, lattice geometry, HRTEM interpretations, and coordination chemistry still provide evidence in favor of these refined structures. The arguments used to invalidate this model and support an alternative one based on the occurrence of tetrahedral six-reversals only and octahedral sheet offsets do not hold.

The structural environment of substitutional Cr 3+ ion in a natural pyrope Mg 3 Al 2 Si 3 O 12 has been investigated by Cr K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) coupled with first-principles computations. The Cr-O distance is close to that in knorringite Mg 3 Cr 2 Si 3 O 12 , indicating a nearly full relaxation of the first neighbors. The local C 3i symmetry of the octahedral Y site is retained during Cr-Al substitution. The second and third shells of neighbors (Mg and Si) relax only partially. Site relaxation is accommodated by strain-induced bond buckling, with angular tilts of the Si-centered tetrahedra around the Cr-centered octahedron, and by a deformation of the Mg-centered dodecahedra.

Thorite and uraninite inclusions are documented in metasomatized regions in three xenotime crystals and one monazite crystal collected from three granitic pegmatites hosted within an anorthosite intrusion located on the island of Hidra, southwestern Norway. Formation of these inclusions is demonstrated to be the result of dissolution-reprecipitation processes that occurred in an effectively closed chemical system. The reaction was initiated by interaction between the early crystallizing orthophosphate minerals and the evolving pegmatite fluid. This fluid was dominated by H 2 O, but also contained F and minor amounts of Cl, and significant quantities of dissolved Na + and K + . This is an example of auto-metasomatism and highlights a natural case where Th-U-Si-enriched orthophosphate minerals have been partially altered such that the Th, U, and Si have been converted to secondary inclusions within the now Th-U-Si-depleted orthophosphate via reaction with fluids rich in alkali elements.

A detailed description of the interlayer site in trioctahedral true micas is presented based on a statistical appraisal of crystal-chemical, structural, and spectroscopic data determined on two sets of trioctahedral micas extensively studied by both X-ray diffraction refinement on single crystals (SCXRD) and X-ray absorption fine spectroscopy (XAFS) at the potassium K-edge. Spectroscopy was carried out on both random powders and oriented cleavage flakes, the latter setting taking advantage of the polarized character of synchrotron radiation. Such an approach (AXANES) is shown to be complementary to crystal-chemical investigation based on SC-XRD refinement. However, the results are not definitive as they focus on few samples having extreme features only (e.g., end-members, unusual compositions, and samples with extreme and well-identified substitution mechanisms). The experimental absorption K-edge (XANES) for potassium was decomposed by calculation and extrapolated into a full in-plane absorption component (σ || ) and a full out-of-plane absorption component (σ ⊥ ). These two patterns reflect different structural features: σ || represents the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets; σ ⊥ is associated with multiple-scattering interactions entering deep into the mica structure, thus also reflecting interactions with the heavy atoms (essentially Fe) located in the octahedral sheet. The out-of-plane patterns also provide insights into the electronic properties of the octahedral cations, such as their oxidation states (e.g., Fe 2+ and Fe 3+ ) and their ordering (e.g., trans- vs. cis-setting). It is also possible to distinguish between F- and OH-rich micas due to peculiar absorption features originating from the F vs. OH occupancy of the O4 octahedral site. Thus, combining crystal-chemical, structural, and spectroscopic information is shown to be a practical method that allows, on one hand, assignment of the observed spectroscopic features to precise structural pathways followed by the photoelectron within the mica structure and, on the other hand, clarification of the amount of electronic interactions and forces acting onto the individual atoms at the various structural sites.

The effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite was investigated using Fourier-transform infrared (FTIR) spectroscopy. Samples were annealed at P = 14-16 GPa and T = 1230-1973 K using Kawai-type multi-anvil apparatus. The effect of oxygen fugacity (f O₂ ) was investigated using three metal-oxide buffers (Mo-MoO 2 , Ni-NiO, and Re-ReO 2 ). The effect of water fugacity (f H₂ ₂ O ) was studied using two different capsule assemblies (“nominally dry” and “dry” assemblies). A range of total OH concentration (COH,Total) of studied wadslyeites varies between <50 H/10 6 Si (<3 wt ppm H 2 O) and 23 000 H/10 6 Si (1400 wt ppm H 2 O). The observed FTIR spectra were classified into four different classes, i.e., peaks at 3620 (“3620”), 3480 (“3480”), and 3205 cm -1 (“3205”) and the others (Group O), where the Group O includes peaks at 3270, 3330, and 3580 cm −1 . The variation in OH concentration corresponding to each peak was analyzed separately. The OH concentrations correspond to “3620,” “3480,” and “3205” were found to be highly dependent on both f H₂ ₂ O and f O₂ ₂. Assuming C OH,Group O = 2[(2H) x M ] (C OH,Group O is OH concentration of Group O), present data were analyzed by using thermodynamic model for concentration of hydrogen-related defects. Based on analytical results, OH concentration of “3620” and “3480” was found to be reasonably explained by q = 1/2 and r = 1/12 (q and r are f H ₂ O and f O₂ exponents, respectively), whereas that of “3205” was consistent with q = 1/2 and r = -1/12. These results suggest that “3620” and “3480” correspond to H M ′ whereas “3205” corresponds to H·, respectively, under the charge neutrality condition of [Fe M ′] = 2[V M ″ ].

Macroscopically bladed kyanite crystals of blue and glassy luster were cleaved along two planes, and the mechanical properties were measured through depth-sensing indentation (DSI). The conventional method to determine fracture toughness (KIC) from indentation is based on radial crack lengths measurements, which is difficult to estimate owing to the ease with which kyanite cleaves. An alternative method is proposed to determine the K IC for the perfect cleavage plane (100) of kyanite based on the estimation of the crack nucleation threshold load from a discontinuity or “pop-in” in the DSI load-unload curve. The toughness value for kyanite in the plane of perfect cleavage (100) determined by the proposed method is K IC =2.1 MPa·m 1/2 . The hardness of 10.7 ± 1.6 GPa for the perfect cleavage plane is lower than the one measured in a plane (010), 18.0 ± 2.9 GPa. The measured elastic modulus for the perfect cleavage plane (100) and for the plane (010) are 297 ± 11 and 405 ± 31 GPa, respectively. These values are in agreement with the published mechanical properties of kyanite, obtained by other techniques. The mechanical behavior is discussed and correlated to fracture patterns during indentation for both crystallographic directions of this mineral.

Members of the natrojarosite-hydronium jarosite [(Na,H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 ] and jarosite-natrojarosite [(K,Na)Fe 3 (SO 4 ) 2 (OH) 6 ] solid-solution series were synthesized and investigated by Rietveld analysis of X-ray powder diffraction data. The synthesized samples have full Fe occupancy, where in many previous studies there were significant vacancies in the B site. Well-defined trends can be seen in the unit-cell parameters across the solid-solution series in the synthetic samples. The majority of the samples in this study were directly synthesized under hydrothermal conditions at 140 °C. End-member natrojarosite was synthesized using a two-step method, where the initial sample was heated in a 1.0 m H 2 SO 4 -0.5Na 2 SO 4 solution at 200 °C for 3 days, yielding a sample with 100% Na occupancy. Many of the samples were initially zoned and required grinding and re-heating in the reactant solution for homogenization. Substitution of H 3 O and K into natrojarosite changes unit-cell parameters in a linear fashion. The unit-cell parameters presented here are significantly different than the majority of previous studies on synthetic samples, as samples in the current study have full Fe occupancy and the Na-K jarosite series has no H 3 O substitution in the A site. Substitution in the A site mainly affects unit-cell parameter c with little change in a. As Na occupancy increases there is a decrease in A-O2 and A-O3 distances and a consequent increase in Fe-O2 and Fe-O3 distance leading to an overall decrease in unit-cell parameter c in both the Na-H3O and Na-K jarosite series. The synthetic samples are compared to natural samples from mine waste deposits in Rio Tinto (Huelva, Spain), Ely Mine (Vermont), and a mineral collecting locality near Sharbot Lake (Ontario), as well as natural and synthetic samples documented in the literature. Based on unit-cell parameters many of the natural samples appear to have full Fe occupancy and correlate well with the synthetic samples from this study. The infrared spectra of the samples were analyzed, and there is a gradual change in the spectral features across the solid-solution series between end-members. The results from this study will aid in the interpretation of the possible chemical compositions of natural jarosite group minerals in mine waste and on Mars.

A new experimental method is presented where transport and partitioning of elements in grain boundaries is investigated by using thin films. As a model system, the element exchange between a nanocrystalline thin film of olivine (Fo 30 ) and a San Carlos olivine single crystal (Fo 89 ), which is mediated by grain diffusion through a polycrystalline ZrO 2 layer was studied. Double layers of ZrO 2 and olivine on polished single crystals have been prepared by pulsed laser deposition and annealed in the temperature range between 900 and 1050 °C at controlled oxygen fugacity. Starting and annealed samples were analyzed by SEM, RBS, and SIMS. From the chemical analysis by RBS and SIMS, the reaction mechanism could be classified according to the reaction mechanism map of Dohmen and Chakraborty (2003). Most of the samples show a combined grain boundary/volume diffusioncontrolled reaction behavior according to this scheme. During the anneal, a linear concentration gradient for the Fe/(Fe + Mg) ratio evolved within the ZrO 2 layer, where Fe strongly prefers the ZrO 2 grain boundary compared to Mg. The depth profiles for the Fe/(Fe + Mg) ratio have been simulated in detail. From the simulation Fe-Mg volume (in olivine) and grain boundary diffusion coefficients, as well as the partition coefficient and the storage capacity for the grain boundary, have been extracted. The chemical diffusion coefficient of Fe-Mg within the ZrO 2 grain boundary at 900 °C is on the order of 1 × 10 -15 m 2 /s. Volume and grain boundary diffusion coefficients obtained using the new method are in good agreement with literature data. An additional observation is that grains within the polycrystalline fayalite-rich olivine layer grow by a grain rotation mechanism until an equilibrium texture is attained.

Albite patches in coarsely mesoperthitic alkali feldspars from the Klokken syenite have oscillatory zoning seen at blue wavelengths using cathodoluminescence. Using a five-spectrometer, high-resolution elemental mapping technique in an electron probe, we show a close correspondence between CL emission intensity and Ti, present at levels up to ~200 ppm. Albite patches were analyzed for major and 16 trace elements by laser-ablation inductively coupled-plasma mass spectrometry. SEM elemental maps acquired simultaneously with the CL showed that a similar zoning pattern is exhibited by Ca, but there is no correlation between CL intensity and Ca concentration. None of the trace elements analyzed correlate with Ti. We conclude that tetrahedral Ti 4+ is the most likely activator of blue luminescence in these albitic alkali feldspars possibly because of a defect associated with Al-O-Ti bridges.

X-ray magnetic circular dichroism (XMCD) experiments were carried out to compare the Fe 2+ /Fe 3+ ratio in nanomagnetite chemically produced from lepidocrocite and nanomagnetite biogenically produced by the Fe-reducing bacterium Shewanella putrefaciens. Together with TEM imaging, these measurements showed that the biotic magnetite nanoparticles were of excellent quality, with small size dispersion and high crystallinity. From the XMCD measurements, it could be shown that the biogenic nanomagnetite contained a higher amount of Fe 2+ than the abiogenic nanomagnetite.

The solubility of andradite garnet was determined at 800 °C and 10 kbar in a solution of 10 mol% NaCl and 90 mol% H 2 O. Experiments were syntheses carried out with high-purity natural wollastonite, reagent hematite or natural specular hematite containing ~1 wt% TiO 2 , and fluid. All experiments were performed with Mn 2 O 3 -Mn 3 O 4 or Fe 2 O 3 -Fe 3 O 4 oxygen buffers in a piston-cylinder apparatus with NaCl pressure media for 1-3 days. Andradite saturation was determined by the presence or absence of garnet in quenched charges. Andradite dissolves incongruently to hematite and fluid with CaSiO 3 molality (m CS ) of 0.0838 ± 0.0015 for the reagent hematite and both buffers. Slightly higher m CS of 0.0895 ± 0.0005 for the natural hematite and Mn-oxide f O₂ buffer is interpreted as due to incomplete equilibration and/or ~9 mol% Ti in the run-product andradite. Dissolved Fe molality could be determined only approximately, but must be at least ten times lower than m CS . Quenched fluids were very basic (pH 11-12). The solubility of Fe 2 O 3 in andradite-saturated H 2 O-NaCl fluids is lower than that of Al2O3 at grossular saturation at the same pressure (P), temperature (T), and fluid composition. The results permit a test of a model of CaSiO 3 dissolution in NaCl solutions to three dominant aqueous species: CaCl + , OH − , and H 3 NaSiO 4 . Combination of the CaSiO 3 molality at andradite saturation with wollastonite solubility at the same conditions (0.1253 ± 0.0047 molal; Newton and Manning 2006) leads to a Gibbs free energy change of the reaction 3 wollastonite + hematite = andradite at 800 °C and 10 kbar of -32.21 ± 2.45 kJ. The good agreement between this value and that derived from previous studies supports the dissolution model of CaSiO 3 in NaCl solutions. The low solubility of the Fe 2 O 3 component of andradite contrasts with the high solubility of magnetite and of Fe in pelitic and granitic mineral assemblages measured in acidic chloride solutions by previous workers at lower P and T. The results imply that Fe 2 O 3 is conserved during metasomatic processes affecting calc-silicates at high metamorphic grades.

The three-dimensional shapes of plagioclase crystals in an experimentally cooled basaltic liquid have been reconstructed, with the aim of (1) better understanding crystal growth processes and the diversity of crystal shapes produced during cooling, and (2) to assess the validity of crystal-size distributions (CSDs) derived from 2D sections. The experimental charge was cooled from above the liquidus (~1175 °C) at a rate of 0.2 °C/h. It contained ~40% crystals at the quenching temperature of ~1120 °C. To quantify the crystals in 3D, photographs of a series of 2D-polished sections were taken under an optical microscope using reflected light. Interpolation and 3D reconstruction of 261 individual crystals was performed using the gOcad geomodeling software, and their short (S), intermediate (I), and long (L) dimensions were measured. Plagioclase crystals are generally tabular, with a nearly constant I/L ratio. On the other hand, S/I and S/L shape factors are more variable, although both are found to be correlated with length of the S axis. These observations are believed to result either from crystal agglomeration and attachment, preferentially along (010) faces, or from varying thermodynamic or kinetic conditions during cooling. Growth rates along the S, I, and L axes have been calculated from the size of the largest crystals and vary from 1.5 × 10 -10 to 5.1 × 10 -10 and 7.2 × 10 -10 m/s, respectively. The CSDs for the maximal length and short axes of 3D crystals are presented and compared with CSDs obtained from 2D sections. Published corrections for cutting effects are found to be generally very satisfactory.

Rudashevskyite, Fe-dominant analogue of sphalerite, is an accessory phase in the Indarch meteorite (enstatite chondrite, EH4). It occurs as xenomorphic polycrystalline grains, 5-120 μm in size, associated with clinoenstatite, kamacite, troilite, oldhamite, niningerite, schreibersite, and roedderite. Macroscopically, rudashevskyite is black with resinous luster. In reflected light, it is gray with brownish tint. Isotropic, no internal reflections. Reflectance in air (%, λ): 19.5(400), 19.5(420), 19.5(440), 19.5(460), 19.6(470), 19.8(480), 19.8(500), 19.9(520), 20.2(540), 20.3(546), 20.5(560), 20.7(580), 20.8(589), 20.9(600), 20.9(620), 21.1(640), 21.1(650), 21.1(660), 21.1(680), and 21.2(700). Brittle. Dc 3.79 g/cm 3 . VHN 353 kg/mm 2 . Chemical composition (electron microprobe, average of 31 analyses on 11 grains, wt%): Fe 37.1, Zn 24.7, Mn 2.4, Cu 0.4, S 35.3, total 99.9. Empirical formula (2 apfu): (Fe 0.61 Zn 0.35 Mn 0.04 Cu 0.01 ) Σ=1.00 S 1.00 , ideally (Fe,Zn)S. Cubic, F-4̄3m, a 5.426(2) Å, V 159.8 (2) Å 3 , Z = 4. X-ray powder diffraction pattern (Debye-Scherrer, FeKα), [d(I)(hkl)]: 3.130(100)(111), 2.714(10)(200), 1.919(50)(220), 1.634(40)(311), 1.359(5)(400), 1.246(30)(331), 1.107(30)(422), 1.045(30)(511, 333). Crystal structure: R1 = 0.050 for 26 unique observed (|F o | ≥ 4σ F ) reflections. It is named in honor for N.S. Rudashevsky, St. Petersburg, Russia, for his contributions to the study of ore minerals.

The new mineral species birchite, idealized formula Cd 2 Cu 2 (PO 4 ) 2 (SO 4 )·5H 2 O, occurs on specimens from the Block 14 Opencut, Broken Hill, New South Wales, Australia, as sprays and aggregates of crystals to 0.75 mm across on a host rock composed of quartz, garnet, galena, chalcopyrite, and fluorapatite. It is a late-stage supergene mineral formed as part of a suite of secondary phosphate minerals under low-temperature conditions. Associated secondary minerals are covellite, cerussite, anglesite, plumbogummite-hinsdalite, pyromorphite, libethenite, and sampleite. Individual crystals are bladed to prismatic and acicular in habit, with a maximum length of 0.3 mm and width of 0.05 mm. The crystals are elongated along [001] and sometimes also flattened on (100). The crystal forms are major {100} and {010}, and minor {101} and {001}. Birchite is orthorhombic, space group Pnma, with unit-cell parameters refined from powder X-ray diffraction data, a = 10.489(6), b = 20.901(7), c = 6.155(5) Å, V = 1349.6(3) Å 3 , and Z = 4. The eight strongest lines in the diffraction pattern are [d(Å)(I)(hkl)]: 10.451(100)(020); 5.146(28)(111); 4.223(38)(131); 3.484(39)(060); 2.902(70)(260); 2.719(33)(132); 2.652(32)(042); 1.919(80)(432). Birchite is translucent (masses) to transparent (crystals); pale blue with a vitreous luster. Optically, birchite is biaxial positive, with n α = 1.624(4), n β = 1.636(5), n γ = 1.669(4), and 2V calc = +63°. The optical orientation is X = b, Y = a, Z = c; the optical axis plane lies within the {100} plane. Birchite shows very faint pleochroism, X = pale bluish, Z = pale greenish, absorption Z ≥ X. Birchite is brittle, has a conchoidal fracture and is nonfluorescent. Hardness (Mohs) is 3.5-4; the measured density is 3.61(4) g/cm 3 , and the calculated density is 3.647 g/cm 3 (from the empirical formula). Average electron microprobe analysis (wt%): CdO 36.79, CuO 21.22, CaO 0.17, MnO 0.17, ZnO 1.07, P 2 O 5 20.21, SO 3 9.70, H 2 O (from crystal-structure analysis) 12.37, total 101.70. The empirical formula, calculated on the basis of 17 O atoms and with H 2 O calculated to give 5H 2 O is (Cu 1.94 ,Zn 0.10 ) Σ2.04 (Cd 2.09 ,Ca 0.02 ,Mn 0.02 ) Σ2.13 P 2.07 S 0.88 O 12 ·5H 2 O. The crystal structure has been refined to an R index of 4.3% for 846 observed reflections measured with MoKα X-radiation. Alternating [CdO 4 (H 2 O) 2 ] octahedra and [CuO 3 (H 2 O) 2 ] square-pyramids share edges to form chains that extend along the a axis, which are linked by (PO 4 ) tetrahedra to form [CdCu(PO 4 )(H 2 O) 2 O] sheets in the (010) plane. Two such sheets are linked via (PO 4 ) tetrahedra vertices to form a layer in the (010) plane. Two layers, which are related by mirror symmetry, are linked via (SO 4 ) tetrahedra vertices to form a heteropolyhedral framework structure. Interstitial channels within the framework extend along both the a and c axes and are occupied by a H 2 O group. The mineral is named for William D. Birch, Senior Curator of Geosciences at Museum Victoria, Australia.

The crystal structure of Al 4 B 2 O 9 , synthesized from Al(NO) 3 ·9H 2 O and B(OH) 3 via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with 11 B and 27 Al MAS NMR spectroscopy. The aluminum borate with a unit-cell composition of Al 32 B 16 O 72 is closely related to the boralsilite (Al 32 B 12 Si 4 O 74 ) structure with Si replaced by B and to mullite (Al 4+2x Si 2-2x O 10-x ). It crystallizes in the monoclinic space group C2/m, a = 14.8056(7) Å, b = 5.5413(2) Å, c = 15.0531(6) Å, β = 90.913(2)°, Z = 8 for Al 4 B 2 O 9 . The main structural units are isolated chains of edge-sharing AlO 6 -octahedra running parallel to b that is a characteristic feature of the mullite-type crystal structures. The octahedral chains are crosslinked by AlO 4 , AlO 5 , BO 3 , and BO 4 groups with two B atoms and one O atom (O5′) disordered on interstitial positions. 27Al and 11B NMR studies confirm the presence of sixfold (octahedral), fivefold, and fourfold (tetrahedral) coordinated Al (sixfold:[fourfold + fivefold] = ~50%:50%) and of threefold and fourfold coordinated B (~80%:20%).

The interaction of arsenate-bearing aqueous solutions with gypsum at a starting pH of 9 and 25 °C results in surface precipitation of guerinite, Ca 5 (HAsO 4 ) 2 (AsO 4 ) 2 ·9H 2 O, sainfeldite, Ca 5 (HAsO 4 ) 2 (AsO 4 ) 2 ·4H 2 O, and occasionally Ca 2 Na(HAsO 4 )(AsO 4 )·6H 2 O, a new arsenate. These three solid phases are characterized by the simultaneous presence of HAsO 4 2- and AsO 4 3- groups in their structure, which is explainable since crystallization occurs within a pH range in which both HAsO 4 2- and AsO 4 3- are available in the aqueous solution. The interaction leads to a decrease in the As(V) concentration in the aqueous phase to reach values controlled by the solubility of these solid phases. The study combines several macroscopic experiments, in which changes in the solution chemistry are monitored as a function of time, with the characterization of solid phases by SEM-EDS and XRD. The crystal morphologies of the precipitating phases are interpreted on the basis of their respective structures. The thermodynamic solubility products of both guerinite and the new arsenate have been determined, being 10 -31.17±0.05 and 10 -13.83±0.03 , respectively. The reaction paths followed by the system and the equilibrium endpoints have been modeled using the geochemical code PHREEQC.

Jahnsite-(NaFeMg), NaFe 3+ Mg 2 Fe 2 3+ (PO 4 ) 4 (OH) 2 ·8H 2 O, space group P2/a, a = 15.0811(16), b = 7.1403(8), c = 9.8299(11) Å, β = 110.445(1)°, V = 991.8(2) Å 3 , and Z = 2, is a new mineral from the Tip Top mine, Custer County, South Dakota. It occurs as the dominant chemical zone in transparent pseudo-orthorhombic twinned prisms up to 1 mm long with shallow wedge-like terminations, both as isolated twinned crystals and in subparallel to divergent intergrowths. The crystals are yellow with orange-red bands just below the terminations. The mineral has a white streak, vitreous luster, Mohs hardness of 4, and good {001} cleavage. The measured density is 2.58(2) g/cm 3 , and the calculated density is 2.608 g/cm 3 . It is optically biaxial (-), with α = 1.632(1), β = 1.669(1), γ = 1.671(1) (589 nm); 2V (meas.) = 25(5)°, 2V (calc.) = 26°; dispersion: r > v, very strong; orientation: Y = b, Z ^ a = +28° (in β obtuse); pleochroism: X = colorless, Y and Z = beige; Y = Z > X. Electron microprobe analyses provided: Na 2 O 2.82, CaO 0.34, MnO 0.32, MgO 10.27, Fe 2 O 3 27.35, P 2 O 5 35.93, H 2 O 21.58 (calc.), total 98.61, which in terms of the general formula for the whiteite-jahnsite group XM1M2 2 M3 2 (PO 4 ) 4 (OH) 2 ·8H 2 O, yields the empirical formula: (Na 0.72 Ca 0.05 Mn 2+ 0.04 )(Fe 3+ 0.72 Mg 0.01 )Mg 2.00 Fe 3+ 2.00 (PO 3.77 OH 0.23 ) 4 (OH) 2 ·8H 2 O. Crystal structure refinement (R1 = 0.041) supports these cation site assignments. Typical crystals have as many as five chemical zones from base to termination corresponding to the following jahnsite species: jahnsite-(NaMnMg), jahnsite-(NaFeMg), jahnsite-(CaMnMg), jahnsite-(NaFeMg), jahnsite-(CaMgMg). The characterization of the new mineral jahnsite-(NaFeMg) is based upon the second zone listed, which corresponds to roughly 80% of the crystal volume. Jahnsite-(NaMnMg) and jahnsite-(CaMgMg) also represent new species; however, the very small volume of these zones is insufficient to permit complete characterization of these species. Jahnsite-(NaFeMg) is the first member of the whiteite-jahnsite group in which a monovalent cation (Na + ) predominantly occupies the X site and a trivalent cation (Fe 3+ ) the M1 site. The structure differs from that determined for jahnsite-(CaMnMg) in that the X site is [8]-coordinated and the M1 and P1 polyhedra exhibit greater distortions caused by cation-cation repulsion across their shared edges. The determination of H positions allows the elucidation of a definitive H-bonding scheme.

Ab-initio molecular dynamics simulation of forsterite (Mg 2 SiO 4 ) melt at 2273 K shows the presence of nearly 40% of the Si atoms as (Si 2 O 7 ) 6- dimers. This result is directly corroborated by the 29 Si nuclear magnetic resonance spectrum of bulk Mg 2 SiO 4 glass, prepared by container-less levitation techniques. The presence of a large excess of bridging O atoms associated with the (Si 2 O 7 ) 6- dimers in forsterite glass and melt is in sharp contrast with their complete absence in crystalline forsterite. Such structural differences between the crystal and the melt can have important implications in understanding the dynamics of crystallization and segregation in a primordial magma ocean and the continuing chemical differentiation of the Earth.

The integrated molar absorption coefficient of isolated and localized OH groups in selected minerals is theoretically investigated within the density functional theory framework. The overall decrease in absorption coefficient of stretching modes observed with increasing frequency is consistent with the experimental observations. It is related to a decrease in the magnitude of the hydrogen Born effective charge tensor projected along the OH bond as a function of increasing H-bonding. The scatter of theoretical data shows that the use of a general calibration of infrared absorbances in minerals cannot lead to accurate water contents. In contrast, the combination of theoretical modeling and experimental measurements should improve the determination of the hydrogen distribution among structurally distinct OH defects in nominally anhydrous minerals.