Volume 102, Issue 10

The substitution of sulfate in apatite is of potential importance in synthetic biomaterials used in bone repair and reconstruction. The counter ion (e.g., Na + , K + , Mg 2+ , Sr 2+ ) in the sulfate reagent may also be used as a source of medically beneficial ions. An understanding of the structural parameters controlling sulfate substitution is also important in expanding our knowledge beyond the substitution of carbonate in apatites. The incorporation of sulfate in calcium and strontium hydroxylapatites, prepared in aqueous solution at pH 9, was verified by combustion analysis of sulfate, infrared and Raman spectroscopy, and by determination of unit-cell parameters. Sulfate could not be incorporated into barium hydroxylapatite because of the preferential formation of BaSO 4 . The amount of sulfate substituted in the apatite was affected by the mole ratio of sulfate to phosphate in the reaction mixture and by the nature of the counter ion in the sulfate reagent. When sodium is the counter ion in the sulfate reagent, the molar amounts of both sodium and sulfate in the product apatite can be explained by assuming charge compensation by sodium ions and sulfate displacement of phosphate and calcium. With lithium as the counter ion, a greater molar amount of lithium than sulfate is incorporated into the apatite, an observation that requires an additional charge-compensation mechanism. With potassium and rubidium as counter ions, less of the counter ion is incorporated than sulfate, probably a result of less favorable accommodation of the larger cation in the apatite structure. The maximum molar amount of sulfate incorporated in hydroxylapatite (prepared in the presence of Na + ) is more than three times lower than the maximum molar amount of carbonate that can be incorporated, a difference that can be explained by the relative solubilities of the substituted apatites. The unit-cell parameters determined for both sulfated calcium and strontium hydroxylapatites synthesized with the sodium counter ion show a slight increase in the a -axis length and a nearly constant c -axis length with increasing sulfate content. The difference in the variation of unit-cell parameters with anion content can be rationalized by the difference in size of the anion. The results indicate that sulfate can be incorporated into biomaterials such as apatite or in composites with calcium sulfate and that the design of new apatites and composites could include the use of medically desirable counter cations.

Autunite, Ca[(UO 2 )(PO 4 )] 2 (H 2 O) 11 , is a common uranyl mineral found in oxidized portions of uranium deposits, as well as subsurface environments contaminated by uranium. Enthalpies of formation of autunite were obtained via high-temperature oxide melt calorimetry using a 3Na 2 O⋅4MoO 3 solvent at 976 K. The synthetic analog of autunite was prepared using slow mixing by diffusion into an aqueous barrier solution at room temperature. Prior to calorimetric measurements, the material was characterized using powder X-ray diffraction (PXRD), inductively coupled plasma optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), and Raman spectroscopy, to ensure purity. The calculated enthalpy of formation from binary oxides of autunite is −579.92 ± 21.68 kJ/mol; the enthalpy of formation from the elements is −8311.32 ± 21.79 kJ/mol. The measured drop solution enthalpy allowed calculation of the enthalpy of the reaction of dehydration of autunite to meta-autunite. The results demonstrate that autunite is a metastable phase and explain the observed rapid dehydration to meta-autunite, a lower hydrate, as well as the common occurrence of the latter mineral in nature.

The crystal structure of turneaureite, ideally Ca 5 (AsO 4 ) 3 Cl, was studied using a specimen from the Brattfors mine, Nordmark, Värmland, Sweden, by means of single-crystal X-ray diffraction data. The structure was refined to R 1 = 0.017 on the basis of 716 unique reflections with F o > 4σ( F o ) in the P 6 3 / m space group, with unit-cell parameters a = 9.9218(3), c = 6.8638(2) Å, V = 585.16(4) Å 3 . The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%; average of 10 spot analyses): SO 3 0.22, P 2 O 5 0.20, V 2 O 5 0.01, As 2 O 5 51.76, SiO 2 0.06, CaO 41.39, MnO 1.89, SrO 0.12, BaO 0.52, PbO 0.10, Na 2 O 0.02, F 0.32, Cl 2.56, H 2 O calc 0.58, O(=F+Cl) −0.71, total 99.04. On the basis of 13 anions per formula unit, the empirical formula corresponds to (Ca 4.82 Mn 0.17 Ba 0.02 Sr 0.01 ) Σ5.02 (As 2.94 P 0.02 S 0.02 Si 0.01 ) Σ2.99 O 12 [Cl 0.47 (OH) 0.42 F 0.11 ] Σ1.00 . Turneaureite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M 1 polyhedra running along c are connected through T O 4 tetrahedra with channels hosting M 2 cations and X anions. Owing to its particular chemical composition, the studied turneaureite can be considered as a ternary calcium arsenate apatite; consequently it has several partially filled anion sites within the anion columns. Polarized single-crystal FTIR spectra of the studied sample indicate stronger hydrogen bonding and less diverse short-range atom arrangements around (OH) groups in turneaureite as compared to the related minerals johnbaumite and svabite. An accurate knowledge of the atomic arrangement of this apatite-remediation mineral represents an improvement in our understanding of minerals able to sequester and stabilize heavy metals such as arsenic in polluted areas.

The formation of porphyry Cu-Mo deposits hinges critically on the ability of an exsolving magmatic volatile phases (MVP) to efficiently extract the available Cu and Mo from the silicate melt and transport them to the site of mineralization. There has been substantial debate about the relative importance of the critical parameters likely to control metal partitioning among silicate melts and supercritical fluids, vapors, and brines. To explore the relative contributions of key MVP parameters ( X NaCleq , f O 2 , f S 2 ), we present felsic magmatic Cu-Mo partitioning experiments at both reduced ( f O 2 = NNO+0.6) and oxidized conditions ( f O 2 = NNO+2), at high f S 2 , and over the full range of salinities ( X NaCleq ) relevant to porphyry deposit formation. The experiments demonstrate that fluid-melt Cu partition coefficients ( DCuf/m$\begin{array}{} D_\text{Cu}^{\text{f/m}} \end{array} $) have a dominantly second-order exponential relationship with X NaCleq at relevant salinities, consistent with a (Na,K)CuCl 2 ion-pair complexation mechanism. We find a strong linear dependence of Cu partitioning on Cl partitioning between coexisting brine and vapor, in good agreement with limited data from unmodified natural fluid inclusions. Whereas H 2 S can increase Cu partitioning via (Na,K)CuCl(HS) formation, SO 2 has no measureable effect on Cu partitioning. These data allow for quantifying the strong partitioning of Cu out of silicate melts at MVP salinities above ~5 wt%, which will become further enriched into tardo-magmatic brines on phase separation. Our data also highlight that low-salinity (<2–3 wt% NaCl eq ) oxidized MVPs are not capable of substantially extracting Cu from evolved silicate melts or transporting it to the site of mineralization. We also show that DMof/m$\begin{array}{} D_\text{Mo}^{\text{f/m}} \end{array} $ is a linear function of X NaCleq , consistent with mono-chloride (e.g., {Na,K}MoO 3 Cl), Na-K molybdate (e.g., {Na,K}HMoO 4 ), or thio-molybdate ({Na,K}HMoO 2 S 2 ) complexation mechanisms at modest salinities (>3 wt%) rather than the Mo-oxy-hydroxy [MoO 2 (OH) 2 ] complexation observed at lower salinities. The f O 2 of the magmatic system has a subordinate effect on DMof/m$\begin{array}{} D_\text{Mo}^{\text{f/m}} \end{array} $, with enhanced partitioning at higher f O 2 . We use the combined data set to produce expressions for DCuf/m$\begin{array}{} D_\text{Cu}^{\text{f/m}} \end{array} $ and DMof/m$\begin{array}{} D_\text{Mo}^{\text{f/m}} \end{array} $ as functions of X NaCleq , X H 2 S , and f O 2 DCuf/m=8.0(± 1.4)× 104[(XNaCleq)2(XH2O)14± 2{1+180(± 60)(XH2S)}]+380(± 50)(XNaCleq)+0.8(± 0.5)DMof/m(at NNO+2)=430(± 60)⋅ (XNaCleq)+1.5(± 0.7)⋅ (XH2O).$$\begin{array}{} D_\text{Cu}^{\text{f/m}}=8.0(\pm 1.4)\times 10^4[(X_{\text{NaCleq}})^2(X_{\text{H}_2{\text O}})^{14\pm2}\{1+180(\pm60)(X_{\text H_2\text S})\}]+380(\pm50)(X_{\text{NaCleq}})+0.8(\pm 0.5)\\ D_\text{Mo}^{\text{f/m}}(\text{at NNO}+2)=430(\pm 60)\cdot (X_{\text{NaCleq}})+1.5(\pm 0.7)\cdot (X_{\text H_2 \text O}). \end{array} $$ These equations provide Cu-Mo fluid-melt partition coefficients for common arc melt-MVP assemblages as their composition evolves through time and space. Quantitative modeling of the contrasting partitioning behavior of Cu and Mo using these equations will allow for significant improvement in understanding metal extraction and porphyry deposit formation.

Plagioclase textures were investigated in the products of the voluminous 1783–1784 CE Laki eruption from the Eastern Volcanic Zone (EVZ) of Iceland to establish whether mush disaggregation occurred solely at the onset of the eight-month eruption or throughout its whole duration. Phase proportions and plagioclase size distributions were determined using standard optical and manual techniques as well as automated approaches based on Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN). Based on optical microscopy and the explicit combination of textural and compositional information in QEMSCAN images, plagioclase crystals were divided into two populations: small (<0.5 mm long), high-aspect ratio (length/width >4) microcrysts with low-anorthite (<An 80 ) cores; and large (>0.5 mm long), low-aspect ratio (length/width = 2–3) macrocrysts with high-anorthite (An 84 −An 92 ) cores. Small microcrysts grew from their carrier liquid during the final phase of pre-eruptive crystallization while large macrocrysts, which are out of geochemical equilibrium with their carrier liquids, were entrained from crystal mushes. Changes in phase proportions and plagioclase size distributions between eruptive episodes demonstrate that macrocryst entrainment efficiency varied substantially during the eruption; material erupted in later episodes contain proportionally more mush-derived material. Using stereologically corrected plagioclase size distributions, we estimate that the pre-eruptive residence times of microcrysts in the Laki carrier liquid were probably of the order of 2–20 days. Because microcryst crystallization was concurrent with macrocryst rim growth, these day-to-week residence times also indicate that macrocryst entrainment occurred on much shorter timescales than the eruption’s eight-month duration. In line with constraints from independent geochronometers, macrocryst entrainment and mush disaggregation thus appears to have continued throughout the eruption. Magmas were assembled on an episode by episode basis, and the volume of eruptible magma in the plumbing system at any given time was probably closer to 1–2 km 3 than the final erupted volume of 15.1 km 3 .

We present a new hydrothermal moissanite cell for in situ experiments at pressures up to 1000 bar and temperature to 850 °C. The original moissanite cell presented by Schiavi et al. (2010) was redesigned to allow precise control of fluid pressure. The new device consists of a cylindrical sample chamber drilled into a bulk piece of NIMONIC 105 super alloy, which is connected through a capillary to an external pressure control system. Sealing is provided by two gold gasket rings between the moissanite windows and the sample chamber. The new technique allows the direct observation of various phenomena, such as bubble nucleation, bubble growth, crystal growth, and crystal dissolution in silicate melts, at accurately controlled rates of heating, cooling, and compression or decompression. Several pilot experiments on bubble nucleation and growth at temperature of 715 °C and under variable pressure regimes (pressure oscillations between 500 and 1000 bar and decompression from 800 to 200 bar at variable decompression rates) were conducted using a haplogranitic glass as starting material. Bubble nucleation occurs in a short single event upon heating of the melt above the glass transition temperature and upon decompression, but only during the first 100 bar of decompression. New bubbles nucleate only at a distance from existing bubbles larger than the mean diffusive path of water in the melt. Bubbles expand and shrink instantaneously in response to any pressure change. The bubble-bubble contact induced during pressure cycling and decompression does not favor bubble coalescence, which is never observed at contact times shorter than 60 s. However, repeated pressure changes favor the diffusive coarsening of larger bubbles at the expense of the smaller ones (Ostwald ripening). Experiments with the haplogranite show that, under the most favorable conditions of volatile supersaturation (as imposed by the experiment), highly viscous melts are likely to maintain the packing of bubbles for longer time before fragmentation. In-situ observations with the new hydrothermal moissanite cell allow to carefully assess the conditions of bubble nucleation, eliminating the uncertainty given by the post mortem observation of samples run using conventional experimental techniques.

High-pressure high-temperature phase relation experiments in Mg 14 Si 5 O 24 were performed using a 6-8 multi-anvil high-pressure apparatus in the pressure range of 12–22 GPa and temperature range of 1673–2173 K. We first found that Mg 14 Si 5 O 24 anhydrous phase B (Anh-B) dissociates to Mg 2 SiO 4 wadsleysite (Wd) and MgO periclase (Per) at about 18 GPa and 1873 K. From the results of the high-pressure experiments, the phase boundaries of 5 Mg 2 SiO 4 forsterite (Fo) + 4 Per = Anh-B and Anh-B = 5 Wd + 4 Per were determined. In addition, the isobaric heat capacity ( C P ) of Anh-B was measured by differential scanning calorimetry in the temperature range of 300–770 K and the thermal relaxation method using a Physical Property Measurement System (PPMS) in the range of 2–303 K. From the measured low-temperature C P , the standard entropy (S298.15o)$\begin{array}{} (S{^{\rm o}_{298.15}}) \end{array} $ of Anh-B was determined to be 544.4(2) J/(mol⋅K). We also performed high-temperature X-ray diffraction measurements in the range 303–773 K to determine the thermal expansivity (α) of Anh-B. The obtained C P and α were theoretically extrapolated to higher temperature region using a lattice vibrational model calculation partly based on Raman spectroscopic data. Thermodynamic calculations by adopting the thermochemical and thermoelastic data for Anh-B obtained in this study and the estimated formation enthalpy for Anh-B of −13 208 kJ/mol gave phase equilibrium boundaries for 5 Fo + 4 Per = Anh-B and Anh-B = 5 Wd + 4 Per that were consistent with those determined by the present high-pressure high-temperature experiments. The results clarified that, in the Mg 14 Si 5 O 24 system, Anh-B is stable between 12 and 18 GPa at the expected temperatures of the Earth’s mantle.

A model for the origin, ascent, and eruption of the lunar A17 orange glass magma has been constructed using petrological constraints from gas solubility experiments and from analyses of the lunar sample 74220 to better determine the nature and origin of this unique explosive eruption. Three stages of the eruption have been identified. Stage 1 of the eruption model extends from ~550 km, the A17 orange glass magma source region based on phase equilibria studies, to 50 km depth in the Moon. Stage 2 extends from ~50 km to 500 m, where a C-O-H-S gas phase formed and grew in volume based on melt inclusion analyses and measurements. The volume of the gas phase at 500 m depth below the surface is calculated to be 7 to 15 vol% of the magma (closed-system) using the minimum and maximum estimates of CO, H 2 O, and S loss from the melt. In Stage 3, depths shallower than ~450 m, the rising magma exsolved an additional 800–900 ppm H 2 O and 300 ppm S, increasing the moles in the gas by a factor of 3 to 4. The closed-system gas phase is calculated to reach ~70 vol% at ~130 m depth, enough to fragment the magma and form pyroclastic beads. However, fragmentation (bead formation) is interpreted to have occurred at depths ranging from 600 to 300 m below the lunar surface based on the pressure necessary to explain the C content of the orange glass beads. The gas volume (70%) required to fragment the ascending magma at this depth is a factor of ~5 greater than the volume determined for closed-system degassing of an orange glass magma at 500 m, strongly implying that the gas was produced by open-system degassing as the magma ascended from greater depths. Formation of the dike carrying the magma up from the ~550 km deep source is considered to occur by a crack propagation mechanism (Wilson and Head 2003, 2017). The rapid dike-propagation process facilitates gas collection by open-system degassing in the upper part of the dike. This is necessary to achieve the gas volumes required for magma fragmentation at 600 m depths, and the magma-ascent velocities to explain the wide areal distribution of the bead deposit. The explosive nature of the picritic orange glass eruption, and the homogeneity of the bead compositions, are consistent with this gas-assisted eruption scenario, as is the evidence of a Fe-metal forming reduction event during Stage 2 followed by a Stage 3 oxidation event in the ascending magma.

Phase relations of magnesioferrite-magnetite solid solutions (Mg,Fe 2+ ) Fe23+ $\rm {Fe_{2}^{3+}}$ O 4 were investigated at pressures of 9–23 GPa and temperatures of 1200–1600 °C. Our new results indicate that the assemblage Mg 2 Fe 2 O 5 + Fe 2 O 3 reconstitutes to a hp-MgFe 2 O 4 phase at 20 GPa and 1300–1500 °C. The stability field of hp-MgFe 2 O 4 begins at ~1300 °C and widens to higher temperature. At lower temperature (1200–1300 °C) Mg 2 Fe 2 O 5 + Fe 2 O 3 breaks down to the new phase assemblage Mg 3 Fe 4 O 9 + Fe 2 O 3 with its stability field expanding to higher pressures and temperatures at the expense of hp-MgFe 2 O 4 . The Mg 3 Fe 4 O 9 phase has the same crystal structure that recently reported for Fe 7 O 9 , and thus represents the Mg-end-member. From powder X-ray diffraction, we find that hp-MgFe 2 O 4 has a structure consistent with an orthorhombic unit cell belonging to the Pmcn space group (no. 62). However, it could have undergone a transformation from a different structure during decompression. Experiments conducted with a Mg0.5Fe0.52+Fe23+O4 $\rm{ Mg_{0.5}Fe^{2+}_{0.5}Fe^{3+}_2O_4 } $ composition demonstrate that the addition of Fe 2+ significantly changes the topology of the phase relations compared to the MgFe 2 O 4 end-member system. At 10–11 GPa and 1000–1600 °C, Mg 0.5 Fe 0.5 Fe 2 O 4 breaks down to the assemblage MgFeFe 2 O 5 + Fe 2 O 3 , with the phase boundary described by: P (GPa) = 2.0 × 10 −3 × T (°C) + 8.2. No stability field for the constituent oxides [i.e., (Mg,Fe)O + Fe 2 O 3 ] exists, in contrast to that observed for the MgFe 2 O 4 end-member. The stability of the assemblage MgFe2+Fe23+O5+Fe2O3 $\rm{ MgFe^{2+}Fe_2^{3+}O_5 + Fe_2O_3 }$ is limited at higher pressures and appears to pinch out to higher temperatures. At 15–16 GPa and temperatures up to 1350 °C, this assemblage reconstitutes to form a hp-Mg 0.5 Fe 0.5 Fe 2 O 4 phase. However, at higher temperatures a new assemblage of (Mg,Fe) 3 Fe 4 O 9 + Fe 2 O 3 appears. The occurrence of such compositions suggests that solid solution may be complete across the Mg 3 Fe 4 O 9 –Fe 7 O 9 binary. Our results further demonstrate that phase relations even in simple Fe-Mg oxides can become complex at high pressures and temperatures and that phases with various novel stoichiometries (i.e., Mg 3 Fe 4 O 9 ) may become stable. In addition, this study has implications for natural samples by helping to place constraints on the range in pressure and temperature at which a given sample formed. For instance, magnetite or magnesioferrite entrapped as inclusions in diamond could have either have crystallized directly, or formed from precursor phases at depths that exceed the stability of the spinel-structured phases. Evidence for such high-pressure transformations can potentially be found by investigating micro-textures.

Sulfur is an important volatile element involved in magmatic systems. Its quantification in silicate glasses relies on state-of-the-art techniques such as electronprobe microanalyses (EPMA) or X-ray absorption spectroscopy but is often complicated by the fact that S dissolved in silicate glasses can adopt several oxidation states (S 6+ for sulfates or S 2− for sulfides). In the present work, we use micro-Raman spectroscopy on a series of silicate glasses to quantify the S content. The database is constituted by 47 silicate glasses of various compositions (natural and synthetic) with S content ranging from 1179 to 13 180 ppm. Most of the investigated glasses have been synthesized at high pressure and high temperature and under fully oxidizing conditions. The obtained Raman spectra are consistent with these f O 2 conditions and only S 6+ is present and shows a characteristic peak located at ~1000 cm −1 corresponding to the symmetric stretch of the sulfate molecular group (ν 1 SO42− $\begin{array}{} \text{SO}_4^{2-} \end{array} $). The intensity of the ν 1 SO42− $\begin{array}{} \text{SO}_4^{2-} \end{array} $ peak is linearly correlated to the parts per million of S 6+ determined by EPMA. Using subsequent deconvolution of the Raman spectra, we established an equation using the ratio between the areas of the ν 1 SO42− $\begin{array}{} \text{SO}_4^{2-} \end{array} $ peak and the silicate network species (Q n ) in the high-frequency region: ppm S6+=34371ASO42− AQn± 609.$$\begin{array}{} \text{ppm S}^{6+}=34371\frac{A\text{SO}_4^{2-}}{A\text Q^{\text n}}\pm609. \end{array} $$ We tested our calibration on several silicate glasses equilibrated under moderately reducing conditions (QFM+0.8 ≤ f O 2 ≤ QFM+1.4) in which S is dissolved as both SO42− $\begin{array}{} \text{SO}_4^{2-} \end{array} $ and S 2− . We also analyzed several olivine-hosted melt inclusions collected from Etna for which the f O 2 and S speciation are unknown. For these samples, the S content estimated by the Raman calibration is systematically lower than the total S measured by EPMA. We combined both methods to estimate the S 2− content not accounted for by Raman and derive the S speciation and f O 2 conditions. The derived f O 2 is consistent with the imposed f O 2 for synthesized glasses and with current assumed f O 2 conditions for basaltic melt inclusions from Etna.

The presence in a magma of fayalite, the iron end-member of the olivine binary series, affects the feldspars at pressure by lowering the temperatures at which they crystallize from the magma. Starting with estimates from published literature it becomes obvious that at pressure, fayalite becomes important because the pressure effects on the melting temperatures are very different: large for albite, and small for anorthite. In this experimental study, a powder of fayalite composition was combined with six finely ground natural feldspars from Ab to An 97 to make six bulk compositions. Using graphite crucibles in piston-cylinder apparatus at a pressure of 5 kbar, a cotectic in the ternary system was found to range from 1141 °C at An(Fa) to 1124 °C at Ab(Fa), with fayalite contents from 68 to 17 wt%, respectively. The results can be used to show that ternary feldspars saturated with fayalite and Fe monoclinic pyroxene will crystallize at a 5 kbar multiphase eutectic 1010 °C—56 °C below a calculated azeotropic point on the Ab-Or join. The results are used to compare the end points of two very different layered intrusions, Skaergaard and Kiglapait, and to illuminate the nature and origins of syenite and trachyte, which are leucocratic rocks unsaturated with mafic minerals. Because fayalite-saturated melts are responsive to pressure unequally on the feldspar end-members, olivine of intermediate composition will have a damped but potentially significant effect on feldspar fractionation in the lower crust of the Earth, possibly affecting the origin of anorthosite and syenite.

The lithology of the 1.305 Ga Kiglapait intrusion is dominated by a Lower Zone of troctolite, succeeded by an Upper Zone of olivine gabbro, ferrodiorite, and syenite with olivine composition of pure fayalite. The feldspar composition of the intrusion varies from An 68 to An 9 over a thickness of 8.4 km from the base to a sandwich horizon under an Upper Border Zone. The anhydrous nature of the Kiglapait syenites is shown by their high temperature, by the loss of minor biotite up-stratigraphy in the intrusion, and the absence of amphibole. The end-stage feldspar of the Kiglapait syenites is that of a solidus embedded in a solvus in a 3 kbar eutectic at 1000 °C. The end-member assemblage at temperature and pressure is invariant. The final bulk composition is relatively An-rich—An ~11%—with a composition of X Or = 1/3 when projected to the Ab-Or sideline. The experimental feldspar solvus when corrected for the effects of An and Ba and referred to 3 kbar penetrates the solidus and fits the experimental tie lines. These conditions precede a stage of local coarsening under subsolidus conditions that is found in colloform symplectites invading mesoperthite. The oligoclase-orthoclase symplectites are iso-compositional with their host mesoperthites. The coarsening is assumed to be related to a plausibly F-rich vapor phase that is locally consumed with time. The observed phase compositions indicate the end of exsolution at ~800 °C at 3 kbar on the binodal solvus.

Exsolution lamellae in pyroxene occurring in mantle lherzolite xenoliths in Cenozoic basalts from the Mingxi area, Fujian Province, China, have been investigated by electron backscattered diffraction (EBSD) to determine epitaxial relationships between host and lamellae. Clinopyroxene (diopside) hosts developed two sets of lamellae: one of orthopyroxene (pigeonite-enstatite) lamellae and the other of clinopyroxene (augite) lamellae. Orthopyroxene (enstatite) hosts developed a single set of clinopyroxene (augite) lamellae. A zone crossing method has been used to determine Miller indices of lamellae, which appear as linear traces on thin sections tested by EBSD. In clinopyroxene hosts, the index of orthopyroxene lamellae is (100) and that of clinopyroxene lamellae is ~(401) at 22° to the c -axis. In orthopyroxene hosts, the index of clinopyroxene lamellae is (100). Published high-pressure crystallographic data for compositions approximating those of the lamellae and host are used to compare cell parameters of lamellae and hosts at different pressures. Exact phase boundary theory is applied to estimate the exsolution pressure, and the data uncertainty of composition, cell parameters and orientation of the lamellae have been analyzed. Uncertainties of composition and cell parameters give rise to only small uncertainties in the exsolution pressure, but that of the orientation of the lamellae generates large uncertainty. Independent high accuracy measurement of the angle between lamellae and c -axis by TEM or other techniques combined with exact phase boundary theory would give more reliable estimates of exsolution pressure.

The spin states of iron in deep magmas are one of the most important properties that affect the partitioning of iron between magmas and minerals and, thus, the gravitational stability of magmas in the Earth. We investigated the spin state and electronic environments of iron in a basaltic glass containing ~70 Fe 3+ /ΣFe at room temperature and pressures from 1 bar to 130 GPa using a diamond-anvil cell combined with energy domain synchrotron 57 Fe Mössbauer source spectroscopy. The basaltic glass represents an analog of a multi-component magma typical for the Earth. The Mössbauer spectra could be fitted by a two pseudo-Voigt doublet model including a high quadrupole splitting (QS) doublet and a low QS doublet, which were assigned to high-spin Fe 2+ and high-spin Fe 3+ , respectively. The high-spin states of Fe 2+ and Fe 3+ remained up to 130 GPa corresponding to the pressure in the lowermost mantle. The center shift values of high-spin Fe 2+ and Fe 3+ did not show large changes with pressure, ruling out sharp electronic changes in the basaltic glass. Therefore, a sharp and complete spin crossover of Fe 2+ from the high-spin to the low-spin state does not appear to occur in the basaltic glass although the possibility of a partial spin transition cannot be fully excluded. The QS values of Fe 2+ increased slightly at 0–20 GPa and above 100 GPa, and the higher value was preserved after decompression to ambient conditions. This behavior may be related to distortion of Fe 2+ polyhedra due to short-range ordering on compression. Such a distortion of Fe 2+ polyhedra could gradually stabilize Fe 2+ in the basaltic glass with pressure compared to bridgmanite according to the Jahn-Teller effect, and thus could gradually enhance the partitioning of iron into deep magmas in the lower mantle.

The crystal structure of the high-pressure Mg 2 Cr 2 O 5 phase was studied by single-crystal X-ray diffraction (XRD) analysis for the recovered samples. The 61 parameters including anisotropic displacement parameters of each atom and site occupancies of Mg and Cr in cation sites were refined with R 1 = 1.26%, wR 2 = 4.33%, and S fit = 1.265 for 470 unique reflections. The results show that the structure of the recovered Mg 2 Cr 2 O 5 phase is the same as modified ludwigite (mLd)-type Mg 2 Al 2 O 5 [space group: Pbam (no. 55)], and the lattice parameters are a = 9.6091(2), b = 12.4324(2), c = 2.8498(1) Å ( Z = 4). The refined structure of the Mg 2 Cr 2 O 5 phase has four (Mg,Cr)O 6 octahedral sites and a MgO 6 trigonal prism site, and is similar to but distinct from that of CaFe 3 O 5 -type Mg 2 Fe 2 O 5 phase, which has two octahedral sites and a bicapped trigonal prism site with two longer cation-oxygen bonds. The isotropic atomic displacement parameter of the trigonal prism site cation in mLd-type Mg 2 Cr 2 O 5 is relatively small compared with that of CaFe 3 O 5 -type Mg 2 Fe 2 O 5 , suggesting that the trigonal prism site is less flexible for cation substitution than that of CaFe 3 O 5 -type structure. To stabilize mLd-type A22+B23+O5$\begin{array}{} A_2^{2+}B_2^{3+}\text{O}_5 \end{array} $ phase, it would be an important factor for the B 3+ cation to have high octahedral-site preference, resulting in only A 2+ cation being accommodated in the tight trigonal prism site. Our study also suggests that mLd-type phase with (Mg,Fe 2+ ) 2 Cr 2 O 5 composition would crystallize as one of decomposed phases of chromitites, when the chromitites were possibly subducted into the mantle transition zone.

The Volyn pegmatites from Volodarsk-Volynskyi in the Zhytomyr Oblast, NW Ukraine, are associated with granites genetically related to the Paleoproterozoic Korosten pluton. Their late-stage evolution is characterized by the formation of opal-cemented breccia. A polymineralic pseudomorph after beryl within the breccia includes bertrandite (±euclase) + F-muscovite (with tobelite component) + buddingtonite + organic matter (OM) + opal (+ traces of K-feldspar, albite, columbite, FeS 2 , barite, REE-minerals). Sector-zoned and platy to fibrous buddingtonite has variable (K+Na)-vs. NH 4 ,-contents (electron microprobe analyses) and some H 2 O or H 3 O + , as indicated by microscope infrared spectroscopy. We suggest that ammonium was produced by decay of OM, which is partly preserved in the pseudomorph. Energy-dispersive electron microprobe data of the OM show with increasing O–decreasing C-N-content due to degassing; the OM contains the high field strength elements Zr (≤7 at%), Y (≤3 at%), Sc (≤0.8 at%), REE (≤0.3 at%), Th (≤0.2 at%), and U (≤1.25 at%), which increase with increasing O-content. Transmission electron microscopy of the OM confirms the presence of N; Zr, Si, and O (with other HFSE) are concentrated in nanometer-sized areas and at the transition from OM to opal in nanometer-sized platy Zr-Si-O crystals. C-rich areas are amorphous but show poorly developed lattice fringes. OM is present in the pseudomorph also as brown pigmentation of opal and in pegmatitic beryl from Volyn as a component in late stage fluid inclusions, identified by C-H vibrational bands in infrared spectra. Stable isotope investigations of C and N of buddingtonite, black opal and kerite (fibrous OM known from the literature to occur in the Volyn pegmatites and interpreted as microfossils) indicate a biogenic origin of the OM. We propose that OM in the pseudomorph is condensed kerite, which achieved the high concentrations of high field strength elements via fluid-pegmatite interaction. Although no age determination of minerals in the pseudomorph is available, textural arguments and phase equilibria indicate its formation in a late stage of the pegmatite evolution, at P-T conditions below ~100 MPa/150 °C. We favor a conceptual model for the formation of the Volyn buddingtonite in analogy to Phanerozoic occurrences of buddingtonite, where over and around the shallow anorthosite-granite Korosten pluton hydrothermal convection cells introduced N-bearing hydrocarbons and its precursors into the cooling igneous rocks. Due to the elevated temperature, the OM disintegrated into degassing volatile and non-volatile residual components analogous to petroleum maturation. Organic N, released as NH 4 , was then incorporated into buddingtonite.

Mengxianminite, ideally Ca 2 Sn 2 Mg 3 Al 8 [(BO 3 )(BeO 4 )O 6 ] 2 , is a new borate mineral from Xianghualing skam, Hunan Province, southern China. It occurs in the hsianghualite vein of Xianghualing skarn, associated with fluorite, phlogopite, hsianghualite, magnetite, dravite, magnesiotaaffeite-2 N ‘2 S , and calcite. Mengxianminite forms subhedral to euhedral green crystals from 20 to 200 mm long, translucent to transparent, with a vitreous luster. The crystals show perfect cleavage on {100} and good cleavage on {010}, and do not fluoresce in long- or short-wave ultraviolet light. The estimated Mohs hardness is 8, and the tenacity is brittle with irregular fracture. The calculated density is 4.170 g/cm 3 . Optically, mengxianminite is biaxial (–), with α = 1.80(2), β = 1.83(2), γ = 1.84(2) (589 nm). The mean chemical composition of mengxianminite is Al 2 O 3 40.00, SnO 2 25.96, MgO 6.57, CaO 8.56, FeO 4.83, B 2 O 3 6.53, BeO 4.37, ZnO 1.81, MnO 1.23, Na 2 O 1.13, TiO 2 0.10, SiO 2 0.04, sum 101.12 with a corresponding empirical formula calculated on the basis of 26 O atoms of (Ca 1.64 ,Na 0.39 ) Σ2.03 (Sn 1.85 ,Zn 0.24 ) Σ2.09 (Mg 1.75 , Fe 0.72 ,Al 0.42 ,Mn 0.19 ,Ti 0.01 ) Σ3.09 Al 8 [(B 1.01 O 3 )(Be 0.94 O 4 )O 6 ] 2 . (Be and B were measured by secondary ion mass spectrometry, average of six electron microprobe analyze points and in wt%.) The strongest eight lines of the powder XRD pattern [ d in Å ( I )( hkl )] are: 3.000 (35)(16.2.0); 2.931 (100)(17.1.1); 2.475 (29)(022); 2.430 (30)(13.3.1); 2.375 (100)(14.0.2/640); 2.028 (52)(21.3.1); 1.807 (35)(913); 1.530 (98)(14.6.0/15.3.3). Mengxianminite is orthorhombic, space group Fdd 2; unit-cell parameters refined from single-crystal X-ray diffraction data are: a = 60.699(4), b = 9.914(1), c = 5.745(1) Å, V = 3457.4(4) Å 3 , and Z = 8. The structure of mengxianminite is characterized by the alternating O-T1-O-T2-O′-T2 layers stacked along the a axis, which are equal to two alternating modules: the module A (O-T1-O) corresponding to the spinel module with an additional O layer (AlO 6 octahedra layer), and the module B (T2-O′-T2) showing the simplified formula CaSnAl(BeO 4 )(BO 3 ), where SnO 6 octahedra are isolated in the T2 layers, connected via BeO 4 and CaO 11 groups, and AlO 6 edge-sharing octahedra in the O′ layer form chains running along the {011} or {0 1 1} direction, connected in the c direction by the BO 3 triangular groups. Mengxianminite is the first borate mineral with both Sn and Be, likely crystallized under F-rich conditions at late stages of the Xianghualing skarn.

Risk assessments that take into account the formation of environmentally dangerous hexavalent chromium in Cr-containing mine tailings, and associated soils and sediments, require an understanding of the occurrence and speciation of Cr in silicate minerals and glasses. Silicates are more soluble and generally more susceptible to weathering than the refractory mineral chromite, the principal ore mineral of Cr. Studies at the nanoscale using a combination of advanced sample preparation via microtoming and focused ion beam techniques, in combination with state-of-the art analytical transmission electron microscopy and electron diffraction, reveal the occurrence of chromite nanoparticles held within clinochlore and lizardite grains in chromitite ore (an igneous cumulate consisting primarily of chromite) from the Black Thor Chromium deposit in Northern Ontario, Canada, and the Mistake Mine, Fresno County, California, U.S.A., respectively. Nanoscale examinations of altered chromitite ore samples from the Black Thor deposit after dissolution experiments in sulfuric acid-bearing solutions of pH 2.5 show that clinochlore alters to amorphous silica depleted in chromite nanoparticles. This observation suggests the release of chromite nanoparticles rather than Cr 3+ aqueous species during the weathering of chromite-bearing silicate minerals. This will in turn have an impact on the environmental behavior of Cr 3+ and its potential oxidation to Cr 6+ . The formation of Craq6+$\begin{array}{} \rm Cr_{aq}^{6+} \end{array} $ species in this case will require either the initial dissolution of the nanoparticles or the oxidation of Cr 3+ species on the surface of the nanoparticles, either process being a rate limiting step in the formation of Craq6+$\begin{array}{} \rm Cr_{aq}^{6+} \end{array} $ species.

Al-Fe-bearing protoenstatite was discovered in Oregon sunstones with unusual pleochroic/dichroic red to green coloration using high-resolution transmission electron microscopy (HRTEM) and X-ray energy-dispersive spectroscopy (EDS). The empirical formula calculated on the basis of 6 O apfu is (Mg 1.17 Fe 0.43 Al 0.26 Ca 0.03 Na 0.10 Ti 0.01 ) Σ2.00 (Si 1.83 Al 0.17 ) Σ2.00 O 6 . The protoenstatite has a space group of Pbcn ; its unit-cell parameters refined from selected-area electron diffraction patterns are a = 9.25(1), b = 8.78(1), and c = 5.32(1) Å. Protoenstatite nanocrystals are quenchable to low temperature. The crystallographically oriented nanocrystals of protoenstatite and clinoenstatite in association with copper nanocrystals are responsible for the unusual green and “watermelon” coloration of the labradorite gemstone.