Volume 99, Issue 5-6

Upon compression, silicate melts in the Earth’s interior are expected to be subject to successive structural transitions with multiple densification mechanisms that are distinct from those of their crystalline analogs. Experimental verification of this phenomenon remains a major target of glass-melt studies. Early studies of Fe spin-state transitions in silicate glasses under compression used synchrotron X-ray emission spectroscopy (XES) to develop seemingly irreconcilable interpretations that vary from complete transitions to low spin states at high pressure to a prevalence of high spin states. In an effort to reconcile the controversy, Mao et al. (February-March 2014 issue of American Mineralogist) showed that the XES data must be properly handled with a correct reference spectrum for each spin state and suggested that the subtle effect of pressure-induced broadening in the spectra be considered. Based on the new analyses, they concluded that no pressure-induced spin-state transitions exist in iron-bearing silicate glasses at high pressure, relevant to Earth’s deep lower mantle. Thanks to several series of experimental studies, the elusive spin state in glasses under compression is being revealed, rendering the spin state of iron among the best understood for glasses at high pressure.

Dolomite and magnesite are the major carbon reservoirs in the subducted oceanic lithosphere. Compositional complexities in dolomite and magnesite solid solutions are often overlooked, but normal and oscillatory zoning in dolomite from mafic eclogites of Tianshan (China) demonstrates that prograde pressure-temperature histories and interactions with coexisting mixed fluids can be recorded in carbonates. Thermodynamic modeling and comparison with experimental results warn against a simplistic approach to carbonate-bearing assemblages and show that magnesite occurrence is not an unambiguous evidence for ultrahigh-pressure metamorphism.

Crystals of laihunite from Xiaolaihe of Liaoning Province, northeast China, were studied using selected-area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), and Z-contrast imaging. Z-contrast images directly reveal ordered vacancies in M1 sites. The results confirm early structural models for 1-layer laihunite (or laihunite-1M) with ideal stoichiometry of □ 0.5 Fe 2+ 0.5 Fe 3+ SiO 4 . 2-layer laihunite and 3-layer laihunite are found to be chemically different from laihunite-1M. The 2-layer laihunite can be viewed as a periodic intergrowth of laihunite and fayalite in the 1:1 ratio. The 3-layer laihunite can be considered to be a periodic intergrowth of laihunite and fayalite in the 1:0.5 ratio along the c -axis. Ideal stoichiometries for the 2-layer structure and the 3-layer structure are □ 0.5 Fe 2+ 2.5 Fe 3+ [SiO 4 ] 2 and □ 1.0 Fe 2+ 3.0 Fe 3+ 2.0 [SiO 4 ]3, respectively. The structural intergrowth of the 3-layer laihunite and the 1-layer lahunite results in chemical compositions falling within the range between the two aforementioned structures, such as the chemical formula of □ 0.4 Fe 2+ 0.8 Fe 3+ 0.8 SiO 4 , reported earlier in the literature. The crystal structures of the 1-layer laihunite (1M), the 2-layer laihunite (2M), and the 3-layer laihunite (3Or) determined from Z-contrast images and ab initio calculations using the density functional theory (DFT) have space groups of P21/b, P21/b, and Pbnm, respectively. The previously reported monoclinic symmetry for the 3-layer laihunite may be an artifact due to overlapping diffraction spots from both, the laihunite-3Or and the laihunite-1M. Our study demonstrates that the method of combining Z-contrast imaging and ab initio calculation can be effectively used for identifying structures of nano-phases in host crystals. Perhaps more importantly, Z-contrast imaging provides a powerful means for direct observation of vacancies and other defects, and may be utilized to map vacancies in Fe 3+ -bearing olivines, the alignments of which can greatly affect anisotropic diffusion in such structures.

Successful synthesis of fluorapatite has been achieved through ion-exchange between NIST hydroxlyapatite SRM 2910a and optical-grade fluorite. Additional intermediate F-OH apatite compositions were made through ion-exchange between the newly synthesized fluorapatite and the original hydroxylapatite. Based on solution calorimetric data collected on seven fluorapatite-hydroxlyapatite crystalline solutions at 50 °C in 20.0 wt% HCl under isoperibilic conditions, fluorine-rich series members display ideal thermodynamic behavior, whereas hydroxyl-rich compositions show negative enthalpies of F-OH mixing. Unit-cell volumes for the series are linear with composition. Relative to enthalpy and volume, therefore, there are no energy barriers to complete solid solution between the F and OH end-members.

In this study, we have used electron-microprobe mapping to investigate plagioclase compositional evolution due to cooling kinetics. We re-analyzed five run-products from a prior study (Iezzi et al. 2011), crystallized by cooling a natural andesitic melt from 1300 to 800 °C at 25, 12.5, 3, 0.5, and 0.125 °C/ min under atmospheric pressure and air redox state. As the cooling rate decreases, the texture of large plagioclases changes from skeletal to hollow to nearly equant. In this study, we use X‑ray map data to obtain a database of 12 275 quantitative chemical analyses. The frequency of An-rich plagioclases showing disequilibrium compositions substantially increases with increasing cooling rate. At 25 and 12.5 °C/min the distribution is single-mode and narrow, at 0.5 and 0.125 °C/min is single-mode but very broad, whereas at the intermediate cooling rate of 3 °C/min two distinct plagioclase populations are present. This intermediate cooling rate is fast enough to cause departure from equilibrium for the crystallization of the An-rich population but also sufficiently slow that An-poor plagioclases nucleate from the residual melt. We interpret our findings in the context of time-temperature-transformation (TTT) diagrams, and infer the crystallization kinetics of plagioclase in the experiments. Compositional trends and our inferences regarding TTT systematics are consistent with two discrete nucleation events that produced separate populations of plagioclase (i.e., An-rich and An-poor populations) at 3 °C/min. Using plagioclase-melt pairs as input data for the thermometric reaction between An and Ab components, we find that plagioclase mirrors very high- (near-liquidus) crystallization temperatures with increasing cooling rate. These results have important implications for the estimate of post-eruptive solidification conditions. Lava flows and intrusive bodies from centimeters to a few meters thick are characterized by a short solidification time and a significant thermal diffusion. Under such circumstances, it is possible to crystallize plagioclases with variable and disequilibrium chemical compositions simply by cooling a homogeneous andesitic melt. X‑ray element maps enrich the study of plagioclase compositional variations generated under conditions of rapid cooling.

This study reports for the first time the lattice parameters and the complete crystal structure evolution with increasing pressure for a thiospinel with composition CuCr 1.7 V 0.3 S 4 (space group Fd3̅m) measured by single-crystal X-ray diffraction as a function of pressure up to 7 GPa. The P-V data are adequately described to a fourth-order Birch-Murnaghan equation of state with the following coefficients: V 0 = 947.86(6) Å 3 , K T0 = 88(1), and K′ = 6.3(9), K″ = -1.1(4). This is the first time that the compressibility behavior of a spinel structure has been described by a fourth-order Birch-Murnaghan equation of state. The unit-cell volume shows a compression of about 6.3% over the entire pressure range investigated. The crystal structure evolution clearly indicates that the main compression mechanism is related to the compression of the CuS 4 tetrahedron, which is significantly greater than the CrS 6 octahedron. The tetrahedral volume decreases by 7.8% over the pressure range studied while the octahedral volume decreases by 5.5%. The change in the octahedral volume is accompanied by a decrease in the angular distortion of the CrS 6 octahedra.

Phosphate minerals, while relatively rare, show a broad range of crystal structure types with linkages among PO4 tetrahedra mimicking the hierarchy of polymerization of SiO 4 tetrahedra seen in silicate minerals. To augment previous Mössbauer studies of individual phosphate species and groups of species, this paper presents new Mössbauer data on 63 different phosphate samples, and integrates them with data on more than 37 phosphate species in 62 other studies from the literature. Variations in Mössbauer parameters of different sites in each mineral are then related to both the local polyhedral environment around the Fe cations and the overall structural characteristics of each species. The entire aggregated Mössbauer data set on phosphate minerals is juxtaposed against parameters obtained for spectra from the MIMOS spectrometers on Mars. This comparison demonstrates that signatures from many different phosphate or sulfate mineral species could also be contributing to Mars Mössbauer spectra. Results underscore the conclusion that unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a wealth of new data on Fe-bearing phosphate minerals to bolster future analyses of Mössbauer spectra acquired on Mars.

A temperature-controlled sample stage with an operational range of ~ 60 °C above or below ambient laboratory temperature ( ~ -35 to 85 °C) was constructed for in situ X-ray diffraction of minerals and materials using a Bruker D8 Discover diffractometer with θ-θ geometry. The stage was primarily designed for characterizing mirabilite-bearing samples from a Mars analog High Arctic perennial cold spring at an in situ temperature. Operation of the stage was demonstrated through the analysis of a synthetic sample of the hydrated sodium sulfate, mirabilite (Na 2 SO 4 ·10H 2 O ). Mirabilite was held at -25 °C for approximately two hours without significant dehydration and then incrementally warmed to ambient laboratory temperature at 5 °C intervals, during the acquisition of in situ diffraction data. At ambient laboratory temperature, the mirabilite dehydrated and only polycrystalline thenardite (Na 2 SO 4 ) remained. Preliminary analysis of the cold spring precipitates demonstrates that when mirabilite is present in the sample the dehydration reaction is occurring between collection and analysis at ambient laboratory temperature. This temperature-controlled stage was designed for versatility and ease of X-ray access, with applications that can extend to many geological and planetary settings, including Mars analog environments.

Natroalunite containing substantial amounts of Fe occurs as a prominent secondary phase during acid-sulfate alteration of pyroclastic basalts in volcanic fumaroles in Nicaragua and elsewhere, and has been observed in laboratory simulations of acid-sulfate alteration as well. Reaction path models constrained by field and experimental observations predict that Fe-rich natroalunite should also form as a major secondary phase during alteration of martian basalt under similar circumstances. Here, we evaluate the potential to use spectroscopic methods to identify minerals from the alunite group with chemical compositions intermediate between natroalunite and natrojarosite on the surface of Mars, and to remotely infer their Fe contents. X-ray diffraction and spectroscopic measurements (Raman, visible/ near infrared, mid-infrared, Mössbauer) were obtained for a suite of synthetic solid solutions with a range of Fe contents ranging from natroalunite to natrojarosite. In the visible/near infrared, minerals with intermediate compositions display several spectral features not evident in end-member spectra that could be used to remotely identify these minerals and infer their composition. In addition, Raman spectra, mid-infrared spectra, and X-ray diffraction peaks all show systematic variation with changing Fe content, indicating that these methods could potentially be used to infer mineral compositions as well. The results suggest that alunite group minerals with intermediate Fe compositions may be able to account for some visible/near-infrared and Mössbauer spectral features from Mars that had previously been unidentified or attributed to other phases. Overall, our findings indicate that consideration of solid solutions may lead to new identifications of alunite group minerals on the surface of Mars, and raise the possibility that minerals with compositions intermediate between natroalunite and natrojarosite may be widely distributed on the planet.

Harmunite, naturally occurring calcium ferrite CaFe 2 O 4 , was discovered in the Hatrurim Complex of pyrometamorphic larnite rocks close to the Jabel Harmun, the Judean Desert, West Bank, Palestinian Autonomy, Israel. The new mineral occurs in larnite pebbles of the pseudo-conglomerate, the cement of which consists of intensely altered larnite-bearing rocks. Srebrodolskite, magnesioferrite, and harmunite are intergrown forming black porous aggregates to the central part of the pebbles. Larnite, fluorellestadite, ye’elimite, fluormayenite, gehlenite, ternesite, and calciolangbeinite are the main associated minerals. Empirical crystal chemical formula of harmunite from type specimen is as follows Ca 1.013 (Fe 3+ 1.957 Al 0.015 Cr 3+ 0.011 Ti 4+ 0.004 Mg 0.003 )S 1.993 O 4 . Calculated density is 4.404 g/cm 3 , microhardness VHN 50 is 655 kg/mm 2 . The Raman spectrum of harmunite is similar to that of the synthetic analog. Harmunite in hand specimen is black and under reflected plane-polarized light is light gray with red internal reflections. Reflectance data for the COM wavelengths vary from ~ 22% (400 nm) to ~ 18% (700 nm). The crystal structure of harmunite [Pnma; a = 9.2183(3), b = 3.0175(1), c = 10.6934(4) Å; Z = 4, V = 297.45(2) Å 3 ], analogous to the synthetic counterpart, was refined from X-ray single-crystal data to R1 = 0.0262. The structure of CaFe 2 O 4 consist of two symmetrically independent FeO 6 octahedra connected over common edges, forming double rutile-type ∞ 1 [Fe 2 O 6 ] chains. Four such double chains are further linked by common oxygen corners creating a tunnel-structure with large trigonal prismatic cavities occupied by Ca along [001]. The strongest diffraction lines are as follows [d hkl , (I)]: 2.6632 (100), 2.5244 (60), 2.6697 (52), 1.8335 (40), 2.5225 (35), 2.2318 (34), 1.8307 (27), 1.5098 (19). Crystallization of harmunite takes place in the presence of sulfate melt. ̃

Many studies have used melt inclusions (MI) to track the pre-eruptive volatile history of magmas. Often, the volatile contents of the MI show wide variability, even for MI hosted in the same phenocryst. This variability is usually interpreted to represent trapping of a volatile-saturated melt over some range of pressures (depths) and these data are in turn used to define a magma degassing path. In this study, groups of MI that were all trapped at the same time (referred to as a melt inclusion assemblage or MIA) based on petrographic evidence, were analyzed to test the consistency of the volatile contents of MI that were all trapped simultaneously from the same melt. MIA hosted in phenocrysts from White Island (New Zealand) and from the Solchiaro eruption on the Island of Procida (Italy) were analyzed by secondary ion mass spectrometry (SIMS). In most MIA, H 2 O, F, and Cl abundances for all MI within the MIA are consistent (relative standard errors <27%, with the exception of two MIA), indicating that the MI all trapped a melt with the same H 2 O, F, and Cl concentrations and that the composition was maintained during storage in the magma as well as during and following eruption. In several MIA, S abundances are consistent (relative standard errors <33%, with the exception of five out of 28 MIA). Conversely, CO2 (White Island and Solchiaro MIA) showed wide variability in several MIA. The result is that some MIA display a wide range in CO 2 content at approximately constant H 2 O. Similar trends have previously been interpreted to represent degassing paths, produced as volatile-saturated melts are trapped over some significant pressure (depth) range in an ascending (or convecting) magma body. However, the CO 2 vs. H 2 O trends obtained in this study cannot represent degassing paths because the MI were all trapped at the same time (same MIA). This requires that all of the MI within the MIA trapped a melt of the same composition (including volatile content) and at the same temperature and pressure (depth). The cause of the variable concentration of CO 2 within some MIA is unknown, but may reflect micrometer-scale heterogeneities within the melt during trapping, heterogeneities within individual MI, post-entrapment crystallization within the MI, or C-contamination during sample preparation. These results suggest that trends showing variable CO 2 and relatively uniform H 2 O obtained from MI may not represent trapping of volatile-saturated melts over a range of pressure, and care must be taken when interpreting volatile contents of MI to infer magma degassing paths. Results of this study have been used to estimate the uncertainties in volatile concentrations of MI determined by SIMS analysis. The H 2 O, F, and Cl contents have an average estimated uncertainty of 11, 9, and 12%, respectively, which is consistent with the SIMS analytical error. In contrast, the S and CO 2 contents have an average estimated uncertainty of 24 and 69%, respectively, which is considerably larger than the SIMS analytical error.

Beryllium is a quintessential upper crustal element, being enriched in the upper crust by a factor of 30 relative to primitive mantle, 2.1 vs. 0.07 ppm. Most of the 112 minerals with Be as an essential element are found in granitic pegmatites and alkalic rocks or in hydrothermal deposits associated with volcanic and shallow-level plutonic rocks and skarns. Because of the extensive differentiation needed to enrich rocks sufficiently in beryllium for beryllium minerals to form, these minerals are relative latecomers in the geologic record: the oldest known is beryl in pegmatites associated with the Sinceni pluton, Swaziland (3000 Ma). In general, beryllium mineral diversity reflects the diversity in the chemical elements available for incorporation in the minerals and increases with the passage of geologic time. Furthermore, the increase is episodic; that is, steep increases at specific times are separated by longer time intervals with little or no increase in diversity. Nonetheless, a closer examination of the record suggests that at about 1700 Ma, the rate of increase in diversity decreases and eventually levels off at ~ 35 species formed in a given 50 Ma time interval between 1125 and 475 Ma, then increases to 39 species at 125 Ma (except for four spikes), before dropping off to ~ 30 species for the last 100 Ma. These features appear to reflect several trends at work: (1) diversifications at 2475, 1775, and 525 Ma, which are associated with highly fractionated rare-element granitic pegmatites and with skarns at Långban and similar deposits in the Bergslagen ore region of central Sweden, and which are inferred to correspond to the collisional phases of the supercontinents Kenorland, Nuna, and Gondwana, respectively; (2) diversification at 1175 Ma due to the rich assemblage of beryllium minerals in the Ilímaussaq peralkaline complex, Gardar Province, West Greenland, in an extensional environment; (3) diversification at 275 Ma, which is largely attributable to granitic pegmatites (Appalachian Mountains, U.S.A., and Urals, Russia) and the Larvik alkalic complex, Norway, but nonetheless related to continental collision; and (4) limited exhumation of environments where beryllium minerals could have formed in the last 100 Ma. That the maximum diversity of Be minerals in any one geologic environment could be finite is suggested by the marked slowing of the increase in the number of species formed in a given 50 Ma time interval, whereas the drop off at 100 Ma could be due to 100 Myr being too short a time interval to exhume the deep-seated occurrences where many Be minerals had formed. The relative roles of chance vs. necessity in complex evolving systems has been a matter of considerable debate, one equally applicable to what extent the temporal distribution of beryllium minerals is a matter of contingency. On the one hand, the appearance of the most abundant Be minerals, such as beryl and phenakite, early in the history of Be mineralization appears to be a deterministic aspect since these minerals only require the abundant cations Al and Si and crystallize at relatively low concentrations of Be in aqueous solution or granitic magmas. On the other hand, it could be argued that the very existence of most other Be minerals, as well as the temporal sequence of their appearance, is a matter of chance since 55 of the 112 approved Be minerals are known from a single locality and many of these phases require an unusual combination of relatively rare elements. Consequently, we cannot exclude the possibility that other equally rare and thus contingent potential Be minerals await discovery in as yet unexposed subsurface deposits on Earth, and we suggest that details of Be mineral evolution on other Earth-like planets could differ significantly from those on Earth.

Yttrium and the rare-earth elements (Y+REEs) are incorporated into aluminosilicate garnet as trivalent ions replacing divalent Mg, Fe, Mn, or Ca (“M 2+ ”) in dodecahedral sites, which requires some form of coupled substitution to maintain electroneutrality. We compare the energetic costs of competing coupled-substitution schemes, using lattice dynamics calculations to assess defect energies and exchange energies for each scheme. Substitutions with relatively low energetic costs introduce menzerite-like components via the exchange vector [YM -1 ·(Mg,Fe)Al -1 ], or alkali components via the exchange vector [Y(Na,Li)M -2 ]. Substitutions with substantially higher energetic costs introduce a vacancy component via the exchange vector [Y 2 □M -3 ], or the yttrogarnet (YAG) component via the exchange vector [YM -1 ·AlSi -1 ], or a component with octahedral Li via the exchange vector [Y 2 M -2 ·LiAl -1 ]. Energetic costs decrease significantly as the host-garnet unit-cell dimension expands, decrease very modestly as temperature rises or pressure falls, and decrease substantially with the contraction in ionic radius across the lanthanide series. These results, combined with critical re-examination of arguments cited in favor of each substitution scheme in natural occurrences, suggest that Y+REE incorporation in natural garnet is dominated by coupled substitutions that introduce menzerite and alkali components, that the YAG substitution plays only a subsidiary role, and that the other schemes are likely to be of very minor importance.

Serpentinization of peridotite in subduction zones and mid-ocean ridges is a key process that controls not only the geodynamic behavior of the mantle but also the activity of modern (and probably primordial) microbial systems on the seafloor. However, there is still controversy about what factors promote the mineralogical reactions of serpentinization in natural ultramafic rocks. Here we report textures, chemistry, and magnetic susceptibility of variably serpentinized harzburgite and dunite samples from Iwanaidake ultramafic body, Japan, which originated from the forearc mantle of the Northeast Japan arc. Successive changes in textures, mineral chemistry, and magnetic susceptibility during serpentinization of harzburgite involved two stages: replacement of olivine by serpentine and brucite, and subsequent formation of magnetite along with more-magnesian serpentine and brucite. The later reactions occurred concurrently with serpentinization of orthopyroxene, which supplied the silica component. In serpentinized dunite, which does not contain orthopyroxene, serpentinization involved replacement of olivine by serpentine and brucite, and the fraction of magnetite did not increase with the progress of serpentinization. These observations suggest that the silica supply from serpentinization of orthopyroxene is an essential factor for the formation of magnetite during serpentinization. Magnetite formation facilitated by addition of silica has often been reported for many serpentinite systems, suggesting that the magnetite formation triggered by silica addition is one of the key reactions for the progress of serpentinization in natural ultramafic rocks.

Ophirite, Ca 2 Mg 4 [Zn 2 Mn 2 3+ (H 2 O) 2 (Fe 3+ W 9 O 34 ) 2 ]·46H 2 O, is a new mineral species from the Ophir Hill Consolidated mine, Ophir district, Oquirrh Mountains, Tooele County, Utah, U.S.A. Crystals of ophirite are orange-brown tablets on {001} with irregular {100} and {110} bounding forms; individual crystals are up to about 1 mm in maximum dimension and possess a pale orange streak. The mineral is transparent, with a vitreous luster; it does not fluoresce in short- or long-wave ultraviolet radiation. Ophirite has a Mohs hardness of approximately 2 and brittle tenacity. No cleavage or parting was observed in the mineral. The fracture is irregular. The density calculated from the empirical formula using the single-crystal cell data is 4.060 g/cm 3 . Ophirite is biaxial (+) with a 2V angle of 43(2)°. Indices of refraction for ophirite are α = 1.730(3), β = 1.735(3), γ = 1.770(3)°. The optic orientation (incompletely determined) is Y ∠ b ≈ 9° and one optic axis is approximately perpendicular to {001}. Dispersion r > v, strong; pleochroism is X = light orange brown, Y = light orange brown, Z = orange brown; X < Y << Z. Chemical analyses of ophirite were obtained by electron probe microanalysis; optimization of that analysis using the results of the crystal-structure analysis yielded the formula (Ca 1.46 Mg 0.50 Zn 0.04 ) Σ2.00 (Mg 3.96 Mn 3+ 0.04 ) Σ4.00 [(Zn 1.16 Fe 3+ 0.68 Ca 0.14 Sb 5+ 0.02 ) Σ2.00 (Mn 3+ 1.42 Sb 5+ 0.32 Fe 3+ 0.24 W 0.02 ) Σ2.00 {(H 2 O) 2 [(Fe 3+ 0.80 Sb 5+ 0.11 Ca 0.07 Mg 0.02 ) Σ1.00 (W 8.71 Mn 3+ 0.29 ) Σ1.00 ] 2 }]·46H 2 O; the simplified formula of ophirite is Ca 2 Mg 4 [Zn 2 Mn 2 3+ (H 2 O) 2 (Fe 3+ W 9 O 34 ) 2 ]·46H 2 O. Ophirite is triclinic, P1, with a = 11.9860(2), b = 13.2073(2), c = 17.689(1) Å, α = 69.690(5), β = 85.364(6), γ = 64.875(5)°, V = 2370.35(18) Å 3 , and Z = 1. The strongest four lines in the diffraction pattern are [d in Å (I)(hkl)]: 10.169(100)(100,110), 11.33(91)(011,010), 2.992(75)(334,341,1̄ 1̄ 5), and 2.760(55)(412,006,1̄ 3 5). The atomic arrangement of ophirite was solved and refined to R 1 = 0.0298 for 9230 independent reflections. The structural unit, ideally { [6] Zn 2 [6] Mn 2 3+ (H 2 O) 2 ( [4] Fe 3+[6] W 9 6+ O 34 ) 2 } 12- , consists of a [Zn 2 Mn 2 3+ (H 2 O) 2 ] octahedral layer sandwiched between opposing heteropolytungstate tri-lacunary ( [4] Fe 3+[6] W 9 6+ O 34 ) Keggin anions. Similar structures with an octahedral layer between two tri-lacunary Keggin anions are known in synthetic phases. Charge balance in the ophirite structure is maintained by the {[Mg(H 2 O) 6 ] 4 [Ca (H 2 O) 6 ] 2 ·10H 2 O} 12+ interstitial unit. The interstitial unit in the structure of ophirite is formed of two distinct Mg(H 2 O) 6 octahedra and a Ca(H 2 O) 6 O 1 polyhedron, as well as five isolated water molecules. The linkage between the structural unit and the interstitial unit results principally from hydrogen bonding between oxygen atoms of the structural unit with hydrogen atoms of the interstitial unit. Ophirite is the first known mineral to contain a lacunary defect derivative of the Keggin anion, a heteropolyanion that is well known in synthetic phases. The new mineral is named ophirite to recognize its discovery at the Ophir Hill Consolidated mine, Ophir District, Oquirrh Mountains, Tooele County, Utah, U.S.A.

Bubble formation during continuous decompression from ~ 400 to ~ 70 MPa was investigated experimentally in hydrous andesitic melts at T = 1030 °C and at an oxygen fugacity (f O2 ) of about log(f O2 /bar) = QFM+1 (QFM: quartz-fayalite-magnetite buffer). Experiments were carried out at variable decompression rates (r), ranging from 0.0005 to 0.1 MPa/s. The samples were directly quenched after decompression, allowing the investigation of the influence of r on the bubble formation. The effect of variable annealing times (t A ) after decompression was also investigated for experiments performed at a decompression rate of 0.1 MPa/s. These samples were annealed for t A = 0 to 72 h at final pressure (70 MPa) to study changes in vesiculation during magma storage at shallow depths after fast ascent. Backscattered electron (BSE) images of the samples were analyzed to determine bubble number densities (BND). The BND values increase strongly with increasing r and vary from about 10 2.2 mm -3 at 0.0005 MPa/s to about 10 4.6 mm -3 at 0.1 MPa/s. After fast decompression (r ~ 0.1 MPa/s), the BND decrease significantly with tA, i.e., from ~ 10 4.6 mm -3 at t A = 0 h to ~ 10 2.9 mm -3 at t A = 72 h. A comparison of the derived BND values with recently published experimental data demonstrates the essential role of the decompression path on bubble formation. The BND are higher in experiments with multi- or single-step decompression when compared to continuous decompression. The new data show that H 2 O-undersaturated andesitic melts are characterized by 1 to 2 log units higher BND values than H 2 O saturated rhyolitic melts after decompression with the same rate, indicating a strong influence of melt composition on bubble nucleation. This compositional effect is not predicted accurately by existing models and the interpretation of the vesicularity of dacitic to andesitic melts may lead to overestimations of magma ascent rates by about an order of magnitude.

The thermodynamic stabilities of different manganese carbonate phases at ambient conditions were determined by acid solution and water adsorption calorimetry. Amorphous manganese carbonate precursor provides a low energy pathway for MnCO 3 crystallization analogous to that observed in (Ca-Mg-Fe)CO 3 systems where crystallization enthalpies appear to be controlled by cation size (become less exothermic with increase in ionic radius). The surface energy of nanophase MnCO 3 (0.64 ± 0.08 J/m 2 for hydrous and 0.94 ± 0.12 J/m 2 for anhydrous surface) is lower than that of nano-calcite and MnCO3 binds surface water less strongly (-65.3 ± 3 kJ/mol) than calcite (-96.26 ± 0.96 kJ/mol). This probably reflects the greater basicity of CaO compared to MnO. Substantial particle size driven shifts in the MnCO 3 -manganese oxide Eh-pH and oxygen fugacity-CO 2 fugacity diagrams were calculated using the measured surface energies. These shifts expand the stability field of hausmannite, Mn 3 O 4 , in both aqueous and anhydrous environments. The particle size driven (caused by differences in surface energy) shifts in oxidation potential (Eh, oxygen fugacity) and pH of phase boundaries could affect stability, a electrochemical and catalytic properties and hence influence geochemical and technological processes. Manganese oxides (mainly hausmannite) dominate at the nanoscale in aerated environments, while manganese carbonate is favored in coarse-grained materials and reducing environments. In supercritical CO 2 , the expansion of the MnCO 3 stability field leads to significant reduction of the Mn 3 O 4 stability field.

Experimental and calculated electron density distributions determined for oxide and silicate crystals and siloxane molecules provide a new basis for addressing the classic foundation of the crystal chemistry of silicates, including atomic/ionic radii, the radius ratio rule and the nexus between the Pauling’s bond strength, resonance bond number, and bond length. The distributions indicate that the charge density of a bonded oxygen atom is highly distorted with its bonded radius decreasing systematically from 1.50 Å when bonded to highly electropositive atoms like sodium to ~ 0.65 Å when bonded to highly electronegative atoms like nitrogen. Rather than a single radius, the atom has as many bonded radii as it has bonded interactions. Bonded radii determined for the metal atoms match the Shannon effective ionic radii for the more electropositive atoms, but they depart and decrease systematically as the electronegativity of the M atoms increases. Pauling’s first rule is considered to be irrelevant given the asphericity and the range of the bonded radii displayed by the O atom. A power law regression expression is formulated between the average M-O bond lengths, <R(M-O)>, and the average value of the electron density, <ρ( r c )> = r[1.41/<R(M-O)>] 4.76 , at the bond critical point, r c , between pairs of bonded M-O atoms. The expression applies to a host of crystals and molecules comprising M-atoms for all rows, r, of the periodic table. The <ρ( r c )> values correlate with bond strength and resonance bond strength for the M-O bonded interactions on a one-to-one basis, demonstrating that the Pauling bond strength is a direct measure of the electron density involved in a bonded interaction and the accumulation of the electron density between the bonded pair. The widespread applications of the Brown-Shannon bond valence model in the Earth sciences and material science owes much of its success to the direct connection that exists between bond strength and the quantum mechanical observable, the electron density distribution. Compelling evidence is presented that supports the argument that the Si-O bonded interactions within siloxane molecules and silicate crystals are fundamentally the same, and that the local Si-O bonded interactions comprising molecules are, at the very core, equivalent to the Si-O bonded interactions observed in silicate crystals. Bond paths between the O atoms comprising shared polyhedral edges are consistent with Pauling’s third rule, the shorter the O-O shared edges, the greater the accumulation of the electron density between the O atoms, the greater the stabilization of the shared edges.

The adsorption of pure sulfur dioxide gas on volcanic ashes has been studied from 0.1 mbar to 1 bar and from -80 to +150 °C. Finely ground synthetic glasses of andesitic, dacitic, and rhyolitic composition served as proxies for fresh natural ash surfaces. Powders from two natural obsidian samples were also studied and yielded results broadly similar to the synthetic model systems. SO 2 adsorption on ash is partially irreversible; it appears that the first layer of SO 2 molecules absorbed on the surface cannot be removed. The pressure and temperature dependence of adsorption can be described by the equation lnc = A T-1 + B lnP + C, where c is the surface concentration of adsorbed SO 2 in mg/m 2 , T is temperature in Kelvin, and P is the partial pressure of SO 2 in mbar. A multiple regression analysis of the experimental data yielded A = 1645, B = 0.29, C = -7.43 for andesite, A = 2140, B = 0.29, C = -9.32 for dacite, and A = 910, B = 0.21, C = -4.48 for rhyolite. These data imply that adsorption strongly decreases with temperature, but only slowly decreases with decreasing partial pressure of SO 2 . Therefore, adsorption primarily occurs in the cool and diluted parts of the volcanic plume. Model calculations show that most of the SO 2 may be removed from the plume, if the initial SO 2 concentration in the volcanic gases is low and the surface area of the ash is high. For high initial SO 2 concentrations, the fraction of SO 2 that is lost by adsorption decreases, since the amount of SO 2 in the gas phase is proportional to the SO 2 partial pressure P, while adsorption is proportional only to P 0.2 - P 0.3 . Adsorption of SO 2 on ash particles may therefore be one reason why the climatic impact of explosive volcanic eruptions does not always scale with total sulfur yield.

We use microscopic and electron microprobe techniques to study samples of ores containing native gold with dark rims of Au-Ag sulfides from six deposits and ore occurrences of Russia: Khopto (Au-Cuskarn), Ulakhan, Yunoe (Au-Ag epithermal), Dorozhnoe, Konechnoe, and Yakutskoe (Au-quartz). Dark rims around native gold are uytenbogaardtite (Ag 3 AuS 2 ) or petrovskaite (AgAuS), or a mixture of acanthite (Ag 2 S) with uytenbogaardtite or uytenbogaardtite with petrovskaite. In the ore samples from the Khopto and Ulakhan deposits, we have found microrims of higher fineness gold at the contact of native gold and Au-Ag sulfide. The reactions of native gold sulfidation occurring in natural processes are proposed based on the compositions of Au-Ag sulfides, their mutual textural relationships. The composition of Au-Ag sulfides rims was found to depend on the primary fineness of native Au: uytenbogaardtite forms after gold of fineness above 380‰, whereas petrovskaite, above 650‰. We propose that the fineness of gold and silver may be used for forecasting presence of uytenbogaardtite or petrovskaite, or a mixture of acanthite with uytenbogaardtite or uytenbogaardtite with petrovskaite in sulfide ores at Au-Ag epithermal, Au-skarn, Au-Cu volcanic-hosted massive sulfide, Au-quartz-sulfide, and other deposits.

The electrical conductivity of 2/1-mullite (approximate composition 2Al 2 O 3 ·SiO 2 ) was measured using plane parallel, polished plates cut perpendicular to [100], [010], and [001] from a large single crystal grown by the Czochralski method. Impedance spectra were recorded in the 1 Hz to 1 MHz frequency range at temperatures from 550 to 1400 °C in air. The conductivity vs. temperature curves display changes of their slope between 850 and 950 °C depending on the crystallographical direction. The low-temperature region (T < 850 °C) of conductivity is characterized by low-electrical conductivities (σ av ≈ 5.4 × 10-9 Ω-1cm-1, average conductivity at 550 °C) with σ [010] > σ [100] > σ [001] and low-activation energies (≈0.66 eV, average value). In the high-temperature region (T > 950 °C) the electrical conductivity is significantly higher (σ av ≈ 1.1 × 10-5 W-1cm-1, average conductivity at 1400 °C) with σ [001] > σ- ≈ σ [010] , and with higher activation energies (≈1.6 eV). While the conductivity in the low-temperature region essentially is electronic, ion conductivity dominates the conductivity in the high-temperature region. We believe that the ionic conductivity is essentially due to hopping of O atoms from structural sites linking the tetrahedral double chains in mullite toward adjacent oxygen vacancies especially in c -axis direction. These oxygen hoppings are associated with complex structural re-arrangements, which control and slow down the velocity of the processes. Thus the electrical conductivity of mullite at high temperature is much lower than, e.g., that of Y-doped zirconia, but is significantly higher than that of a-alumina.

The chemical forms of sulfur in geological fluids control the behavior of this element and associated base and precious metals in magmatic, hydrothermal, and metamorphic environments. However, these forms are insufficiently known at elevated temperature (T) and pressure (P). In this study, sulfur speciation in model aqueous solutions of thiosulfate and sulfur ( ~ 3 wt% of total S) was examined by in situ Raman spectroscopy on synthetic fluid inclusions at T-P-pH-redox conditions typical of porphyry Cu-Au-Mo deposits. Fluid inclusions were entrapped at 2 kbar and 600 or 700 °C in quartz that served as a container for the high T-P fluid. Then, the inclusion-bearing quartz samples were re-heated and examined by Raman spectroscopy as a function of T and P (up to 500 °C and ~1 kbar). At T < 200 °C, all fluid inclusions show sulfate (SO 4 2- ± HSO 4 - ) and sulfide (H 2 S ± HS-) in the aqueous liquid phase and elemental sulfur (S 8 ) in the solid/molten phase; these results agree both with thermodynamic predictions of sulfur speciation and the common observation of these three S forms in natural fluid inclusions. At T > 200-300 °C, in addition to these S species, the S 3 - ion was found to appear and grow with increasing temperature to at least 500 °C. The formation of S 3 - is rapid and fully reversible; its Raman signal disappears on cooling below 200 °C, and re-appears on heating. These new data confirm the recent findings of S 3 - in similar aqueous solutions at P of 5-50 kbar and T > 250 °C; they suggest that S 3 - may account for some part of dissolved sulfur and serve as a ligand for chalcophile metals in fluids from subduction zones and related Cu-Au-Mo deposits. This work demonstrates that in situ approaches are required for determining the true sulfur speciation in crustal fluids; it should encourage future spectroscopic investigations of natural fluid and melt inclusions at high temperatures and pressures close to their formation conditions.

Pressure-temperature divariant melting phase relations of model carbonated peridotite in the system CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 from 8 to 12 GPa are reported. From 8 to 12 GPa, melting temperatures on the studied pressure-temperature divariant surface rise quite rapidly. Liquids, in equilibrium with forsterite, orthopyroxene, clinopyroxene, and garnet, on the low-temperature side of the divariant surface are magnesiocarbonatitic in composition. With increasing temperature along an isobar on the pressuretemperature divariant surface, and with the same crystalline phase assemblage, liquids gradually become kimberlitic in their composition. Given the model system data reported here, Group IB kimberlites and perhaps some kimberlites from Greenland, Canada, and South Africa could be generated from direct melting of carbonated peridotite in the pressure range of approximately 6-8 GPa. Some kimberlites from Canada and Russia could have formed by partial melting of carbonated mantle peridotite at pressures of about 10-12 GPa. From 8 to 12 GPa, liquid compositions on the studied divariant surface show a limited compositional range, which implies that the divariant surface is essentially flat. The implied flatness of the divariant surface, and so long as the liquid is in equilibrium with the crystalline assemblage of forsterite + orthopyroxene + clinopyroxene + garnet on the divariant surface, indicate that kimberlites belonging to Group IA and those more magnesian than Group IA cannot have their origin in only being a melting product of carbonated mantle peridotite. The absence of topography of the studied pressure-temperature divariant surface in all likelihood limits the generation of kimberlites in the Earth’s upper mantle only.

Fission track (FT) thermochornology is essentially based on empirical fits to annealing data of FTs revealed by chemical etching, because, until now, unetched, latent FTs could not be examined analytically at the atomic-scale. The major challenge to such an analysis has been the random orientation of FTs and their extremely small diameters. Here we use high-energy ions (2.2 GeV Au or 80 MeV Xe) to simulate FT formation along specific crystallographic orientations. By combining results from transmission electron microscopy (TEM) of single tracks and small-angle X-ray scattering (SAXS) for millions of tracks, a precise picture of track morphology as a function of orientation is obtained. High-resolution analysis reveals that orientation affects the shape of tracks in apatite and zircon through the preferential creation of damage along directions with highest atomic density. However, track radius does not depend on orientation, contradicting previous reports. Independent of track orientation, track radii, as measured at each point along the entire length of 80 MeV Xe ion tracks in apatite, can be understood using the thermal spike model of Szenes. Thus, the well-known track annealing anisotropy of apatite is not due to track radius anisotropy. The combination of ion-irradiations with TEM and SAXS analysis provides a unique opportunity to understand and model track formation and annealing under various geologic conditions.

More than 150 samples of pyromorphite, mimetite, vanadinite, and minerals of the hedyphane-group (which collectively are summarized here under the term PyGM for pyromorphite-group minerals) from the Schwarzwald mining district, southwest Germany, have been analyzed by electron microprobe and LA-ICP-MS. In this largest study of its kind, the relations of PyGM composition to host rock and fluid compositions and the amount of solid solution between the various end-members were investigated. In addition, we report the colors of the many analyzed mineral compositions. Here is a list of the most important results. • Pyromorphite and mimetite are completely miscible. • At conditions of the oxidation zone in ore deposits, the solvus between pyromorphite and vanadinite appears to asymmetrical with up to about 3 mol% of vanadinite component in pyromorphite and up to about 39 mol% of pyromorphite component in vanadinite in cases where both minerals coexist. • Due to a lack of suitable samples, the solvus between vanadinite and mimetite could not be completely constrained, but we report vanadinite analyses with up to 12 mol% mimetite and 8 mol% pyromorphite component. • There is complete miscibility between pyromorphite and phosphohedyphane and between mimetite and hedyphane. • F-rich varieties appear only to exist in hedyphanes and phosphohedyphanes, while pyromorphites, mimetites, and vanadinites are Cl- or OH-dominated. • We report for the first time analyses suggesting the occurrence of an OH-end-member corresponding to mimetite. • While pyromorphites are preferably green and mimetites yellow, this is not at all a diagnostic feature, as many exceptions exist; major elements are not correlated at all with the color of a specific crystal. • Most PyGM are strongly zoned and display, for example, significant variations in Ca that are likely to be related to variations of the compositions of various fluid pulses from which the crystals formed. • PyGM composition is generally uncorrelated with host rock composition, but PyGM enrich other metals like REE, Cr, Sb, Bi, or U up to a factor of 10 6 ; therefore, they can be regarded as very sensitive recorders of the metal inventory of an oxidation zone and they even record metals only present as traces in the primary ore deposit very reliably. • REE patterns of PyGM show significant variability even at one location; this may suggest that each zone of a PyGM crystal records the REE pattern of a single fluid pulse or it may indicate fractionation of the REE during PyGM growth. In the absence of conclusive data, the former possibility appears the more likely one. • PyGM are extremely efficient filters for heavy metals from supergene solutions in and in the vicinity of ore deposits.

The isothermal bulk modulus, together with its temperature dependence, and the thermal expansion of diamond at various pressures were calculated from first principles in the [0, 30 GPa] and [0, 3000 K] pressure and temperature ranges, within the limits of the quasi-harmonic approximation (QHA). The hybrid HF/DFT functional employed (WC1LYP) proved to be particularly effective in providing a very close agreement between the calculated and the available experimental data. In particular, the bulk modulus at 300 K was estimated to be 444.6 GPa (K′ = 3.60); at the same temperature, the (volume) thermal expansion coefficient was 3.19×10 -6 K -1 . To the authors’ knowledge, among the theoretical papers devoted to the subject, the present one provides the most accurate thermo-elastic data in high-pressure and temperature ranges. Such data can confidently be used in the determination of the pressure of formation using the “elastic method” for minerals found as inclusions in diamonds (recently applied on different minerals included in diamonds), thus shedding light upon the genesis of diamonds in the Earth’s upper mantle.

Vanadio-oxy-chromium-dravite, NaV 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, Cr-V-bearing tremolite and muscovite-celadonitechromphyllite- roscoelite, diopside-kosmochlor-natalyite, Cr-bearing goldmanite, escolaite-karelianite, dravite-oxy-vanadium-dravite, V-bearing titanite and rutile, ilmenite, oxyvanite-berdesinskiite, shreyerite, plagioclase, scapolite, zircon, pyrite, and an unnamed oxide of V, Cr, Ti, U, and Nb. Crystals are emerald green, transparent with a vitreous luster, pale green streak, and conchoidal fracture. Vanadio-oxy-chromium-dravite has a Mohs hardness of approximately 7½, and a calculated density of 3.3 g/cm 3 . In plane-polarized light, vanadio-oxy-chromium-dravite is pleochroic (O = dark green, E = pale green) and uniaxial negative: ω = 1.767(5), ε = 1.710(5). Vanadio-oxy-chromium-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1260(2), c = 7.3759(1) Å, V = 1661.11(4) Å 3 , Z = 3. Crystal chemistry analysis resulted in the empirical structural formula: X (Na 0.89 K 0.06□0.05 ) Y (V 3+ 2.77 Mg 0.17 Fe 3+ 0.06 ) Z (Cr 3+ 1.85 Al 1.59 V 3+ 0.78 Mg 1.78 ) T [(Si 5.95 Al 0.05 )O 18 ] B (BO 3 ) 3 V (OH 2.91 O 0.09 ) W (O 0.86 F 0.14 ). The crystal structure of vanadio-oxy-chromium-dravite was refined to a statistical index R1 of 1.16% using 2543 unique reflections collected with MoKa X-radiation. Ideally, vanadio-oxy-chromiumdravite is related to oxy-chromium-dravite and oxy-vanadium-dravite by the homovalent substitution V 3+ ↔ Cr 3+ . Tourmaline with chemical compositions classified as vanadio-oxy-chromium-dravite can be either Cr 3+ -dominant or V 3+ -dominant as a result of the compositional boundaries along the solid solution between Cr 3+ and V 3+ that are determined at Y+Z (V 5 Cr 2 ), corresponding to Na Y (V 3 ) Z (V 2 Cr 2 Mg 2 ) Si 6 O 18 (BO 3 ) 3 (OH) 3 O, and Y+Z (V 1.5 Cr 5.5 ), corresponding to Na Y (V 1.5 Cr 1.5 ) Z (Cr 4 Mg 2 )Si 6 O 18 (BO 3 ) 3 (OH) 3 O.

The new mineral graţianite, MnBi 2 S 4 , is described from the Bǎiţa Bihor skarn deposit, Bihor County, Romania. Graţianite occurs as thin lamellae, intimately intergrown with cosalite and bismuthinite, or as flower-shaped blebs within chalcopyrite, where it is associated with cosalite and tetradymite. Graţianite displays weak to modest bireflectance in air and oil, respectively, and strong anisotropy. The mean empirical composition based on 20 electron probe microanalyses is: (Mn 0.541 Fe 0.319 Pb 0.070 Cu 0.040 Cd 0.009 Ag 0.001 )S 0.980 (Bi 1.975 Sb 0.018 )S 1.993 (S 4.008 Se 0.012 Te 0.007 )S 4.027 , corresponding to the ideal formula MnBi2S4. Graţianite crystallizes in the monoclinic system (space group C2/m). Single-crystal X-ray studies of material extracted by the focused ion beam-scanning electron microscopy (FIB-SEM) technique, and carried out on the MX2 macromolecular beamline of the Australian Synchrotron determined the following cell dimensions: a = 12.6774(25) Å, b = 3.9140(8) Å, c = 14.7581(30) Å, b = 115.31(3)°, V = 662.0(2) Å3, and Z = 4. The six strongest X-ray reflections and their relative intensities are: 3.448 Å (100), 2.731 Å (77), 2.855 Å (64), 3.637 Å (55), 3.644 Å (54), and 3.062 Å (51). Graţianite is the monoclinic analog of berthierite (FeSb 2 S 4 ), garavellite [Fe(Bi,Sb) 2 S 4 ] and clerite [Mn(Sb,As) 2 S 4 ] (Nickel-Strunz class 02.HA.20). It is isostructural with synthetic sulfides and selenides in the MnBi 2 S 4 -MnSb 2 S 4 and MnBi2Se 4 - MnSb 2 Se 4 series, and with grumiplucite (HgBi 2 S 4 ) and kudriavite, [(Cd,Pb)Bi 2 S 4 ], 3P members of the pavonite homologous series. The mineral is named for Graţian Cioflica (1927-2002), formerly Professor in Mineralogy and Ore Deposits at the University of Bucharest, Romania. The Băiţa Bihor skarn, like others within the same belt, is geochemically complex. The availability of Cu, Zn, and Pb, but also Ag, Bi, Mo, and B, as well as a wide range of minor elements, has created an environment allowing for crystallization of an unusually diverse range of discrete minerals. Graţianite is part of the peculiar associations of Bi-Pb-sulfosalts and Bi-chalcogenides that are genetically related to Au-enrichment. This study demonstrates the versatility of FIB-SEM techniques for in situ extraction of small volumes of well-characterized material, coupled with single-crystal X-ray analysis using synchrotron radiation, for the characterization of new minerals

Recent experiments have shown that microporous gold can be obtained via the oxidative dealloying of Au(Ag)-tellurides such as calaverite (AuTe 2 ), krennerite (Au 3 AgTe 8 ), and sylvanite [(Au,Ag) 2 Te 4 ] under mild hydrothermal conditions. The same Au textures have been found in natural gold-telluride ores from the Late Miocene epithermal Aginskoe Au-Ag-Te deposit in Kamchatka, Russia. This confirms that natural microporous gold can form via the replacement of telluride minerals. This replacement may take place under hydrothermal conditions, e.g., during the late stage of the ore-depositing event, explaining the wide distribution of “mustard gold” in some deposits. At Aginskoe, the oxidation of Au-tellurides appears to have resulted only in local redistribution of Au and Te, because the associated oxidation of chalcopyrite scavenged the excess Te, inhibiting the crystallization of secondary Te minerals more than a few micrometers in size. Such cryptic mobility may explain the lack of reported secondary Te minerals in many Te-bearing deposits.

A martian basalt (Yamato 980459) composition was used to synthesize olivine, spinel, and pyroxene at 1200 °C at five oxygen fugacities: IW-1, IW, IW+1, IW+2, and QFM. The goal of this study is to examine the significant variation in the value of DV pyroxene/melt with changing Wo content in pyroxene. While most literature on this subject relies on electron microprobe data that assumes that if the Wo component (CaSiO 3 ) is <4 mol%, the pyroxene is in fact orthopyroxene, we’ve made a more robust identification of orthopyroxene using appropriate Kikuchi diffraction lines collected during electron backscatter diffraction analysis. We compare augite (Wo ~ 33), pigeonite (Wo ~ 13), orthopyroxene (Wo <4), and olivine. In augite (Wo ~ 33), the M2 site is 8-coordinated, while in pigeonite (Wo ~ 13), the site is 6-coordinated. The larger (8-coordinated) M2 site in augite requires structural expansion along the chain direction. The longer chain is enabled by the substitution of the larger Al for Si. The Al 3+ substitution for Si 4+ causes a charge deficiency that is made up, in part, by the substitution of V 4+ and V 3+ in the pyroxene M1 site. This rationale does not fully explain the dramatic decrease in DV orthopyroxene/melt . In monoclinic pyroxenes, the TOT stacking is characterized by + + + + (indicating the direction), a stacking pattern that produces a monoclinic offset. In orthopyroxene, the stacking is + + - -, which produces an orthorhombic structure. The M2 site is located between the reversed TOT units and is highly constrained to 6-coordination and thus cannot contain significant Ca that requires 8-coordination. Because the M2 site in orthopyroxene is small and constrained, it accommodates less Al in the tetrahedral chains and thus less V in the pyroxene M1 site.