Volume 97, Issue 11-12

A multi-analytical approach using electron microprobe analysis, X-ray structural refinement, and optical absorption spectroscopy was applied to characterize short-range and long-range structures of synthetic spinel single crystals along the MgAl 2 O 4 -CuAl 2 O 4 solid-solution series. Site populations, derived from the results of site-scattering refinement and stereochemical analysis, show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Cu 2+ , while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a significant degree of inversion, i.e., occurrence of Al at T counterbalanced by occurrence of divalent cations at M, which increases slightly from 0.24 to 0.29 for the highest Cu 2+ contents. Short-range information derived from optical spectra suggests that the local TCu 2+ -O distances remain constant at increasing Cu 2+ content, whereas local MCu 2+ -O distances are ca. 0.02 Å shorter in Cu-poor MgAl 2 O 4 spinels as compared to MCu 2+ -O distances in end-member CuAl 2 O 4 . The observed splitting of an absorption band, at ca. 7000 cm -1 , caused by electron transitions in TCu 2+ as well as the anomalous broadness of an absorption band, at ca. 13 500 cm -1 , caused by electron transitions in MCu 2+ indicates the occurrence of local Jahn-Teller distortions at T and M. Long-range information, however, shows no violation of Fd3m symmetry. From refinements of our single-crystal XRD data we could for the first time derive for a cubic spinel phase a MCu 2+ -O distance of 2.080 Å and a TCu 2+ -O of 1.960 Å. The very limited variations in the unit-cell parameter a from 8.079 to 8.087 Å are mainly related to the disordering of Al. Because of the very similar size of Cu 2+ and Mg at the T and M sites, the spinel structure responds to the Cu 2+ → Mg substitution by increasing cation disordering in such a manner that mean M-O distances remain constant and the mean T-O distances decrease slightly. This results in increasing length of shared octahedral edges and thereby increase of the octahedral cation-cation repulsion. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure is demonstrated to be valid also in the MgAl 2 O 4 -CuAl 2 O 4 solid-solution series.

Natural blue Co-bearing spinel crystals are rare and actively sought as gemstones, while synthetic blue Co-bearing spinel powders are largely used as ceramic pigments. High-quality spinel single crystals with compositions closely corresponding to the solid-solution series spinel sensu stricto (MgAl 2 O 4 )-cobalt spinel (CoAl 2 O 4 ) were produced by flux growth method, with Na 2 B 4 O 7 as flux. Lowcooling rates (2 °C/h) and linear temperature profiles were applied in the thermal interval 1200-800 °C, followed by rapid cooling. Thermal runs were performed in reducing atmosphere (f O₂ 10 −8 -10 −15 bars) created by a continuous flow of a CO 2 :H 2 mix with a ratio of 100:4 (cm 3 /min). Ten experiments were successfully carried out and hundreds of inclusion-free gem-quality single crystals (up to 1 mm large) were produced in each of them, sometimes together with crusty aggregates and microcrystalline powder. Selected crystals were investigated by SEM/EDS X-ray mapping to check for compositional homogeneity and by electron-microprobe analysis to obtain the chemical formula. Crystals were found to be chemically homogeneous and entirely representing the MgAl 2 O 4 -CoAl 2 O 4 solid-solution series, with the latter component ranging from 7 to 100%. With increasing Co 2+ contents, the crystals vary in color from light blue to intensely dark blue in daylight. The unit-cell parameter a increases from 8.084 to 8.105 Å along the solid-solution series, and the observed increase is determined more by the inversion degree than by the variation in Co contents. The composition of crystal products does not correspond to the composition of the starting oxide mixture, being cobalt enriched in the crystals. A tentative explanation of this behavior is suggested by considering possible ionic potential as well crystal field stabilization effects.

Optical absorption spectroscopy and X-ray structural refinements were used to characterize short-range and long-range structures of 10 gem-quality, blue spinel single crystals synthesized on the (Mg 1-x Co x ) Al 2 O 4 solid solution (x = 0.07-1.00). The site distributions of Mg, Co 2+ , and Al show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Co 2+ , with a marked preference of Co 2+ for tetrahedral coordination with respect to Mg, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a certain degree of inversion, i.e., occurrence of Al at T counterbalanced by the occurrence of divalent cations at M, which decreases from 0.24 to 0.13 with increasing Co 2+ content. Short-range information based on the crystal field splitting parameter Dq derived from single-crystal optical spectra suggests that the local Co 2+ -O bond length at the T-site may increase marginally at increasing Co 2+ content. An almost constant value for the Racah B-parameter, also derived from optical spectra, for tetrahedrally coordinated Co 2+ suggests that any influence of substitutional second nearest neighbor cations on the ionicity of Co 2+ -O bonds at the T-site is very small. Long-range information shows that variations in the unit-cell parameter from 8.084 to 8.105 Å along the solid-solution series are mainly related to the ordering of Al at the M site as a result of the replacement of Mg by Co 2+ . Therefore, the spinel structure responds to the chemical variation by ordering of Al in such a manner that M-O remains almost constant and T-O increases. In this way, the lengths of shared octahedral edges are reduced and the destabilization effect due to the increased octahedral cation-cation repulsion is minimized. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure has also been shown to be valid in the MgAl 2 O 4 -CoAl 2 O 4 solid-solution series.

To determine the influence of basalt composition on the CO 2 solubility in martian lavas, we investigated experimentally a synthetic melt based on the martian meteorite Yamato 980459 (Y 980459), an olivine-phyric shergottite and a picritic rock (19 wt% MgO) thought to be a near-primary liquid derived from high-temperature (>1540 °C) partial melting of the martian mantle. Experiments were performed in a piston-cylinder apparatus at 1-2 GPa and 1600-1650 °C. CO 2 contents in quenched glasses were determined using Fourier transform infrared spectroscopy (FTIR) and range from 0.45-1.26 wt%. Despite large differences in FeO* and MgO contents, the CO 2 solubilities in Y 980459 are similar to that in a less primitive synthetic martian basalt based on the Humphrey rock and to a Hawaiian tholeiite. The lack of enhanced solubility in Fe 2+ - and Mg 2+ -rich melts is likely owing to the complex structural role of these cations in silicate melts, acting partly as network formers, rather than network modifiers. The small sensitivity of CO 2 solubility to compositional variations among martian and tholeiitic basalts means that the experimentally determined solubilities may be applicable to a wide spectrum of martian magmatic products. Using experimentally determined CO 2 solubilities of Y 980459 and Humphrey allows the calibration of the thermodynamic parameters governing dissolution of CO 2 vapor as carbonate in martian basalts. This relation facilitates calculation of the CO 2 dissolved in magmas derived from graphite-saturated martian basalt source regions as a function of P, T, and f O₂ . The hot conditions in the source of Y 980459, 1540 ± 10 °C, and 1.2 ± 0.5 GPa, are plausible for plume-related magmas forming the giant Tharsis volcanic complex, which accounts for 50% of martian igneous activity since stabilization of the primordial crust. If oxygen fugacity in the sources of hot Tharsis magmatism were equivalent to that at the iron-wüstite buffer (IW) or 1 log unit above (IW+1), respectively, then the entire Tharsis event would outgas 30-300 mbars of CO 2 to the martian atmosphere, which is far from the 2 bars required to stabilize an equable climate in the late Noachian and early Hesperian epochs. This mismatch could be reconciled if significant martian igneous activity derived from comparatively oxidized mantle sources (i.e., IW+2) similar to those responsible for the nakhlite meteorites.

The structure of the synthetic high-pressure sheet-disilicate Phase-X (PhX), a possible host of H 2 O and K in the mantle, has been determined for a crystal synthesized at 16 GPa/1300 °C/23 h. The composition of the sample is close to K 1.5 Mg 2 Si 2 O 7 H 0.5 , which is 50% PhX/50% Anhydrous-PhX and has 25% of interlayer K sites vacant. The structures of four crystals were determined by singlecrystal X-ray diffraction and had very similar diffraction characteristics and structural results; the structure of one of the larger crystals is reported here. Reflection intensity statistics strongly indicate that PhX is centrosymmetric, space group P6 3 /mcm, in contrast to other studies that have reported non-centrosymmetric space group P63cm. While it was possible to obtain good agreement indices for refinements in P63cm, there were strong correlations between atoms that are equivalent in P6 3 /mcm, suggesting that the correct structure is centrosymmetric. Full anisotropic refinement in space group P6 3 /mcm gave R 1 = 0.036, wR 2 = 0.079, GoF = 1.467. As with all previous studies of PhX, the H atom was not located. Difference-Fourier maps of the residual electron density indicated that the K atom is displaced from the 4c site lying on the sixfold axis on to three split 12j sites 0.2 Å away, each having ¼ occupancy, giving a total of 3 K atoms per unit cell and corresponding to 1.5 K apfu, in good agreement with the content derived from electron microprobe analysis. Diffraction patterns of all four crystals examined, reconstructed from the full-intensity data collection, consistently show the presence of a large hexagonal superstructure with dimensions 8a sub × 8a sub × c sub , having Z = 128, compared with Z = 2 for the two-layer subcell. Complex arrays of superlattice reflections occur in layers with l = 2n, but are absent from l = 2n + 1 layers. Unpolarized infrared spectra of single crystals of PhX were obtained that are similar to those reported previously in the literature. Spectra in the OH-stretching region consist of a major absorption band at 3595 cm -1 and three much weaker bands at 3690, 3560, and 3405 cm -1 . Bond-valence analysis of PhX indicates that O1 is very over-bonded, whereas O2 is slightly under-bonded and a possible site for protonation. We present geometrical and crystal-chemical arguments that exclude O1 as a candidate for protonation, whereas a much better case can be made for O2. In PhX structures, H must be located at a partially occupied site with a multiplicity 4 ≤ m ≤ 24 in P6 3 /mcm or 4 ≤ m ≤ 12 in P6 3 cm. Such low occupancies for H sites are the likely reason for their invisibility to diffraction. We outline a model for the incorporation of H into PhX of composition K 1.5 Mg 2 Si 2 O 7 H 0.5 that suggests a mechanism for ordering based upon avoidance of H and K, coupled with K-site vacancies. Such behavior may also be the origin of the superstructure. The P6 3 /mcm structure and the presence of an underlying superstructure may well be characteristic of ordered intermediate compositions at or near PhX 50 /Anhydrous-PhX 50 . Identification of a new space group and recognition of a previously unobserved superstructure point to new possibilities for PhX and its derivatives that may bear significantly upon their stability at mantle conditions.

Borate minerals composed of [Bφ 3 ] triangles and/or [Bφ 4 ] tetrahedra (φ = O or OH) commonly exhibit complex polymerizations to form diverse polyanion groups. High-resolution solid-state magic angle spinning (MAS) 11 B and 25 Mg NMR spectroscopy at moderate to ultrahigh magnetic fields (9.4, 14.1, and 21.1 T) allows for very accurate NMR parameters to be obtained for the borate dimorphs, inderite, and kurnakovite, [MgB 3 O 3 (OH) 5 ·5H 2 O]. Improved agreement between experimental results and ab initio density functional theory (DFT) calculations using Full Potential Linear Augmented Plane Wave (FP LAPW) with WIEN2k validates the geometry optimization procedures for these minerals and permits refinements of the hydrogen positions relative to previous X-ray diffraction crystal structures. In particular, the optimized structures lead to significant improvements in the positions of the H atoms, suggesting that H atoms have significant effects on the 11 B and 25 Mg NMR parameters in inderite and kurnakovite. This study shows that combined high-resolution NMR spectroscopy and ab initio theoretical modeling provides an alternative method for the refinement of crystal structures, especially H positions.

The crystal structure, including H positions, of the monoclinic zeolite mineral parthéite (space group C2/c) of framework type -PAR from Denezhkin Kamen (Urals, Russia) was refined, from single-crystal X-ray data to R 1 = 3.46%. In addition, in situ single-crystal X-ray data have been measured in steps of 25 °C up to 375 °C to analyze dehydration behavior. In situ Raman spectra of the natural (room temperature) and partly dehydrated varieties of parthéite have been recorded at 100, 150, and 275 °C. The structure of parthéite, Ca 2 Al 4 Si 4 O 15 (OH) 2 ⋅4H 2 O, is characterized by a tetrahedral framework interrupted by an OH-group forming the apex of one AlO 4 tetrahedron. In addition, this OH-group coordinates extraframework Ca together with two H 2 O molecules and four framework oxygen sites. The structure has four strong hydrogen bonds with H···O interactions below 2 Å. The fifth hydrogen associated with a H 2 O molecule, exhibits two potential acceptors of weak hydrogen bonds with H···O distances of ca. 2.5 Å. This softly bound H 2 O molecule is released at 150 °C without severe impact on framework distortions but with decrease of Ca coordination from seven to six. Concurrently with loss of the second H 2 O molecule at 250 °C, the structure further compacts and becomes severely distorted. The space group C2/c and the tetrahedral connectivity are preserved but β changes from 91 to 79° and the volume drops from 1730 to 1600 Å 3 . Ca is still six-coordinated by five framework O atoms and OH.

To track dehydration behavior of cavansite, Ca(VO)(Si 4 O 10 )·4H 2 O [space group Pnma, a = 9.6329(2), b = 13.6606(2), c = 9.7949(2) Å, V = 1288.92(4) Å 3 ] single-crystal X-ray diffraction data on a crystal from Wagholi quarry, Poona district (India) were collected up to 400 °C in steps of 25 °C up to 250 °C and in steps of 50 °C between 250 and 400 °C. The structure of cavansite is characterized by layers of silicate tetrahedra connected by V 4+ O5 square pyramids. This way a porous framework structure is formed with Ca and H 2 O as extraframework occupants. At room temperature, the hydrogen bond system was analyzed. Ca is eightfold coordinated by four bonds to O of the framework structure and four bonds to H 2 O molecules. H 2 O linked to Ca is hydrogen bonded to the framework and also to adjacent H 2 O molecules. The dehydration in cavansite proceeds in four steps. At 75 °C, H 2 O at O9 was completely expelled leading to 3 H 2 O pfu with only minor impact on framework distortion and contraction [V = 1282.73(3) Å 3 ]. The Ca coordination declined from originally eightfold to sevenfold and H 2 O at O7 displayed positional disorder. At 175 °C, the split O7 sites approached the former O9 position. In addition, the sum of the three split positions O7, O7a, and O7b decreased to 50% occupancy yielding 2 H 2 O pfu accompanied by a strong decrease in volume [V = 1206.89(8) Å 3 ]. The Ca coordination was further reduced from sevenfold to sixfold. At 350 °C, H 2 O at O8 was released leading to a formula with 1 H 2 O pfu causing additional structural contraction (V = 1156(11) Å 3 ). At this temperature, Ca adopted fivefold coordination and O7 rearranged to disordered positions closer to the original O9 H 2 O site. At 400 °C, cavansite lost crystallinity but the VO 2+ characteristic blue color was preserved. Stepwise removal of water is discussed on the basis of literature data reporting differential thermal analyses, differential thermo-gravimetry experiments and temperature dependent IR spectra in the range of OH stretching vibrations.

The REE enrichment process in fluorapatite and the REE redistribution among fluorapatite, monazite, and allanite were studied in a series of three sets of experimental runs at P-T conditions of 0.5 to 4 GPa and 650 to 900 °C. The first two sets of experimental runs utilized fluorapatite as a P-source, synthetic monazite or allanite as the REE sources, albite, quartz, and NaF-H 2 O or NaCl-H 2 O. The third set of runs was carried out with powdered Ca 3 (PO 4 ) 2 , allanite, quartz, (±Al 2 O 3 ), and a NaF-H 2 O solution. In all runs REE-bearing fluorapatite with up to 28 wt% ΣREE 2 O 3 formed at the expense of monazite or allanite; either as narrow zones at the margin of synthetic fluorapatite in runs 1 and 2 or as discrete grains in run 3. The REE-enrichment of fluorapatite in melt-bearing runs is explained in terms of the high solubility of monazite in the presence of alkali-rich melts together with the high partitioning values for REEs among fluorapatite and alkali-rich melts. The formation of REE-enriched fluorapatite in melt-absent runs implies that the solubility of monazite and the REE-uptake of fluorapatite are similarly high in both alkali-rich melts and fluids and depends foremost on the activity of alkalis in fluids or melts. The results from this study show the importance of fluorapatite as a REE-carrier in rocks whose petrogenesis involved alkali-bearing fluids/melts. In metamorphic rocks, alkali-enriched fluids or melts will likely form under higher-grade conditions, explaining the preferential occurrence of REEenriched fluorapatite in granulite and eclogite-facies rocks.

The crystal chemistry and crystal structure of hambergite from the Anjanabonoina mine, Madagascar [Be 2 BO 3 (OH) 0.96 F 0.04 , Z = 8, a = 9.762(2), b = 12.201(2), c = 4.430(1) Å, V = 527.6(2) Å 3 , space group Pbca], were reinvestigated by means of electron microprobe analysis in wavelength-dispersive mode, secondary-ion mass spectrometry, single-crystal X-ray and neutron Laue diffraction. Chemical analyses show only a small amount of F (0.7-0.8 wt%, approximately 0.04 atoms per formula unit) substituting OH and no other substituent at a significant level. An anisotropic neutron structural refinement has been performed with final agreement index R 1 = 0.0504 for 76 refined parameters and 1430 unique reflections with F 0 > 4σ(F 0 ). The geometry of the hydroxyl group and hydrogen bonding in hambergite is now well defined: (1) only one independent H site was located and the O4-H distance, corrected for “riding motion,” is ∼0.9929 Å; (2) only one hydrogen bond appears to be energetically favorable, with a symmetry-related O4 as acceptor and with O4···O4 = 2.904(1) Å, H···O4 = 1.983(1) Å, and O4-H···O4 = 157.5(1)°. In other words, O4 sites act both as donor and as acceptor of the hydrogen bond, with a zigzag chain of H-bonds along [001]. The hydrogen-bonding scheme in hambergite found in this study is consistent with the pleochroic scheme of the infrared spectra previously reported, with two intensive modes ascribable to stretching vibrations of the hydroxyl group, at 3415 and 3520 cm -1 , respectively. The two modes suggest at least two distinct hydrogen-bonding environments, ascribable to the presence of oxygen and fluorine at the acceptor site.

Fourier-transform infrared (FTIR) spectroscopy is commonly used to measure volatile contents dissolved in glass inclusions in minerals. The standard method is to doubly polish the crystal to fully expose the inclusion to allow infrared light to pass through the inclusion only. Glass inclusions are often a few tens of micrometers thick, which can limit the usefulness of FTIR because of how fragile samples become when thinned enough to doubly expose inclusions. Here, we test whether unexposed inclusions can be feasibly analyzed by measuring the dissolved volatile contents of a population of rehomogenized quartz-hosted glass inclusions at variable exposure levels. We analyzed 118 unexposed inclusions in 46 crystals. Of those, we analyzed the 74 inclusions in 38 crystals that survived being singly exposed. Of those, only 24 inclusions in 18 crystals remained to be analyzed when doubly exposed. Measuring the path length of light through the inclusion is critical to FTIR analyses. That length can be measured directly for doubly exposed inclusions. For those inclusions we find that water contents vary from 1.6 to 2.6 wt%, averaging 2.2 ± 0.3 wt%. Path length is difficult to measure, however, in singly exposed or unexposed inclusions. Indeed, we find that path length is variably underestimated when measured using a well-calibrated optical method. Despite that difficulty, the average water contents for the populations at each exposure level are statistically the same. But, on an inclusion-by-inclusion basis volatile contents at various exposure levels are highly discrepant because the typically underestimated thicknesses for non-doubly exposed inclusions result in anomalously high volatile contents. One way to measure path length in those inclusions is to reorient the host crystal to align the path length horizontally so it can be measured with the eyepiece reticle. Often, however, that technique proves impractical because small samples are difficult to handle. When path length cannot be measured directly, we find that using the average of the dimensions of the inclusion orthogonal to the path length can be used as a proxy for path length. That proxy allows volatile contents in unexposed inclusions to be analyzed accurately, which significantly reduces difficulties of sample preparation and can dramatically increase the number of potential target inclusions.

Deuterated boyleite ZnSO 4 ⋅4D 2 O, was synthesized and the atomic structure, including D positions, was successfully refined in a combined histogram neutron diffraction refinement. The cell dimensions for boyleite are a = 5.9144(2), b = 13.5665(4), c = 7.8924(2) Å, and β = 90.668(2)° with space group P2 1 /n and Z = 4. The atomic structure including D positions of the isostructural mineral ilesite, MnSO 4 ⋅4D 2 O, was refined and the cell dimensions are a = 5.9753(1), b = 13.8186(3), c = 8.0461(1) Å, and β = 90.826(2)°. Deuterated bianchite ZnSO 4 ⋅6D 2 O was synthesized and the atomic structure, including D positions, was successfully refined with a unit cell of a = 9.969(1), b = 7. 2441(7), c = 24.249(3) Å, and β = 98.488(5)° in space group C2/c and Z = 8. A comparison of the hydrogen bonding in M 2+ SO 4 ⋅4D 2 O with that in M 2+ SO 4 ⋅6D 2 O shows that bifurcated hydrogen bonds are common in the tetrahydrate sulfates but nonexistent in the hexahydrate structures. This is a result of the packing constraints of the rings of the sulfate and metal-containing octahedra in the tetrahydrates. In the hexahydrate sulfates there is no direct linkage between the sulfate and metal-containing octahedra and hydrogen bonds are optimized without packing constraints, and no bifurcated hydrogen bonds are observed.

We have synthesized magnesium-iron silicate perovskites with the general formula Mg 1-x Fe 3+ x+y Si 1-y O 3 , in which the iron cation is exclusively trivalent. To investigate the crystal chemistry of Fe 3+ -bearing perovskite, six samples (both with and without Al) were analyzed using scanning electron microscopy, electron microprobe, X-ray diffraction, and Mössbauer spectroscopy. Results indicate that Fe 3+ substitutes significantly into both the octahedral and dodecahedral sites in the orthorhombic perovskite structure, but prefers the octahedral site at Fe 3+ concentrations between 0.04 and 0.05 Fe per formula unit, and the dodecahedral site at higher Fe 3+ concentrations. We propose a model in which Fe 3+ in the A/B site (in excess of that produced by charge coupled substitution) is accommodated by Mg/O vacancies. Hyperfine parameters refined from the Mössbauer spectra also indicate that a portion of dodecahedral sites undergo significant structural distortion. The presence of Fe 3+ in the perovskite structure increases the unit-cell volume substantially compared to either the Mg end-member, or Fe 2+ -bearing perovskite, and the addition of Al did not significantly alter the volume. Implications for increased compressibility and a partially suppressed spin transition of Fe 3+ in lower mantle perovskite are also discussed.

The <1 μm fraction of a trans-vacant 1M illite (RM30) was studied by conventional and synchrotron X-ray diffraction (XRD) techniques, combined with thermogravimetric (TG, DTG) methods to investigate the structural transformation of illite at different temperatures and degrees of dehydroxylation (D T ). The oriented specimens preheated at 300 and 680 °C correspond to the non-dehydroxylated (D T = 0) and completely dehydroxylated (D T = 100%) 1 M illite structures. Deviation of the basal reflection positions from rationality, expressed by the coefficient of variation of d(00l) values, progressively increase from 0.05 at D T = 4%, to 0.14 at D T = 51%, and then decrease to 0.06 at D T = 95%. Similarly, for each 00l reflection the full width at half-maximum (FWHM) shows a bell-like evolution with increasing preheating temperature. Both of these features are characteristic for mixed-layered structures. The experimental profiles of 00l reflections from the oriented partially dehydroxylated specimens perfectly matched the profiles from XRD pattern simulations calculated in terms of a mixed-layered structure in which the non-dehydroxylated (ND) and completely dehydroxylated (CD) illite layers are interstratified with a strong tendency to segregation. The content of the CD layers in the modeled mixed-layered structures of the preheated specimens show a significant linear correlation with the corresponding D T values (R 2 = 0.99). In random powder XRD patterns collected with synchrotron radiation, the preheated specimens show a distinctive trend in the unit-cell parameters. However, the accuracy in determination of the unit-cell parameters at first decreases up to D T = 61% and then increases with a further increase in D T . The evolution of FWHM values of individual hkl reflections is also similar to that observed for each 00l peak from oriented sample preparations. The unexpected evolution of the unit-cell parameters during progressive dehydroxylation is explained by the interstratification of ND and CD layers in illite. The formation of the mixed-layered structures during Al-rich 1M illite dehydroxylation is in agreement with the prediction from the kinetic model of partially dehydroxylated dioctahedral 2:1 clay structures where dehydroxylation of each portion of the initial OH groups corresponds to a zero-order reaction that is independent of the structural and chemical composition. The reaction is homogeneous and during partial dehydroxylation of the illite structure, ND layers transform into the CD layers without formation of an intermediate phase. A layer-by-layer dehydroxylation mechanism is suggested for thermally induced illite structural transformation.

An improved algorithm has been elaborated for computing atomic coordinates in K-dioctahedral micas-2M 1 from the experimental data on cation composition and unit-cell parameters. The structure modeling procedure is based on regression equations relating the structural features and chemical composition of micas that were obtained from the analysis of published data on 27 refined structures of dioctahedral micas of various compositions including 20 K-dioctahedral micas-2M 1 , 3 paragonites-2M 1 , 2 margarites-2M 1 , and 2 celadonites-1M. The empirical relationships accurately describe the observed structural distortions in dioctahedral micas, such as tetrahedral tilt and rotation, tetrahedral elongation, octahedral flattening, hydroxyl depression, etc. The majority of the regressions have r 2 > 0.8 and p-values <0.05, which means that the results are statistically significant. The predicted structural parameters are used to calculate the atomic coordinates for K-dioctahedral micas-2M 1 with disordered distribution of tetrahedral and octahedral cations. The estimated standard deviations (e.s.d.) for modeled atomic coordinates vary for different atomic positions and range from 0.0001 to 0.003 (fractional units); the e.s.d. values for structural characteristics obtained from the calculated atomic coordinates are 0.002-0.007 Å for mean and individual tetrahedral bond and edge lengths, 0.004-0.013 Å for mean and individual octahedral bond and edge lengths, 0.013-0.015 Å for K-O distances, and 0.5° for the tetrahedral ditrigonal rotation angle. Computation of atomic coordinates for additional three dioctahedral mica-2M 1 structures that were not included in the derivation of the empirical structure-composition relationships used in the algorithm yielded close agreement between the modeled and observed structural characteristics. The structure modeling algorithm can be used as an inexpensive and express method for evaluation of fine structural features in large collections of K-dioctahedral mica samples of diverse compositions.

We present 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of Al- and Fe-bearing, high-pressure pyroxene and perovskite samples, synthesized in a multi-anvil apparatus at 26 GPa and 1900 °C at targeted compositions of (Mg 1-x Fe x )(Si 1-x Al x )O 3 (x = 0.01, 0.025, and 0.05). 27 Al MAS-NMR spectra of the perovskite samples indicate that Al 3+ replaces both Si4+ in the octahedral site and Mg 2+ in the larger 12-coordinated site. NMR signal loss caused by paramagnetic interactions is often a severe complication when performing NMR on materials containing Fe 2+,3+ ; however, careful measurement of signal loss and comparison to total Fe content in these samples sheds light on the nature of Al and Fe incorporation. NMR signal loss for the pyroxenes is linearly related to total Fe content as would be expected in the case of uncorrelated substitution of randomly distributed Al and Fe. However, 27 Al signal loss for the perovskite samples increases only slightly between samples with x = 0.01 and 0.025 indicating similar coordination of Al by Fe and non-random distribution. Complete signal loss at Fe/(Fe + Mg) = 0.05 suggests the upper limit of Fe 2+ and Fe 3+ concentration at which useful NMR data can be obtained for this system.

A temperature-dependent in situ micro-FTIR and micro-Raman spectroscopic investigation was performed on powdered (FTIR, Raman) and single-crystal (Raman) lizardite-1T samples between room temperature and 819 °C. Between room temperature and 665 °C, the OH stretching bands shift to lower wavenumbers, demonstrating a weak expansion of the O3-H3…O2 interlayer distance. Band deconvolution of FTIR and Raman spectra at room temperature show differences in the number of bands in the OH stretching region with respect to group theory: four (FTIR) and six (Raman) OH stretching bands, respectively. This number reduces to four (Raman) at non-ambient temperatures either caused by LO-TO splitting, the presence of non-structural OH species or the presence of different lizardite polytypes and/or serpentine polymorphs as well as defects. During dehydroxylation, the evolution of the integrated intensity of the OH bands suggests a transport of hydrogen and oxygen as individual ions/molecule or OH - . A significant change in Raman spectra occurs between 639 and 665 °C with most lizardite peaks disappearing contemporaneously with the appearance of forsterite-related features and new, non-forsterite bands at 183, 350, and 670 cm -1 . A further band appears at 1000 cm -1 at 690 °C. The long stability of Si-O-related bands indicates a delayed decomposition of the tetrahedral sheet with respect to the dehydroxylation of the octahedral sheet. Moreover, evidence for a small change in the ditrigonal distortion angle during heating is given. In general, all appearing non-forsterite-related frequencies are similar to Raman data of talc and this indicates the presence of a talc-like intermediate.

The structural relationships between the new mineral markascherite, ideally Cu 3 (MoO 4 )(OH) 4 , and the related minerals szenicsite, antlerite, deloryite, flinkite, retzian, and cahnite are analyzed using hypothetical ideal closest-packed equivalents. Markascherite and the first three related minerals are based on cubic closest-packing (CCP) of anions, flinkite is based on stacking sequence ABAC, and retzian and cahnite are based on hexagonal closest-packing (HCP). However, models that are more realistic than those based on CP can be constructed for retzian and cahnite using small but systematic alterations of CP monolayers. A regular pattern of slight dislocations of some of the spheres in the monolayers creates dodecahedral interstitial sites when the monolayers are stacked, a feature not seen in perfect CP. The use of ideal crystals removes all distortion from polyhedra in closest-packed minerals, allowing for comparison of structural similarities and differences. CCP minerals can have up to four nonequivalent stacking directions. Corresponding stacking directions in the minerals of interest are identified and used to compare the layers of cation coordination polyhedra perpendicular to these zones or face poles (stacking directions are presented in both direct space and reciprocal space). Such layers are natural structural subunits and provide insight into the relationships among these minerals.

Sellaite (MgF2) forms from melts, fluids, and gases under variable temperature, pressure, f O₂ , and fluid salinity conditions. It is typically associated with, but much rarer, than fluorite (CaF 2 ). The Clara mine near Oberwolfach (Schwarzwald, Germany) is an extensive hydrothermal vein-type deposit, where sellaite occurs in huge quantities (thousands of tons) in veins of mostly Jurassic/Cretaceous age. The sellaite mineralization, occurring in gneisses altered prior to and during sellaite mineralization, represents a stockwork-like network of veins and fissures, which is overlain and sealed by sediments, preventing the inflow of and fluid-mixing with sedimentary formation waters. The occurrence of sellaite is unique among the more than 1000 hydrothermal vein-type occurrences of the Schwarzwald ore district. The favored formation of sellaite compared to fluorite requires the initial Ca/Mg-ratio of the mineralizing fluid to be unusually low. These conditions are possible if fluids equilibrate with pre-altered rocks that lost some or much of their Ca during an earlier hydrothermal alteration event. Indeed, calculations demonstrate that rock-buffered fluids of pre-altered rocks (i.e., gneiss around the Clara mine altered during prior hydrothermal events) show significantly lower Ca/Mg-ratios than fluids equilibrated with unaltered gneisses, because Ca-phases (e.g., the anorthite component of plagioclase) are more prone to hydrothermal destruction. Due to the network-like structure of the sellaite-bearing portion of the Clara fluorite vein, the fluid is shielded from sedimentary formation water, resulting in fractionation processes of the repeatedly ascending mineralizing fluid. In addition, fluid cooling and formation of water-bearing phases like illite that consume fluids, favor sellaite, and later fluorite precipitation. The rarity of this combination of prerequisites explains the limited occurrence of sellaite in hydrothermal vein-type deposits.

Members of the edgrewite Ca 9 (SiO 4 ) 4 F 2 -hydroxyledgrewite Ca 9 (SiO 4 ) 4 (OH) 2 series, structural analogues of clinohumite-hydroxylclinohumite series, Mg 9 (SiO 4 ) 4 (F,OH) 2 , were discovered in xenoliths of carbonate-silicate rock altered to skarn within ignimbrites of the Upper Chegem volcanic structure, Kabardino-Balkaria, Northern Caucasus, Russia. The new minerals occur sparingly in zones containing bultfonteinite, hillebrandite, jennite, and chegemite, as well as rare relics of larnite and rondorfite enclosed in a matrix of hydroxylellestadite. Edgrewite and hydroxyledgrewite are largely altered to jennite in places with admixed zeophyllite and trabzonite, and are preserved as elongate relics mostly 0.1-0.4 mm long in the central part of atoll-like pseudomorphs. The new minerals form a solid-solution series Ca 9 (SiO 4 ) 4 (F,OH) 2 , in which the content of the edgrewite end-member Ca 9 (SiO 4 ) 4 F 2 ranges from 74% (F = 3.64 wt%) to 31% (F = 1.52 wt%). Structure refinement of crystals containing 51% and 37% of the edgrewite end-member gave, respectively, R 1 = 3.03%, space group P2 1 /b11 (no. 14), Z = 2, a = 5.06870(10), b = 11.35790(10), c = 15.4004(2) Å, α = 100.5980(10)°, V = 871.47(3) Å 3 ; and R 1 = 1.61%, space group P2 1 /b11 (no. 14), Z = 2, a = 5.06720(10), b = 11.35450(10), c = 15.3941(2) Å, α = 100.5870(10)°, and V = 870.63(2) Å 3 . Minerals of the edgrewite-hydroxyledgrewite series are colorless, optically biaxial (+), 2V meas = 80(5)°; 2V calc = 78.7°; dispersion r > v, medium; orientation: Z = a, X ^ c = 12(2)°; edgrewite: α = 1.621(2), β = 1.625(2), γ = 1.631(2); hydroxyledgrewite: α = 1.625(2), β = 1.629(2), γ = 1.635(2) (589 nm). The micro-hardness VHN 50 = 352-366 kg/mm 2 corresponds to the Mohs scale of 5.5-6. 5. FTIR spectra of edgrewite and hydroxyledgrewite show resolved bands at (edgrewite/hydroxyledgrewite, cm -1 ): 3558 and 3551 and 3543/3554, absent/3486, 1075/1075, 996/996, 980/982, 934/933, 917/918, 904/903, 890/884, 864/864, 842/842, 818/820. Raman spectra are characterized by the following bands (edgrewite/hydroxyledgrewite, cm -1 ) at: 921/923, 889/890, 839/840, and 815/814 (SiO 4 stretching), at: 556/559, 527/527, 423/419, 406/404, and 394/394 (SiO 4 bending), 309/295, 269/256, and 163/166 (CaO 6 ). In the OH stretching region three bands are noted at 3554, 3547, and 3540 cm -1 for edgrewite and two - 3550 and 3475 cm -1 for hydroxyledgrewite confirming the corresponding IR spectra. The major difference in Raman and IR spectra of edgrewite and hydroxyledgrewite is the presence of two resolved peaks in the OH stretching region at ca. 3550 and 3480 cm -1 for hydroxyledgrewite.

The new mineral whelanite, Cu 2 Ca 6 [Si 6 O 17 (OH)](CO 3 )(OH) 3 (H 2 O) 2 , was approved by the Commission on New Minerals and Mineral Names (IMA) in 1977, but until now a description has not been published. The mineral is orthorhombic with space group Pn2n and cell parameters a = 5.6551(4), b = 3.683(3), c = 27.1372(7) Å, V = 565.3(5) Å 3 , and Z = 1. The mineral occurs with thaumasite, stringhamite, and kinoite in a copper-rich, diopside-garnet-magnetite skarn at the Bawana mine, Beaver County, Utah, and has also been confirmed to occur at other localities. At the Bawana mine, it is found as irregular clusters and radial aggregates of platy to lath-like crystals up to 1 mm in length, flattened on {001} and elongated on [100]. The color and streak are pale blue and the luster is vitreous. The laths are flexible, but not elastic. Cleavage is perfect on {001} and good on {010}, producing splintery fracture. The Mohs’ hardness is about 2½. The measured density is 2.74(3) g/cm 3 and the calculated density is 2.737 g/cm 3 based upon the empirical formula. The mineral is biaxial (-), α = 1.612(2), β = 1.622(calc), and γ = 1.626(2), and 2V meas = 64(1)°. The pleochroism is weak: X = Y (pale blue) < Z (light blue). The optical orientation is X = a, Y = c, Z = b. A combination of electron microprobe analyses and thermogravimetric analyses (for CO 2 and H 2 O) yielded: CaO 34.46, CuO 12.09, FeO 1.52, SiO 2 37.96, CO 2 5.93, H 2 O 8.86, total 100.82 wt%, providing the empirical formula (based on O = 26): Cu 1.41 Fe 0.20 Ca 5.68 Si 5.84 C 1.25 O 26 H 9.09 . Raman spectroscopy shows evidence of (CO 3 ) 2- , (OH) − , and H 2 O. The strongest powder X-ray diffraction lines are [d(hkl)I]: 6.79(004)52, 3.072(111)43, 3.013(112)100, 2.921(113)39, 2.802(114)45, 2.522(116,205)44, and 1.839(020,1.1.12)37. The crystal structure (R 1 = 3.89% for 567 F 0 > 4σF) contains two different types of polyhedral layers parallel to {001}, which alternate along [001] and are linked to one another by sharing corners with wollastonite-like silicate chains running parallel [010]. One polyhedral layer, consisting of edge-sharing CaO 7 polyhedra, is identical to that in the structures of the tobermorites (tobermorite 9Å, tobermorite 11Å, tobermorite 14Å, and clinotobermorite). The other layer is brucite-like, with alternating ribbons of edge-sharing Cu 2+ O 6 and CaO 6 octahedra. Disordered CO 3 and H 2 O groups are also located in the interlayer region. The crystal structure of whelanite exhibits OD character.

The 2M 1 -phlogopite from the potassic gabbronorite (Black Hill, Australia) has been studied by electron microprobe and single-crystal X-ray diffraction analyses. The crystal-chemical formula was (K 0.95 Na 0.01 )(Al 0.15 Mg 1.27 Fe 2+ 1.16 Fe 3+ 0.04 Ti 4+ 0.38 )(Si 2.85 Al 1.15 )O 10.76 F 0.11 Cl 0.03 OH 1.10 . The structural analysis has shown that the crystal has the cell parameters a = 5.352(1), b = 9.268(1), c = 20.168(1) Å, and β = 95.10(1)° and exhibits symmetry lowering from the ideal space group C2/c to C1. An octahedral cation ordering pattern was revealed from the refined site-scattering powers. Specifically, using the scattering species Mg vs. Fe, it was found that the M1 site at z = 0 was occupied principally by Mg (~77%) and subordinately by Fe (~23%), whereas that at z = 0.5 was completely occupied by Fe; the M2 sites at z = 0 displayed ~88% Mg and ~12% Fe, whereas those at z = 0.5 were occupied by ~86% Fe and ~14% Mg. The analysis of geometrical features shows that the Ti uptake in the structure via the Ti-oxy mechanism induces structural distortions of different extents on the z = 0 and z = 0.5 layers, with stronger effects for the layer at z = 0. Minor chemical and structural differences, instead, affect the T sheets at z = 0 and z = 0.5.

Oxy-chromium-dravite, NaCr 3 (Cr 4 Mg 2 )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with quartz, calcite, chromphyllite, eskolaite, chromite, uvarovite, chromian phlogopite, and pyroxenes of the diopside-kosmochlor series, Cr-bearing tremolite, Cr-bearing titanite, Cr-bearing rutile, and pyrite. Crystals are emerald green, transparent with a vitreous luster, green streak, and conchoidal fracture. Oxy-chromium-dravite has a VHN hardness of 14 540 MPa, a Mohs hardness of approximately 7½, and a calculated density of 3.3 g/cm 3 . In plane-polarized light, oxy-chromiumdravite is pleochroic (O = dark green, E = yellow green) and uniaxial negative: ω = 1.765(5), ε = 1.715(5). Oxy-chromium-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1121(3), c = 7.3701(1) Å, V = 1656.95(5) Å 3 , Z = 3. The chemical characterization resulted in: SiO 2 = 31.73, TiO 2 = 0.31, B 2 O 3 = 9.35, Al 2 O 3 = 3.61, Cr 2 O 3 = 36.25, V 2 O 3 = 5.81, MgO = 7.49, Na 2 O = 2.78, K 2 O = 0.08, F = 0.78, H 2 O = 2.16, sum 100.01 wt%. The unit formula is X (Na 1.00 K 0.02 ) Σ1.02 Y (Cr 3+ 1.95 V 3+ 0.87 Mg 0.14 Ti 4+ 0.04 ) Σ3.00 Z (Cr 3+ 3.37 Al 0.69 Mg 1.93 ) Σ6.00 [T(Si 5.90 Al 0.10 ) Σ6.00 O 18 ] B (BO 3 ) 3 V(OH 2.67 O 0.33 ) Σ3.00 W (O 0.54 F 0.46 ) Σ1.00 . The crystal structure of oxy-chromium-dravite was refined to statistical index R1 for all reflections equal to 1.54% using MoKα X-ray intensity data. Oxy-chromium-dravite is related to chromium-dravite, ideally NaMg 3 Cr 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 OH, by the heterovalent substitution Cr 3+ +O 2- → Mg 2+ +OH 1- .

We have carried out equilibrium molecular dynamics simulations of CaMgSi 2 O 6 (diopside) liquid as a function of pressure (up to 150 GPa) and temperature (2200 to 6000 K) using density functional theory. Self-diffusion of Mg/Ca atoms decouples most from that of framework (Si/O) atoms at 2200 K and zero pressure, and all diffusivities become increasingly similar as temperature and pressure increase. The predicted temperature variations of all transport coefficients at zero pressure closely follow the Arrhenian law with activation energies of 107 to 161 kJ/mol. However, their pressure variations show significant deviations from the Arrhenius behavior. Along the 3000 K isotherm, the Si and O self-diffusivities show non-monotonic variations up to 20 GPa and then rapidly decrease upon further compression. The melt viscosity also shows a weak anomaly in the low-pressure regime before it starts to increase rapidly with pressure. Our results agree favorably with experimental observations of low-pressure non-uniform variations of Si and O self-diffusivities and viscosity. The predicted complex dynamical behavior requires pressure-volume dependent activation volumes and can be associated with structural changes occurring on compression.

Browneite (IMA 2012-008), MnS, is a new member of the sphalerite group, discovered in Zakłodzie, an ungrouped enstatite-rich achondrite. The type material occurs as one single crystal (-16 μm in size) in contact with and surrounded by plagioclase; enstatite and troilite are nearby. Low-Ni iron, martensitic iron, tridymite, quartz, cristobalite, sinoite, schreibersite, buseckite, keilite, and graphite, are also present in the type sample. Browneite is yellowish brown and translucent. The mean chemical composition, as determined by electron microprobe analysis of the type material, is (wt%) S 36.46, Mn 62.31, Fe 0.62, Ca 0.10, sum 99.49, leading to an empirical formula calculated on the basis of 2 atoms of (Mn 0.993 Fe 0.010 Ca 0.002 )S 0.995 . Electron back-scatter diffraction patterns of browneite are a good match to that of synthetic β-MnS with the F4̅3m structure, showing a = 5.601 Å, V = 175.71 Å 3 , and Z = 4. Browneite is a low-temperature (<200 °C) phase, metastable relative to alabandite, that postdates the impact melting and subsequent crystallization of an enstatite-rich rock.

Various silica minerals form in hydrothermal settings; however, the main controls on mineralogical variations in such environments remain poorly understood. In this study, we investigate the effects of Al and Na on the mineralogy of silica precipitates by hydrothermal flow-through experiments for silica precipitation at 430 °C and 31 MPa. We used solutions with elevated Si concentrations, exceeding the solubility of amorphous silica. As the Al and Na concentrations in the input solution were raised from 0 to 7 ppm, the dominant silica minerals systematically changed from amorphous silica to cristobalite to quartz. The positive correlation between Al and Na contents in precipitated quartz indicates that Al coupled with Na substitutes for Si. Our results suggest a possible mechanism for quartz vein formation in which quartz nucleates directly without precursor amorphous silica or cristobalite due to the presence of feldspar-derived Al and Na in hydrothermal fluids.