Volume 102, Issue 1

Melt inclusion data from primitive arc basalts from Mexico and Kamchatka show clear positive correlations of “fluid mobile element”/H 2 O ratios with the Cl/H 2 O ratio, suggesting that the trace element content of subduction zone fluids is strongly enhanced by complexing with chloride. This effect is observed for large-ion lithophile (LILE) elements, (e.g., Rb and Sr), but also for the light rare earth elements (REE, e.g., La and Ce) as well as for U. The correlations of these elements with Cl/H 2 O cannot be explained by the addition of sediment melts or slab melts to the mantle source, since Cl has no effect on the solubility or partitioning of these elements in silicate melt systems. On the other hand, the observed relationship of trace element abundance with Cl is consistent with a large body of experimental data showing greatly enhanced partitioning into aqueous fluid upon addition of chloride. Accordingly, it appears that a dilute, Cl-bearing aqueous fluid is the main carrier of LILE, light REE, and U from the slab to the source of melting in arcs. Moreover, elevated Ce/H 2 O ratios clearly correlate with fluid salinity and therefore are not suitable as a “slab geothermometer.” From a synopsis of experimental and melt inclusion data, it is suggested that the importance of sediment or slab melting in the generation of arc magmas is likely overestimated, while the effects of trace element scavenging from the mantle wedge may be underestimated. Moreover, establishing reliable data sets for the fluid/mineral partition coefficients of trace elements as a function of pressure, temperature, and salinity requires additional efforts, since most of the commonly used experimental strategies have severe drawbacks and potential pitfalls.

Intermediate plagioclase feldspars are the most abundant minerals in the Earth’s crust. Their incommensurately modulated structure has puzzled geologists and crystallographers for decades since the phenomenon in a labradorite was reported in 1940. Solving the structure is a necessary step toward mapping the complex subsolidus phase relations of plagioclase solid solution. The structure of a homogeneous labradorite (An 51 ) single crystal from a metamorphic rock is solved and refined from single-crystal X-ray diffraction. The result structure can be simplified as alternating I 1-like lamellae domains related by inversion twins. The inversion boundary shows an anorthite-like structure with I1¯ $I\overline 1 $ symmetry and is richer in Ca than the neighboring domains with opposite polarity. No albite-like subunits appear in the e-plagioclase structure. The modulated structure displays a unique Al-Si ordering pattern. A density modulation with a variation of 17 mol% in composition is also observed and can be properly described only by applying second-order harmonic waves for the atomic modulation functions. The modulated structure reveals details that cannot be observed from refinement with only main reflections and may be used to assess the ordering state and cooling rate of its host rock. The homogeneity of the crystal indicates the closure of the solvus for Bøggild intergrowth at low temperature. The highly ordered modulation supports the thermodynamic stability of e -plagioclase. Both Al-Si ordering and Ca-Na ordering are the driving force for formation of the incommensurately modulated structure.

Clay minerals found in and near the surface of Mars contain unique information about the geo-chemical environment in the martian near-surface in the ancient past. To interpret this information, it is necessary to fully understand the environments in which different clay minerals form. Studies of terrestrial analog materials and environments are a useful way to address such questions, and some terrestrial materials are also important standards for remote sensing and in situ chemical and mineralogical analyses. This study presents new information on the formation environment of an unusual standard clay, the Clay Minerals Society source clay SWa-1 ferruginous smectite of Grant County. The SWa-1 collection locality is in the Columbia River Basalts (CRB), at the contact between a paleosol and a capping basalt flow. Features at the contact indicate the paleosol soil was wet when the capping flow was emplaced, that lava-sediment mixing occurred, and that both the soil and the capping lava were hydrothermally altered. The soil was hydrothermally enriched in Fe, Mn, and Si. The SWa-1 sample was collected from within the altered zone, suggesting it formed through alteration of paleosol clays by addition of Fe. Similar environments are widespread in the CRB, particularly at the plateau margins, suggesting that altered clays may occur frequently at lava-sediment contacts. Such environments are likely to occur wherever basalt flows are emplaced under warm, wet conditions promoting weathering—such as Mars >3.5 Ga before the present, when clay minerals were forming at its surface. This information has important implications for the use of clay compositions to inform clay formation environments on Mars.

In deep and cold subduction such as that experienced by the UHP Units of the Western Alps, carbon dissolution is a relevant mechanism for carbon transfer from the slab into the mantle. The UHP impure Cal-Dol-marbles from the Dora-Maira Massif are studied to investigate the poorly known evolution of dolomite during deep subduction. Dolomite shows four stages of growth, from pre-Alpine to early-retrograde Alpine, coupled with chemical variations and distinct included mineral assemblages. To explain the evidence for growth and partial reabsorption of dolomite through HP prograde, UHP peak, and UHP early-retrograde Alpine metamorphism, a chemically simple marble (Cal, Dol, Di, Fo, and retrograde Atg, Tr, Mg-Chl) has been studied in detail. Microstructural relationships, coupled with mineral chemistry, indicate the growth of the assemblage dolomite+diopside+forsterite±aragonite during HP prograde, UHP peak, and UHP early-retrograde evolution. Mixed-volatile P-T projection modeled in the simple CaO-(FeO)-MgO-SiO 2 -H 2 O-CO 2 system and T-P-X CO 2 petrogenetic grids and pseudosections predict the prograde (1.7 GPa, 560 °C) growth of dolomite in equilibrium with diopside and forsterite through the breakdown of antigorite+aragonite. In a H 2 O-CO 2 -saturated system, the subsequent HP-UHP evolution is predicted in the Di+Fo+Dol+Arg stability field in equilibrium with a dominantly aqueous COH fluid [0.0003 < X CO2 < 0.0008], whose composition is internally buffered by the equilibrium assemblage. Thermodynamic modeling indicates that neither the consumption nor the growth of new dolomite generations at UHP conditions can have been induced by metamorphic reactions. The abundant primary H 2 O+Cal+Dol+Cl-rich Tr+Cl-rich Tlc±chloride fluid inclusions present in UHP Cpx indicate that a dominantly aqueous, saline (salinity >26.3 wt% of NaCl eq ) COH fluid, containing Ca, Mg, and Si as dissolved cations was present during the growth of the UHP assemblage Dol+Cpx+Ol+Arg. The complex zoning of dolomite is therefore interpreted as due to protracted episodes of dissolution and precipitation in saline aqueous fluids at HP/UHP conditions. Kinetics of dolomite dissolution in aqueous fluids is poorly known, and experimental and thermodynamic data under HP conditions are still lacking. Data on calcite indicate that dissolution at HP is enhanced by a prograde increase in both P and T , by high salinity in aqueous fluids, and/or low-pH conditions. In the studied marble, the P-T path and the occurrence of free high-saline fluids represent favorable conditions: (1) for the inferred dissolution-precipitation processes of the stable dolomite in a closed system, and (2) for possible migration of the dissolved carbonate, if the system would have been open during subduction.

Hydrothermal experiments were conducted with fluid- and apatite-saturated, high-silica rhyolitic melts at ca. 700–1000 °C and 50–200 MPa to determine the distribution of H 2 O/OH, Cl, and F between melt, apatite, aqueous vapor, brine, or vapor plus brine. Seed grains of fluorapatite (1–3 µm diameter) were added to starting charges to serve as apatite nucleation sites. CaHPO 4 and Ca(OH) 2 were used to stimulate apatite crystallization, and temperature was cycled daily, ±10 to ±15 °C, to promote growth of relatively equant apatite crystals large enough for electron probe microanalysis (EPMA). The experiments were conducted with gold capsules and run in cold-seal pressure vessels on a hydrothermal line and an internally heated gas pressure vessel for durations of 165 to 1149 h. The run-product glasses were analyzed by EPMA and Fourier transform infrared spectroscopy, apatites by EPMA, and most fluid phases by chloridometer; Cl contents of fluids were also estimated by mass-balance calculations. The fluids contained 0.3–39 wt% Cl at run conditions. Most experiments were conducted at 50 MPa, and these glasses contain 0.02–0.42 wt% Cl, 1.8–3.1 wt% H 2 O, and 0.01–0.19 wt% F. The molar Al 2 O 3 /(CaO+Na 2 O+K 2 O) (=A/CNK) and molar Na 2 O/(Na 2 O+K 2 O)(=N/NK) ratios of the 50 MPa glasses range from 0.88 to 1.04 and 0.48 to 0.68, respectively, and straddle the A/CNK and N/NK of the starting glass (0.99 and 0.59, respectively). The measured wt% Cl and F in the 50 MPa apatites range from 0.14 to 3.8 ( XClAp $X_{{\text{Cl}}}^{{\text{Ap}}}$ of 0.02 to 0.56) and 0.32 to 2.4 ( XFAp $X_{\text{F}}^{{\text{Ap}}}$ of 0.08 to 0.63), respectively. Stoichiometrically constrained XOHAp $X_{{\text{OH}}}^{{\text{Ap}}}$ ranges from 0.14 to 0.7. Partition and exchange coefficients were determined for OH, Cl, and F distribution between apatite and melt±fluids. The distribution of these volatile components varies with pressure and melt and apatite compositions. The exchange of F and Cl between apatite and melt, for example, fluctuates with the Si, P, Mg, Na, Ce, Fe, and S±Ca contents of the apatite and with the molar A/CNK and N/NK ratios of the melts. Water and hydroxyl exchange between experimental apatite and melt was also investigated. It is determined empirically that the: XH2Omelt/XClmelt $X_{{{\text{H}}_{\text{2}}}{\text{O}}}^{{\text{melt}}}{\text{/}}X_{{\text{Cl}}}^{{\text{melt}}}$ = [(−19.66) + (39.13) XOHAp/XClAp $ X_{{\text{OH}}}^{{\text{Ap}}}{\text{/}}X_{{\text{Cl}}}^{{\text{Ap}}} $ ] for felsic melts at 50 to 200 MPa, having molar A/CNK ratios between 0.88 and 1.1, N/NK ratios >0.55, and containing ca. 2–6 wt% H 2 O. The apatites are characterized by per formula unit (6 > Si/Mg > 0.3). We test this relationship by comparing H 2 O contents measured in melt inclusions from Augustine volcano, Alaska, with calculated H2O concentrations of melts based on compositions of apatites from 9 samples from 7 of its felsic eruptive units. The results for both approaches are consistent within precision for 6 of the samples. The empirical volatile exchange relationships determined for melt-apatite, melt-fluid, and apatite-fluid pairs are applicable to various magmatic systems. One implication of this study is that the H 2 O concentrations of felsic melts may be calculated from apatite compositions for volcanic systems involving equilibrium between these phases at 50 to 200 MPa, if estimates for the Cl contents of the melts are available. This approach, however, will require additional experimentation and testing. The compositions of igneous apatites could also provide fundamental constraints on the concentrations of H 2 O and other volatiles in mineralizing plutonic systems for which melt inclusions are small, rare, and/or crystallized. Magmatic apatites may also support assessment of H 2 O concentrations in melts derived from melt inclusion compositions.

Apatite is an excellent tracer of petrogenetic processes as it can incorporate a large range of elements that are sensitive to melt evolution (LREE-MREE, Sr, Pb, Mn, halogens, Nd isotopes). Recent advances in the understanding of trace element concentrations and isotope ratios in apatite provide a novel tool to investigate magmatic petrogenesis and sediment provenance. Recent experimental work has better characterized trace element partition coefficients for apatite, which are sensitive to changes in magma composition (e.g., SiO 2 and the aluminum saturation index value). The chemistry of apatites from granitoids has been suggested to reflect the composition of the host magma and yield information about petrogenetic processes that are invisible at the whole-rock scale (mixing, in situ crystal fractionation, metasomatism). Nd isotopes in apatite can now be analyzed by LA-MC-ICP-MS to constrain mantle and crustal contributions to the source(s) of the studied magma. These recent advances highlight exciting new horizons to understand igneous processes using accessory minerals. In this contribution, we use a compilation of recent data to show that apatite in the matrix and as inclusions within zircon and titanite is useful for providing insights into the nature and petrogenesis of the parental magma.Trace element modeling from in situ analyses of apatite and titanite can reliably estimate the original magma composition, using appropriate partition coefficients and careful imaging. This provides a new way to look at magmatic petrogenesis that have been overprinted by metamorphic processes. It also provides the rationale for new investigations of sedimentary provenance using detrital accessory minerals, and could provide a powerful new window into early Earth processes if applied to Archean or Hadean samples.

We have studied synthetic Fe-substituted hydroxyapatite Ca 5-x Fe x (PO 4 ) 3 OH and the corresponding deuterated samples with varying Fe concentrations x (0 ≤ x ≤ 0.3) by Raman and IR spectroscopy at room temperature. In the IR spectra, substitution of deuterons for protons affects the OH internal mode in a way consistent with the mass difference of the substituting ions, as well as a mode attributed to vibrations of the Ca 3 -(OH) unit. In the Raman spectra, the frequency of all modes is not noticeably affected by the Fe substitution. Raman bands show increased width and substantial reduction in intensity with increasing amount of Fe, presumably related to disorder introduced by the substitution. We find that the disorder is smaller in the hydroxyapatites compared to the deuterated ones.

Geophysical and geochemical observations point to the presence of a light element in the lunar core, but the exact abundance and type of light element are poorly constrained. Accurate constraints on lunar core composition are vital for models of lunar core dynamo onset and demise, core formation conditions (e.g., depth of the lunar magma ocean or LMO) and therefore formation conditions, as well as the volatile inventory of the Moon. A wide range of previous studies considered S as the dominant light element in the lunar core. Here, we present new constraints on the composition of the lunar core, using mass-balance calculations, combined with previously published models that predict the metal–silicate partitioning behavior of C, S, Ni, and recently proposed new bulk silicate Moon (BSM) abundances of S and C. We also use the bulk Moon abundance of C and S to assess the extent of their devolatilization. We observe that the Ni content of the lunar core becomes unrealistically high if shallow (<3 GPa) LMO scenarios are assumed, and therefore only deeper (>3 GPa) LMO scenarios are considered for S and C. The moderately siderophile metal–silicate partitioning behavior of S during lunar core formation, combined with the low BSM abundance of S, yields only <0.16 wt% S in the core, virtually independent of the pressure ( P ) and temperature ( T ) conditions during core formation. Instead, our analysis suggests that C is the dominant light element in the lunar core. The siderophile behavior of C during lunar core formation results in a core C content of ~0.6–4.8 wt%, with the exact amount depending on the core formation conditions. A C-rich lunar core could explain (1) the existence of a present-day molten outer core, (2) the estimated density of the lunar outer core, and (3) the existence of an early lunar core dynamo driven by compositional buoyancy due to core crystallization. Finally, our calculations suggest the C content of the bulk Moon is close to its estimated abundance in the bulk silicate Earth (BSE), suggesting more limited volatile loss during the Moon-forming event than previously thought.

Phosphorus-rich olivine (P 2 O 5 > 1 wt%) is a mineral that has been reported only in a few terrestrial and extraterrestrial occurrences. Previous investigations suggest that P-rich olivine mainly forms through rapid crystallization from high-temperature P-rich melts. Here, we report a new occurrence of P-rich olivine in an ungrouped carbonaceous chondrite Dar al Gani (DaG) 978. The P-rich olivine in DaG 978 occurs as lath-shaped grains surrounding low-Ca pyroxene and olivine grains. The lath-shaped olivine shows a large variation in P 2 O 5 (0–5.5 wt%). The P-rich olivine grains occur in a chondrule fragment and is closely associated with chlorapatite, merrillite, FeNi metal, and troilite.Tiny Cr-rich hercynite is present as inclusions within the P-rich olivine. The lath-shaped texture and the association with Cr-rich hercynite indicates that the P-rich olivine in DaG 978 formed by replacing low-Ca pyroxene precursor by a P-rich fluid during a thermal event, rather than by crystallization from a high-temperature melt. The large variation of P 2 O 5 within olivine grains on micrometer-scale indicates a disequilibrium formation process of the P-rich olivine. The occurrence of P-rich olivine in DaG 978 reveals a new formation mechanism of P-rich olivine.

Minerals containing cobalt as an essential element display systematic trends in their diversity and distribution. We employ data for 66 approved Co mineral species (as tabulated by the official mineral list of the International Mineralogical Association, http://rruff.info/ima, as of 1 March 2016), representing 3554 mineral species-locality pairs (www.mindat.org and other sources, as of 1 March 2016). We find that cobalt-containing mineral species, for which 20% are known at only one locality and more than half are known from five or fewer localities, conform to a Large Number of Rare Events (LNRE) distribution. Our model predicts that at least 81 Co minerals exist in Earth’s crust today, indicating that at least 15 species have yet to be discovered—a minimum estimate because it assumes that new minerals will be found only using the same methods as in the past. Numerous additional cobalt minerals likely await discovery using micro-analytical methods. Primary Co minerals include 26 species, most of which are hydrothermally deposited chalcogenides. We identify 33 additional plausible as yet undiscovered primary cobalt chalcogenide minerals, including 28 phases with spinel, nickeline, pyrite, and marcasite structural topologies. All 40 secondary cobalt minerals are oxides, and 37 of these phases also incorporate hydrogen. We tabulate an additional 117 plausible secondary Co minerals that are related compositionally and/or structurally to known species. New cobalt minerals are likely to be discovered in specimens collected at the 10 most prolific Co localities, all of which are mining districts with hydrothermal Co mineralization and hosting at least 10 different primary and secondary Co species.

ABSTRACT High-temperature Fourier transform infrared (HT-FTIR) spectroscopy was used to characterize the deprotonation process of synthetic potassic-ferro-richterite of composition A (K 0.90 Na 0.07 ) B (Ca 0.54 Na 1.46 )cFe4.222+Fe0.783+TSi8O22WOH1.70O0.302−$ ^{\rm{c}}{\left( {{\rm{Fe}}_{4.22}^{2 + }{\rm{Fe}}_{0.78}^{3 + }} \right)^{\rm{T}}}{\rm{S}}{{\rm{i}}_8}{{\rm{O}}_{22}}^{\rm{W}}\left( {{\rm{O}}{{\rm{H}}_{1.70}}{\rm{O}}_{0.30}^{2 - }} \right)$. Unpolarized single-crystal spectra were collected both in situ and on quenched samples, and heating experiments were conducted in air, at a rate of 10 °C/min. The room-T spectrum measured before annealing shows a main band at 3678 cm -1 and a minor band at 3622 cm -1 ; these are assigned to local configurations involving Fe 2+ at M (1) M (1) M (3) and facing a filled and an empty alkali-site, respectively. At 400 °C, a new band grows at 3656 cm -1 ; this is the most intense feature in the pattern at 450 °C. At T ≥ 500 °C, all peaks decrease drastically in intensity, and finally disappear at T > 600 °C. The total absorbance measured in situ increases significantly in the 25 < T < 450 °C range, although the spectra collected on quenched samples show no OH loss in the same T range. This feature is consistent with an increase of the absorption coefficient (ε) with T, the reason for which is still unclear. However, this feature has significant implications for the quantitative use of FTIR data in HT experiments. Examination of the relevant OH-stretching bands shows that iron oxidation occurs preferentially at the M (1,3) sites associated with occupied A sites. The deprotonation temperature indicated by FTIR for single-crystals is around 100 °C higher that that obtained by HT-X-ray diffraction (XRD) on single crystal by Oberti et al. (2016), whereas that obtained by HT-XRD on powders is intermediate. This unexpected observation can be explained by considering that: (1) the iron oxidation process, which is coupled to deprotonation and is probed by XRD, occurs preferentially at the crystal surface where it is triggered by the availability of atmospheric oxygen; (2) the proton diffusion, probed by FTIR, is slower that the electron diffusion probed by XRD; thus, the temperature shift may be explained by a much longer escape path for H in single-crystals than in powders. These results suggest that possible effects due to crystals size should be carefully considered in HT experiments done on Fe-rich silicates.

Understanding the mechanism and kinetics of the replacement of carbonates by fluorite has applications in Earth sciences and engineering. Samples of Carrara marble were reacted with an ammonium fluoride (NH 4 F) solution for different reaction times and temperatures. The microstructure of the product phase (fluorite) was analyzed using SEM. The kinetics of replacement was monitored using Rietveld refinements of X-ray powder diffraction patterns of the products. After reaction, all samples preserved their size and external morphology (a pseudomorphic replacement). The grain boundaries of the original marble were preserved although each calcite grain was replaced by multiple fine crystals of fluorite creating inter-crystal porosity. The empirical activation energy E a (kJ/mol) of the replacement reaction was determined by both model-fitting and model-free methods. The isoconversional method yielded an empirical activation energy of 41 kJ/mol, and a statistical approach applied to the model-fitting method revealed that the replacement of Carrara marble by fluorite is better fitted to a diffusion-controlled process. These results suggest that the replacement reaction depends on the ion diffusion rate in the fluid phase through the newly formed porosity.

We report here new experimental kinetic data on the structural evolution of carbonaceous material (CM) to graphite during heating at various temperatures (1000 to 1450 °C) for various durations (10 min to 115 h) under a pressure of 1 GPa. Natural CMs extracted from sedimentary rocks in the Shimanto accretionary complex and the Hidaka metamorphic belt of Japan transformed in morphology and crystallinity with increasing temperature and annealing duration to become fully ordered graphite ( d 002 spacing ~3.36 Å). Transmission electron microscopy showed that both samples have undergone microstructural evolution from amorphous carbon to platy graphitic carbon. These changes match the evolution of the samples’ X-ray diffraction (XRD) patterns and micro-Raman spectra. The time–temperature relations of crystal parameters obtained by XRD and micro-Raman spectroscopy demonstrated a sigmoidal transformation curve from an amorphous to a graphitic structure, suggesting complexity of these successive and/or parallel chemical reactions are responsible for graphitization. To assess these complex chemical processes, we adopted three different approaches for formulating the graphitization kinetics using a power rate model, a Johnson-Mehl-Avrami (JMA) model and a superposition method. Irrespective of the models employed, the effective activation energies were estimated to lie between 259 and 339 kJ/mol, which are much lower than those reported previously for graphitization. Summarizing the previous studies and our results between 0.1 and 1000 MPa, we found that the effective activation energies systematically decrease as a function of pressure. Based on the experimental results in this study, the sigmoid functions obtained from the time–temperature relations can be extrapolated to low-temperature conditions at 1 GPa. Our kinetic model using unit-cell height c predicts that CM undergoing metamorphism for about 1 m.y. will begin to crystallize at ~410 °C, and will transform to fully ordered graphite at over ~520 °C. Thus, natural graphitization undergoes a much faster transformation than reported in previous studies at 1 atm and could be explored in laboratory experiments using natural precursor materials under pressure conditions and time spans that reflect natural conditions in the Earth’s crust.

The complex asymmetric stretch ( ν 3 ) region infrared (IR) spectrum of synthetic sodium- and carbonate-bearing hydroxylapatites (CHAP) has been interpreted using overlapped Gaussian distributions for individual carbonate ion species. There is now good agreement for the distribution of carbonate ions between phosphate (type B) and c -axis channel (type A) positions using three independent methods: X-ray structure site occupancies, out-of-plane bend ( ν 2 ) band areas, and asymmetric stretch ( ν 3 ) band areas; B/A ratios for a well-crystallized CHAP sample being 0.77, 0.78, and 0.75, respectively. The reported dominance of type B carbonate ions in bone mineral and dental enamel is attributed to the anomalous shift of type A band frequencies into the spectral region of type B, resulting from the substitution of Ca 2+ by Na + in the nearest-neighbor cation shell of the channel carbonate ions. The infrared spectra show that the hydrogencarbonate (bicarbonate) ion in apatite crystals is a channel species, as are its room-temperature decomposition products, type A carbonate and labile (type L) carbonate. The research suggests that bone mineral crystals may actively communicate with body fluids through the apatite channel, pointing to a possible role for the apatite channel in mediating acid-base reactions in the body.

Garnet is a common and important mineral in metamorphic systems, but the mechanisms by which it incorporates Ti—one of the major elements in the crust—are not well constrained. This study draws upon garnets synthesized at a range of temperatures and pressures to understand Ti solubility and the substitution mechanisms that govern its incorporation into garnet at eclogite and granulite facies conditions. Garnets from these synthesis experiments can incorporate up to several wt% TiO 2 . Comparison of Ti content with deficits in Al and Si in garnet indicates that Ti is incorporated by at least two substitution mechanisms ( VI Ti 4+ + VI M 2+ ↔ 2 VI Al 3+ , and VI Ti 4+ + IV Al 3+ ↔ VI Al 3+ + IV Si 4+ ). Increasing Ti solubility is correlated with increasing Ca and Fe/Mg ratios in garnet, clinopyroxene and melt. The complexity of the substitution mechanisms involved in Ti solubility in garnet makes practical Ti-in- garnet thermobarometry infeasible at present. However, a model fit to Ti partitioning between garnet and melt can be used to predict melt compositions in high-grade metamorphic systems. Additionally, the solubility and substitution mechanisms described here can help explain the presence of crystallographically aligned rutile needles in high-grade metamorphic systems.

Titanium can be incorporated either tetrahedrally ( IV Ti) or octahedrally ( VI Ti) in most silicate minerals. Ti K -edge X-ray absorption fine structure (XAFS) spectroscopy enables observation of Ti coordination in minerals and melts. In this study, XAFS is used to determine the coordination of Ti in synthetic and natural garnets. Garnets grown synthetically at eclogite- and granulite-facies conditions can contain several wt% TiO 2 , most of which is incorporated as VI Ti. This observation aligns with major element trends in these garnets. In natural garnets grown at lower temperatures and pressures, on the other hand, Ti is observed to occupy both the octahedral and tetrahedral sites in garnet—in some cases Ti is almost entirely fourfold coordinated. Combined with previous research (see Ackerson et al. 2017, this issue) on substitution mechanisms for VI Ti, the results of this study demonstrate that Ti is incorporated on two crystallographic sites in garnet by at least three primary substitution mechanisms. In both natural and synthetic garnets, there is a discernible increase in VI Ti content in garnet with increasing temperature and pressure, suggesting a significant role for these two parameters in determining Ti solubility. However, a continuous increase in VI Ti with increasing grossular content also suggests that the Ca content of the garnet plays a critical role.

Skiagite-Fe-majorite garnets were synthesized using a multianvil apparatus at 7.5–9.5 GPa and 1400–1600 K. Single-crystal X-ray diffraction at ambient conditions revealed that synthesized garnets contain 23 to 76% of an Fe-majorite component. We found that the substitution of Fe 2+ and Si 4+ for Fe 3+ in the octahedral site decreases the unit-cell volume of garnet at ambient conditions. Analysis of single-crystal X-ray diffraction data collected on compression up to 90 GPa of garnets with different compositions reveals that with increasing majorite component the bulk modulus increases from 159(1) to 172(1) GPa. Our results and literature data unambiguously demonstrate that the total iron content and the Fe 3+ /Fe 2+ ratio in (Mg, Fe)-majorites have a large influence on their elasticity. At pressures between 50 and 60 GPa we observed a significant deviation from a monotonic dependence of the molar volumes of skiagite-Fe-majorite garnet with pressure, and over a small pressure interval the volume dropped by about 3%. By combining results from single-crystal X-ray diffraction and high-pressure synchrotron Mössbauer source spectroscopy we demonstrate that these changes in the compressional behavior are associated with changes of the electronic state of Fe in the octahedral site.

Growth histories recorded in garnet grains in metasedimentary rocks from the Sanbagawa belt in Japan and the Mogok belt in Myanmar were analyzed using an effective combination of electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) data. Garnet in the Sanbagawa metapelite has inner and outer zones that formed in the eclogite and epidote-amphibolite facies stages, respectively. Based on EPMA element mapping, this garnet appears to have grown as a single crystal with a temporal break in growth between the inner and outer zones that occurred during exhumation. The EBSD data, however, document that the garnet grain is composed of four domains. The misorientation angles of crystallographic orientations between the domains are as large as 59°, and domain boundaries crosscut the growth zoning and the compositional boundary between the inner and outer zones. Sets of quartz grains included in the garnets on either side of the domain boundaries sometimes share the same crystallographic orientation with misorientation angles less than 4°. The garnet grains formed via a three-step process of prograde crystallization of polycrystalline garnet during the eclogite facies stage (inner zone) → resorption around garnet rims and along domain boundaries during exhumation → crystallization of the outer zone and in the domain boundaries during the prograde epidote-amphibolite facies stage. The garnet porphyroblasts in the Mogok pelitic gneisses, which formed during prograde metamorphism to the upper amphibolite-granulite facies (0.6–1.0 GPa/780–850 °C), are now separated into segments of various sizes by mosaic or symplectite aggregates of biotite, plagioclase, and quartz or monomineralic biotite veins. The segment texture formed at about 0.3–0.4 GPa/610–650 °C or lowergrade conditions. The EBSD analysis shows that most of the segments share the same crystallographic orientation with misorientation angles less than 4° and show no evidence of deformation and/or rotation processes after segmentation. These data suggest that the Mogok sample did not experience dynamic deformation of the garnet grains after the resorption and segmentation stage and may have been exhumed under static conditions from depths of 9–12 km.

We propose a new formula for the mineral nickelskutterudite, based on our observation that either (or both) Co or Fe 3+ are essential structure constituents. The crystal structure of nickelskutterudite, (Ni,Co,Fe)As 3 , cubic, Im3¯ , $Im\overline 3, $ Z = 8: a = 8.2653(6) Å, V = 564.65(7) Å 3 , has been refined to R 1 = 1.4% for 225 unique reflections I > 2σ( I ) collected on a Bruker X8 four-circle diffractometer equipped with fine-focus, sealed tube MoKα radiation and an APEX-II CCD detector. This is the first report of the crystal structure of nickelskutterudite. Nickelskutterudite, a member of the skutterudite group of isostructural minerals, adopts a distorted perovskite structure with notably tilted octahedra and an unoccupied to partially occupied icosahedral metal site. In the structure of nickelskutterudite, there is one metal ( B ) site occupied by Ni, Co, or Fe in octahedral coordination with six As atoms. Procrystal electron density analysis shows each As anion is bonded to two cations and two As anions, resulting in a four-membered ring of bonded As with edges 2.547 and 2.475 Å. The extreme tilting of B As 6 octahedra is likely a consequence of the As-As bonding. The nickelskutterudite structure differs from the ideal perovskite structure ( A 4 B 4 X 12 ) in that As 4 anion rings occupy three of the four icosahedral cages centered on the A sites. There are reported synthetic phases isomorphous with skutterudite with the other A site completely occupied by a cation ( AB 4 X 12 ). Electron microprobe analyses of nickelskutterudite gave an empirical chemical formula of (Ni 0.62 Co 0.28 Fe 0.12 ) Σ1.02 (As 2.95 S 0.05 ) Σ3.00 normalized to three anions. Pure NiAs 3 nickelskutterudite, natural or synthesized, has not been reported. In nature, nickelskutterudite is always observed with significant Co and Fe, reportedly because all non-bonded valence electrons must be spin-paired. This suggests that nickelskutterudite must contain Co 3+ and Fe 2+ , consistent with previous models since Ni 4+ cannot spin-pair its seven non-bonded electrons, Co 3+ and Fe 2+ , which can spin-pair all non-bonded electrons, are required to stabilize the structure. No anion deficiencies were found in the course of this study so, including the structurally necessary Co and Fe, the chemical formula of nickelskutterudite (currently given as NiAs 3–x by the IMA) should be considered (Ni,Co,Fe)As 3 .

Dolomite is one of the major mineral forms in which carbon is subducted into the Earth’s mantle. End-member CaMg(CO 3 ) 2 dolomite typically breaks down upon compression into two carbonates at 5–6 GPa in the temperature range of 800–1200 K (Shirasaka et al. 2002). However, high-pressure X-ray diffraction experiments have shown that dense high-pressure polymorphs of dolomite may be favored over single-cation carbonates (Santillán et al. 2003; Mao et al. 2011; Merlini et al. 2012). Here we compare calculated dolomite structures to experimentally observed phases. Using density functional theory interfaced with a genetic algorithm that predicts crystal structures (USPEX), a monoclinic phase with space group C 2/ c was found to have lower energy at pressures above 15 GPa than all previously reported dolomite structures. It is possible that this phase is not observed experimentally due to a large activation energy of transition from dolomite I, resulting in the observed second-order phase transition to a metastable dolomite II. Due to the complex energy landscape for candidate high-pressure dolomite structures, several structurally unique metastable polymorphs exist. We calculate the equation of state of a set of lowest-energy dolomite polymorphs with space groups P1¯ $P\bar 1$ , P 2/ c , and C 2/ c up to 80 GPa. Our results demonstrate a need for calculations and experiments on Fe-Mn bearing high-pressure carbonate phases to extend our understanding of Earth’s deep carbon cycle and test whether high-pressure polymorphs of double-cation carbonates represent the main reservoir for carbon storage within downwelling regions of Earth’s mantle.

Plastic deformation in chromite is not frequently reported in literature. We present a detailed microstructural analysis of this mineral from the massive chromitite of the Neoarchaean Sittampundi Complex, southern India. The study reveals intracrystalline plasticity is dominantly active in this mineral, and it produces distinctive features corresponding to at least two different microstructural regimes. The Regime 1 is deformation-related and it commenced with recovery of strained grains and formation of new grains, corresponding to subgrain rotation recrystallization. This was followed by nucleation of strain-free new grains in regions of high strain. The Regime 2 appears to be post-deformational and dominantly temperature-controlled, producing distinctive features of static annealing of already deformed grains. This regime corresponds to grain boundary migration recrystallization resulting coarsening of strain-free facetted, new grains at the expense of high-dislocation density subgrains. The resultant micro-features resemble closely what is known as “abnormal grain growth,” not yet documented for chromites. These coarse grains, in places, also feature accommodation of deformation by displaying very low-angle subgrain boundaries. The movement of high-angle grain boundaries in Regime 2 through precursor strained grains provided high diffusivity paths for the rapid exchange of components, producing compositional heterogeneity between grains dominated by deformation features and facetted new grains. These microstructural observations coupled with chemical heterogeneity provide new directions in interpreting the deformation mechanism of chromite and its annealing history at the post-deformation stage.

A new Ti-bearing bridgmanite-like phase with a threefold commensurate superstructure of the ideal MgSiO 3 -perovskite structure was observed in a [Mg 5/6 Al 1/6 ][Si 1/2 Ti 1/3 Al 1/6 ]O 3 crystal synthesized in the model system Mg 3 Al 2 Si 3 O 12 –MgTiO 3 at 20 GPa and 1600 °C. The compound was found to be orthorhombic, space group Pnma , with lattice parameters a = 14.767(3), b = 6.958(1), c = 4.812(1) Å, V = 494.4(2) Å 3 , which represents a 3 a × b × c superstructure of the typical Pnma perovskite structure. The structure was refined to R = 0.024 using 846 independent reflections. The superstructure mainly arises from the ordering of titanium in one of the octahedral positions. Crystal-chemical details of the different polyhedra in the superstructure are discussed in comparison to pure MgSiO 3 . This is the first documented superstructure of a bridgmanite phase, and Ti-rich bridgmanite in the lower mantle arising from local Tienrichments may exhibit different physical properties and elemental partitioning behavior from Ti-poor, peridotitic bridgmanite. The study also shows that large amounts of Ti can stabilize bridgmanite-like compounds at considerably lower pressure than lower mantle conditions.