Volume 100, Issue 10

In-situ U-Th dating is among the most challenging analytical techniques, requiring extreme sensitivity to quantify the low-abundance 230Th. Consequently, not all mineral phases are suitable for this technique. In this issue, Wu et al. have demonstrated that baddeleyite in basic rocks can provide meaningful U-Th ages when analyzed using secondary ionization mass spectrometry.

Spinel is a very important rock-forming mineral that is found in basalts from Earth, Mars, the Earth’s Moon, and basaltic meteorites. Spinel can be used as a sensitive indicator of petrologic and geochemical processes that occur in its host rock. This paper highlights the role of increasing f O₂ (from IW-1 to FMQ+2) in converting a >90% normal spinel to an ~25% magnetite (inverse) spinel, the trajectory of D V spinel/melt as it relates to the ratio of V 3+ /V 4+ in the melt, and the crystal chemical attributes of the spinel that control the intrinsic compatibility of both V 3+ and V 4+ . This work examines the nuances of the V partitioning and provides a crystal chemical basis for understanding Fe 3+ , Cr, and V substitution into the octahedral sites of spinel. Understanding this interplay is critical for using spinels as both indicators of planetary parentage and reconstructing the redox history of magmatic systems on the terrestrial planets. Three potential examples for this use are provided. In addition, this work helps explain the ubiquitous miscibility gap between spinels with changing ülvospinel contents.

Diffusion modeling in olivine is a useful tool to resolve the timescales of various magmatic processes. Practical olivine geospeedometry applications employ 1D chemical transects across sections that are randomly sampled from a given 3D crystal population, but the accuracy and precision with which timescales can be retrieved from this procedure are not well constrained. Here, we use numerical 3D diffusion models of Fe-Mg to evaluate and quantify the uncertainties associated with their 1D counterparts. The 3D diffusion models were built using both simple and realistic olivine morphologies, and incorporate diffusion anisotropy as well as different zoning styles. The 3D model crystals were sectioned along ideal or random planes, which were used to perform 1D models and timescale comparisons. Results show that the timescales retrieved from 1D profiles are highly inaccurate and can vary by factors of 0.1-25 if diffusion anisotropy is not taken into account. Even when anisotropy is corrected for, timescales can still vary between 0.2-10 times the true 3D diffusion time due to crystal shape and sectioning effects. Simple grain selection procedures are described to reduce the misfit between calculated and actual diffusion times, and achieve an accuracy and precision of ~5% and ~15-25% relative, respectively. Provided that the grains are carefully selected, about 20 concentration profiles and associated 1D models suffice to achieve this accuracy.

Phosphoran olivine (1-7 wt% P 2 O 5 ) is a metastable phase known from fewer than a dozen meteoritic or terrestrial occurrences. We have thoroughly examined phosphoran olivine in the Springwater pallasite to characterize its distribution, textural relationships, and geochemistry. Phosphoran olivine is abundant in Springwater as randomly distributed millimeter-scale partial overgrowths on the P-free olivine crystals. Geochemical analyses support the substitution mechanism of P into the tetrahedral Si site with octahedral site vacancies for charge balance; observed trace element variations, on the other hand, are not related to P substitution. Element mapping reveals fine-scale oscillatory P zoning in unusual serrate patterns, indicating rapid crystal nucleation from a melt as proposed by Boesenberg and Hewins (2010) and a subsequently variable rate of crystallization. The timing of phosphoran olivine formation in Springwater is constrained to after the period of macroscopic olivine rounding but before the cooling of the metal matrix; because the phosphoran overgrowths overprint specific host grain boundary modifications, we suggest that the episode of extremely rapid cooling necessary to crystallize and preserve this rare phase may have been triggered by an additional impact to the parent body.

The diffusivity of phosphorus in San Carlos olivine (SCO) was measured at near-atmospheric pressure and 650-850 °C by in-diffusion of P from a surface powder source consisting of pre-reacted SCO and AlPO 4 . The experiments were conducted in evacuated silica-glass ampoules at oxygen fugacities fixed by solid-state buffers, generally Ni-NiO but also including two experiments buffered at wüstite-magnetite. Phosphorus uptake profiles were characterized by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). The temperature dependence of P diffusion in SCO conforms to the expected Arrhenius relation D = D 0 exp(-E a /RT), where the constants are as follows: log(D 0 , m 2 /s) = -10.06 ± 0.80 and E a = 229 ± 16 kJ/mol. These values characterize P as a relatively slow diffuser in olivine-slower by about an order of magnitude than Cr and Ca at basalt near-liquidus temperatures-but substantially faster than Si. With a view toward modeling P uptake during rapid growth of natural olivines, P diffusion was also characterized in dry MORB basalt melt over the temperature range 1250-1500 °C at 1 GPa, using traditional diffusion couples contained in graphite. Phosphorus diffusion profiles in the quenched and depressurized samples were quantified by laser-ablation ICP/MS. Phosphorus diffusion in basaltic melt is similar to that of Si, with log(D 0 , m 2 /s) = -6.30 ± 0.7 and E a = 147 ± 22 kJ/mol. The new data for P diffusion in olivine and basalt melt can be used to explore the acquisition of finescale zoning in natural olivine phenocrysts through kinetic models, as well as the survival of P zoning in olivine with time spent at elevated temperature. Models of growth entrapment of a P-enriched near-surface layer in the olivine lattice indicate that crystal growth at plausible sustained rates is indeed likely to result in regions of anomalously high P content in the resulting crystal. Phosphorus concentrations above the equilibrium partitioning value can also result from development of a diffusive boundary layer in the melt against a rapidly growing crystal, but this mechanism is ineffective at typical sustained olivine growth rates, requiring dendrite-forming growth speeds. Preservation of P zoning on the scale of a few micrometers apparently requires cooling within a few months of formation of the zoning.

Helium isotopes are used as a tracer for primitive reservoirs that have persisted in the Earth’s mantle. Basalts erupted at several intraplate oceanic islands, including Hawaii, Iceland, Galapagos, and Samoa, have hosted the highest 3 He/ 4 He ratios (>30 Ra, where Ra is atmospheric 3 He/ 4 He ratio) globally that are far in excess of the 3 He/ 4 He typical of the upper mantle sampled at mid-ocean ridges (8 Ra). These lavas have been suggested to be melts of a primitive, or possibly slightly depleted, mantle reservoir, i.e., either fertile or a depleted peridotite. Here we report evidence for geochemical enrichment in the high- 3 He/ 4 He mantle sampled by lavas with the highest 3 He/ 4 He from Hawaii, Samoa, and possibly Galapagos. The titanium concentrations in high- 3 He/ 4 He lavas from Samoa are too high to be explained by melts of a mantle peridotite, even at infinitesimally small degrees of melting, and the elevated Ti corresponds to elevated Pb-isotopic ratios. The highest 3 He/ 4 He lavas from Loihi, Hawaii, also have Ti concentrations that are too high to be melts of primitive mantle peridotite at the degrees of melt extraction proposed for this ocean island. Thus, Ti-rich material must have been added to the high- 3 He/ 4 He mantle reservoir, and this material is likely to be recycled mafic crust similar to MORBlike eclogite, which is consistent with the elevated Pb-isotopic ratios. We show that fractionation corrected, major element compositions of high- 3 He/ 4 He alkalic lavas can be satisfactorily modeled by melting and melt-rock interaction scenario in a fertile peridotite-MORB-eclogite hybrid system. Primitive peridotitic and recycled eclogitic reservoirs are suggested to be intimately associated in the deepest mantle and high- 3 He/ 4 He lavas from several localities may sample a mantle source that hosts a component of recycled oceanic crust.

Baddeleyite is a frequently found accessory mineral in silica undersaturated lavas. Because it is typically enriched in uranium, while having low initial lead, baddeleyite has long been a prime target for U-Pb geochronology of mafic rocks. The difficulties in retrieving small baddeleyite grains from volcanic samples and the lack of a detailed understanding of baddeleyite occurrence, however, have limited baddeleyite chronology largely to coarse-grained mafic intrusive rocks. Here, the development of U-Th in situ baddeleyite analysis using secondary ionization mass spectrometry (SIMS) is presented together with an assessment of baddeleyite occurrence in Quaternary silica-undersaturated lavas from Campi Flegrei (Naples, Italy). Samples studied comprise the pre- and post Campanian Ignimbrite (ca. 40 ka) lava domes of Cuma and Punta Marmolite, and Astroni and Accademia, respectively. The in situ sample preparation required initial identification of baddeleyite crystals from sawed and polished rock billets using scanning electron microscope (SEM) backscatter imaging and energy-dispersive X‑ray analysis. U-Th baddeleyite isochron ages for intra-caldera Astroni and Accademia lava domes are 5.01 +2.61 -2.55 ka (MSWD = 2.0; n = 17) and 4.36 +1.13 -1.12 ka (MSWD = 2.9; n = 24), respectively. The ages for Punta Marmolite (62.4 +3.9 -3.8 ka; MSWD = 1.2; n = 11) and Cuma (45.9 +3.6 -3.5 ka; MSWD = 2.2; n = 11) predate the Campanian Ignimbrite. Rapid baddeleyite crystallization at the time of emplacement is supported by petrologic observations that >50% of the baddeleyite crystals documented in this study occur either in vesicles or in vesicle-rich regions of the host lavas whose textures developed over timescales of a few years to a few decades based on microlite crystal size distribution (CSD) analysis. Radiometric U-Th baddeleyite ages are mostly in agreement with previously determined K-Ar eruption ages, except for the Punta Marmolite lava dome whose U-Th baddeleyite age predates the K-Ar age by ca. 15 ka. Baddeleyite thus records eruptive emplacement with little evidence for significant pre-eruptive crystal residence, and has potential as an eruption chronometer for Quaternary silica-undersaturated volcanic rocks.

We demonstrate in this study that natural green rust nanoparticles and their synthetic analogs can be complex polycrystalline phases composed of crystallites only a few nanometers in size, and they often include nanometer-sized regions of amorphous material. The natural green rusts are Zn-bearing pseudo-hexagonal platelets previously identified by us in the contaminated mine drainage of the former Ronneburg uranium mine in Germany (Johnson et al. 2014). We also identified Ni- and Cubearing green rust platelets in the sediment underlying the drainage outflow 20 m downstream, and, using dark-field transmission electron microscopy (DF-TEM), found that these natural green rusts are not usually structurally coherent single crystals. Synthetic sulfate green rusts are also polycrystalline and composed of crystallites of only a few nanometers in size, though different synthesis conditions produced different patterns of polycrystallinity. While pseudo-hexagonal platelets are the typical morphology of green rust, we also synthesized green rust nanorods, which have not previously been reported. In addition to the known characteristics of green rusts (including a very large aspect ratio and surface area to volume ratio, and the redox properties allowed by the structural mixture of Fe 2+ and Fe 3+ ), these polycrystalline platelets exhibit a high abundance of defect sites and likely a rough surface topography. The combination of these characteristics has important implications for the reactivity of green rust with biogeochemical interfaces in natural and anthropogenic systems.

Capsule pre-saturation has been traditionally employed to circumvent Fe loss from the charge to the container in petrological experiments. However, the method is time-consuming and fraught with theoretical and practical difficulties. An alternative method, based on the use of starting materials pre-enriched in Fe, is presented. Test experiments on two natural basalts, both non Fe-enriched and Fe-enriched with the addition of Fe oxides, have been carried out at 1 atm and 50 MPa (H 2 O-saturated), 1200 and 1250 °C, between NNO and NNO-1 and in Pt and Au 80 Pd 20 capsules. Glasses and capsules were analyzed by electron microprobe. Fe-concentrations in the capsule near the glass interface strongly depend on the capsule material, being 5-10 times less for Au 80 Pd 20 than for Pt. For non Fe-enriched compositions, Fe loss reaches -15% (Au 80 Pd 20 ) and -60% (Pt). Increasing the level of Fe-enrichment reduces Fe loss, the amount of Fe alloyed with the capsule being compensated by the amount of Fe added to the starting composition. FeO t concentrations in high-pressure glasses bracket the nominal FeO t of the two starting basalts, demonstrating that Fe alloying has been successfully counteracted. Combination of AuPd containers with Fe pre-enriched starting materials offers excellent perspectives to solve the Fe loss issue in high-pressure experiments on basaltic compositions.

We have measured Fe-Mg interdiffusion rates (D Fe-Mg ) in synthetic Mg-Al spinel and a natural (Mg,Fe) aluminous spinel from Sri Lanka (X Fe ~ 0.07) at atmospheric pressure over a range of different oxygen fugacities {log 10 (f O₂ [Pa]) = -14 to -10} and temperatures (750-900 °C). Diffusion couples made of single-crystal spinel and thin films of hercynitic composition (X Fe ~ 0.5) were used for the diffusion anneals. The experimentally induced diffusion profiles were analyzed by Rutherford backscattering Spectroscopy to retrieve true depth concentration profiles for Fe. These were fitted numerically by an explicit finite difference scheme that allows compositionally dependent interdiffusion processes to be modeled by relating self- and interdiffusion coefficients. Synthesis of data from the two diffusion couples indicate that: (1) D Fe-Mg depends on X Mg of spinel, with increasing diffusion rates with increasing X Mg . This behavior is opposite of that found in silicates. (2) Self-diffusion coefficients could not be determined from these experiments, but the results indicate that D Fe /D Mg > 100. (3) D Fe-Mg in Mg-spinel is independent of oxygen fugacity, whereas it depends strongly and nonlinearly on oxygen fugacity for the natural spinel. This observation indicates that the mechanisms of diffusion are different in the two kinds of spinel (Fe-bearing vs. Fe-free), which is also seen in the difference in activation energy obtained for these. Moreover, the nonlinear dependence on oxygen fugacity indicates that diffusion occurs by an interstitial mechanism at low-oxygen fugacities and by a vacancy mechanism at highoxygen fugacities in natural, Fe-bearing spinel. (4) Simple Arrhenius relations that describe the data within the range of experimental conditions are: Synthetic magnesium spinel: Q Fe-Mg = 219 ± 19 kJ/mol, log 10 D Fe-Mg = -7.76 ± 0.90 [m 2 /s]. Natural Fe-bearing spinel for log 10 (f O₂ [Pa]) = -12}: Q Fe-Mg = 139 ± 18 kJ/mol, log 10 D Fe-Mg = -12.33 ± 0.85 [m 2 /s]. A model based on point defect considerations that describes the temperature as well as oxygen fugacity dependence of D Fe-Mg in Fe-bearing spinel is: D [m 2 /s] = D v [m 2 /s]f O₂ [Pa] m exp{-Q v [J/mol]/RT [K]} + D i [m 2 /s] f O₂ [Pa]- m exp(-Q i [J/mol]/RT [K]), with D v = 1.07 × 10 -9 ± 1.55 × 10 -9 m 2 /s, Qv = 131 ± 66 kJ/mol, D i = 1.03 × 10 -17 ± 7.32 × 10 -17 m 2 /s, Q v = 130 ± 80 kJ/mol and m = 0.34 ± 0.18. Poor coverage of T-f O₂ space by available experimental data results in large uncertainties in the fit parameters. As a result, these expressions are useful for understanding the diffusion behavior in spinels, but not for extrapolation and calculation of diffusion coefficients for cooling rate or other related calculations. Until the parameters can be better constrained through the availability of more data, we recommend that for such calculations, the parameters noted above for Fe-bearing spinels be used for compositions and f O₂ conditions that are close to those of the experiments. (5) D Fe-Mg in spinel is faster than D Fe-Mg in olivines, pyroxenes, and garnets at most conditions.

An unusual rare earth element (REE) mineralization occurs at a locality known as the “Rusty Gold” within the anorogenic 1.4 Ga Longs Peak-St. Vrain monzo- to syenogranite Silver Plume-type intrusion near Jamestown, Colorado (U.S.A.). Irregular-shaped centimeter- to decimeter-sized mineralized pods and veins consist of zoned mineral assemblages dominated by fluorbritholite-(Ce) in a gray-colored core up to 10 cm thick, with monazite-(Ce), fluorite, and minor quartz, uraninite, and sulfides. The core zone is surrounded by a black, typically millimeter-thick allanite-(Ce) rim, with minor monazite-(Ce) in the inner part of that rim. Bastnäsite-(Ce), törnebohmite-(Ce), and cerite-(Ce) appear in a thin intermediate zone between core and rim, often just a few hundreds of micrometers wide. Electron microprobe analyses show that the overall REE content increases from rim to core with a disproportionate increase of heavy REE (∑ HREE increases 10-fold from 0.2 to 2.1%) compared to light REE (∑ LREE increases twofold from 21.3 to 44.3%). The fluorbritholite-(Ce) contains minor U, Th, Fe, Mn, and Sr (total 0.10 apfu), with Al, Mg, Na, K, Ti, Pb, S, and Cl below instrument detection limits. Cerite-(Ce) is a minor constituent of the thin zone between the inner rim and the core. The cerite-(Ce) is Fe-rich with low Ca, and minor Al, Mg, and Mn, whereas törnebohmite-(Ce) is Al-rich and Ca-poor. Monazite-(Ce) and uraninite U-Th-Pb microprobe ages yield 1.420(25) and 1.442(8) Ga, respectively, confirming a co-genetic relationship with the host ca. 1.42(3) Ga Longs Peak-St. Vrain granite. We suggest the origin of the REE mineralization is a F-rich and lanthanide-rich, either late-magmatic hydrothermal fluid or residual melt, derived from the granite. This late-stage liquid, when becoming progressively enriched in REE as it crystallized, could explain the observed concentric mineralogical and geochemical zoning.

Laser-induced breakdown spectroscopy (LIBS) offers an efficient method for qualitative and semiquantitative analysis of light elements (Z < 10), including lithium. This relatively inexpensive analytical tool provides very rapid analysis with little sample damage, requiring minimal sample preparation. In principle, LIBS is a form of atomic emission spectroscopy, relying on characteristic spectra emitted from plasma generated by a high-energy laser pulse striking a sample (solid, liquid, or gas). In this study, LIBS mapping was applied to petrographically characterized samples of hydrothermally altered spodumene from the Neoarchaean Mt. Cattlin lithium pegmatite deposit. Spodumene (LiAlSi 2 O 6 ) is the ore mineral in this deposit, but lithium is distributed in variety of minerals including primary micas and tourmaline, as well as in the alteration mineralogy of spodumene. Mapping was carried out using a grid of analysis spots of 125 μm diameter, spaced at 200 μm intervals, on a sample surface cut by a diamond saw blade without further preparation. Results from mapping of lithium in these samples effectively discriminated between spodumene, its alteration mineralogy, and matrix silicate minerals of the matrix. However, quantification of LIBS results using lithium-doped borosilicate glasses as standards was limited due to issues with the sensitivity of matrix matching of standards and self-absorption effects at Li 2 O values greater than ~2 wt%, especially at values greater than ~6 wt%. The results of this study testify to the effectiveness of LIBS as a mapping tool for light elements, which may be used as a complement to other mapping techniques. Mapping of lithium in pegmatite minerals has important applications in exploration, evaluation, and beneficiation of lithium pegmatite ore bodies.

Niobium (Nb) K-edge and tantalum (Ta) L III -edge XANES spectra were acquired at the part-permillion concentration level in silicate glasses quenched from chondritic melts equilibrated at 5 GPa and under moderately to highly reducing conditions (IW-1, IW-4.5, IW-7.9). Standard materials have also been analyzed for Nb and Ta, and the data were used to construct the calibration curves of E 0 (threshold energy) vs. valence. Under moderately reducing conditions our results are consistent with niobium and tantalum being mainly pentavalent in the silicate melts as also suggested by previous studies. We do not exclude that at IW-1, a small fraction of Nb and Ta could be reduced, leading to a mean formal valence slightly lower than five. At IW-4.5, Ta is mainly in the form Ta 3+ , and at IW-7.9, Ta appears to be Ta 1+ , whereas Nb is divalent (Nb 2+ ). The possibility for Nb and Ta to be present in reduced forms has implications for the behavior of the two elements during the processes of differentiation on planetary bodies formed in the reduced parts of the early Solar System. Element partitioning is a function of size and valence, and our results show that high field strength elements could be reduced, which could change their chemical affinity. This may also be important for the Earth and Moon formation and early differentiation, as exemplified by the “Nb paradox.”

Elastic properties of leucite have been investigated using resonant ultrasound spectroscopy over a temperature range from 300 to 1400 K. According to these measurements, elastic moduli soften by ~50% at the Ia3d-I4 1 /acd ferroelastic transition temperature T c1 = 940 K relative to the value at 1400 K. A second softening is observed at T c2 = 920 K, corresponding to the structural change from the space group I4 1 /acd to I4 1 /a. These elastic anomalies are analyzed in a simple model under the assumption that the transitions observed at T c1 and T c2 can be approximated by a single pseudoproper ferroelastic transition. The two phase transitions are accompanied by a single peak in mechanical damping attributed to the high mobility of twin walls in the intermediate phase followed by pinning in the low-temperature phase. To determine whether twin walls in tetragonal leucite are polar, resonant piezoelectric spectroscopy and second harmonic generation measurements were performed, but no evidence of polarity was found.

Tests show that terrestrial mineral+liquid geothermobarometers are not well equipped for use on martian rocks, which tend to have much higher FeO and lower Al 2 O 3 . Here, we present new calibrations of thermometers and barometers using experimental data on martian samples from the literature. These new models recover P-T conditions with a greater accuracy compared to models calibrated using terrestrial compositions. We applied these new calibrations to primitive martian mantle-derived melts Yamato 980459 (Y98) and Northwest Africa (NWA) 6234 and several surface basalts (Gusev). Our new models yield similar P-T conditions for NWA and Y98 compositions of 1.4-1.7 GPa and 1500-1550 °C, which are close to estimates by most prior studies. Our models yield somewhat lower P estimates compared to Lee et al. (2009), apparently because our Si-activity model (from Beattie 1993) includes an Al 2 O 3 -correction (where lower Al 2 O 3 , as in martian samples, leads to lower P estimates). For Gusev basalt compositions, our new models yield P-T estimates of 1.0-1.3 GPa and 1340-1390 °C; furthermore, we also obtain P = 1.03 GPa and T = 1340 °C, for a Gusev composition from Monders et al. (2007), which comes very close to the Monders et al. (2007) estimate for multiple saturation, of 1.0 GPa and 1325 °C, derived from phase saturation relationships. Given the different ages of these meteorites, with Gusev at 3.65 Ga (Greeley et al. 2005) and Y98 at 4.3 Ga (Bouvier et al. 2005, 2008, 2009; Werner et al. 2014), their thermal contrasts may represent secular cooling of Mars. We estimate a mantle potential temperature difference of ~200 °C, with mantle potential temperatures of 1450 ±50 °C for Gusev and 1650 ±50 °C for Y98; this implies a cooling rate of 300 °C/Ga. This would appear to be a much more rapid rate of cooling compared to Earth, as may be expected by Mars’ higher surface/volume ratio.

An updated and expanded data set that consists of 214 plagioclase-liquid equilibrium pairs from 40 experimental studies in the literature is used to recalibrate the thermodynamic model for the plagioclase- liquid hygrometer of Lange et al. (2009); the updated model is applicable to metaluminous and alkaline magmas. The model is based on the crystal-liquid exchange reaction between the anorthite (CaAl 2 Si 2 O 8 ) and albite (NaAlSi 3 O 8 ) components, and all available volumetric and calorimetric data for the pure end-member components are used in the revised model. The activities of the crystalline plagioclase components are taken from Holland and Powell (1992). Of the 214 experiments, 107 are hydrous and 107 are anhydrous. Four criteria were applied for inclusion of experiments in the final data set: (1) crystallinities <30%; (2) pure-H 2 O fluid saturated; (3) compositional totals (including H 2 O component) of 97-101% for hydrous quenched glasses and 98.5-101 for anhydrous quenched glasses; and (4) melt viscosities ≤5.2 log 10 Pa·s. The final data set spans a wide range in liquid composition (45-80 wt% SiO 2 ; 1-10 wt% Na 2 O+K 2 O), plagioclase composition (An 17-95 ), temperature (750-1244 °C), pressure (0-350 MPa), and H 2 O content (0-8.3 wt%). The water solubility model of Zhang et al. (2007) was applied to all hydrous experiments. The standard error estimate on the hygrometer model is 0.35 wt% H 2 O, and all liquid compositions are fitted equally well. Application of the model as a thermometer recovers temperatures to within ±12°, on average. Tests of the hygrometer on anhydrous piston-cylinder experiments in the literature, not included in the regression, show that the model is accurate at all pressures where plagioclase is stable. Applications of the hygrometer are made to natural rhyolites (Bishop Tuff, Katmai, and TobaTuff) with reported H 2 O analyses in quartz-hosted melt inclusions from the literature; the results show agreement. Applications of the hygrometer/thermometer are additionally made to natural rhyolites from Iceland and Glass Mountain, California. The updated model can be downloaded either as a program in Excel format or as a MatLab script from the Data Repository.

To quantify the effects of hydration on anharmonicity of olivine thermodynamics, we have measured in situ Raman spectra of an extremely hydrous forsterite with 4500 ppm (wt) H 2 O at temperatures up to 1273 K. All the Raman modes in hydrous forsterite shift linearly to lower wavenumbers with increasing temperature. The calculated isobaric mode Grünesien parameters related to SiO 4 internal stretching and bending vibrations are much lower than lattice vibrations. Additionally, compared with anhydrous forsterite, except for the modes at 919, 858, and 227 cm -1 , water greatly reduces the isobaric mode Grüneisen parameters of the Raman modes in forsterite. Water also has a large effect on the anharmonic parameters related to lattice vibrations, whereas it has little effect on the anharmonic parameters related to SiO 4 internal stretching and bending vibrations. Those results have the implications to the variations of local structure with temperature and estimation of water effects on the thermodynamics of forsterite.

A structural derivative of quartz with the composition Mg 0.5 AlSiO 4 has been grown from glass and characterized using synchrotron X‑ray diffraction (XRD), transmission electron microscopy (TEM), and 29 Si nuclear magnetic resonance (NMR) spectroscopy. Rietveld analysis of the XRD data indicates that the framework of Mg 0.5 AlSiO 4 is isostructural with a-quartz, rather than b-quartz, as is consistent with previous theoretical modeling (Sternitzke and Müller 1991). Al and Si exhibit long-range disorder over the framework tetrahedral sites, indicated by the absence of the superlattice reflections corresponding to the doubling of c relative to that of quartz. Nevertheless, 29 Si NMR measurements show that Al and Si exhibit partial short-range order with an ordering degree of 56%. Electron diffraction reveals superlattice reflections indicative of doubled periodicities along the a-axes. Fourier electron density maps show that Mg occupies channel sites that each are bonded to six O atoms, in contrast to the tetrahedral coordination of Li in the β-quartz-type framework for β-eucryptite, LiAlSiO 4 . Furthermore, the concentrations of Mg in adjacent channels are different, resulting in framework distortions that generate the superstructures along a.

The 3.65 Å phase [MgSi(OH) 6 ] is likely to be formed by decomposition of the hydrous 10 Å phase [Mg 3 Si 4 O 10 (OH) 2 ·H 2 O] at pressures of 9-10 GPa. In this study, we use a combination of X-ray diffraction and first-principles simulations to constrain the equation of state and elasticity of the 3.65 Å phase. We find that the equation of state results for the 3.65 Å phase, from X-ray diffraction data are well represented by a third-order Birch-Murnaghan formulation, with K 0 = 83.0 (±1.0) GPa, K 0 ′ = 4.9 (±0.1), and V 0 = 194.52 (±0.02) Å 3 . Based on the first-principles simulations, the full single-crystal elastic constant tensor with monoclinic symmetry shows significant anisotropy with the compressional c 11 = 156.2 GPa, c 22 = 169.4 GPa, c 33 = 189.3 GPa, the shear components c 44 = 55.9 GPa, c 55 = 58.5 GPa, c 66 = 74.8 GPa, and c 46 = 1.6 GPa; the off-diagonal components c 12 = 38.0 GPa, c 13 = 26.5 GPa, c 23 = 22.9 GPa, c 15 = 1.5 GPa, c 25 = 1.5 GPa, and c 35 = -1.9 GPa at zero pressure. At depths corresponding to 270-330 km, the seismological X-discontinuity has been observed in certain regions. We find that the formation of 3.65 Å from layered hydrous magnesium silicates (LHMS) such as 10 Å angstrom phases occurs at around 9 GPa, i.e., coinciding with the seismic X-discontinuity. The LHMS phases have significant seismic anisotropy. Based on the full elastic constant tensor, although among the dense hydrous magnesium silicate (DHMS) phases, the 3.65 Å phase reveals considerably larger elastic anisotropy, it is significantly smaller than the LHMS phases. This change in seismic anisotropy in hydrous phases might be one of the plausible explanations for the seismic X-discontinuity.

The effect of Zn substitution on a series of clinopyroxenes along the join CaZnSi 2 O 6 -Zn 2 Si 2 O 6 was studied. The pyroxenes were synthesized at P = 4-5 GPa and T = 1000-1200 °C by a multi-anvil apparatus. SEM-EDS and XRD analysis showed complete solid solution; all of the samples have the C2/c space group. No miscibility gap between clino- and orthopyroxene nor phase transition to the P2 1 /c space group was found. Moreover, the cell volume of Ca-Zn pyroxenes decreases less than expected from the decrease of the average cation size for the substitution of Zn for Ca. The crystal structures of three synthetic pyroxenes of composition (Ca 0.5 Zn 0.5 )ZnSi 2 O 6 , (Ca 0.3 Zn 0.7 ) ZnSi 2 O 6 , and (Ca 0.2 Zn 0.8 )ZnSi 2 O 6 were refined by single-crystal X‑ray diffraction (R4s between 3 and 4.5%). It was observed that the Ca-Zn substitution occurs in the M2 polyhedron, with a sub-site splitting of Zn in a position at approximately 0.7 Å from Ca. In this position, Zn retains a highly distorted fourfold coordination; moreover, the tetrahedral chain configuration is little changed along the series, and the M1 polyhedral size increases with Zn substitution in M2. An implication of the present work is the interpretation of the partitioning of Zn between clinopyroxene and melt. The distribution coefficients of Zn and Co are quite different in rocks of the same composition, despite their very similar ionic radius, and the difference is related to the preference of Zn for the M2 site, where Zn may find a suitable atomic coordination.

27 Al magic-angle spinning nuclear magnetic resonance (MAS NMR) and 27 Al triple quantum (3Q) MAS NMR spectroscopy have been performed at 21.1 Tesla (T), as a direct probe of environment around Al in synthetic Ca-Tschermak’s clinopyroxene, CaAlAlSiO 6 , henceforward referred to as CaTs. For comparison, 27 Al MAS NMR of CaTs has also been performed at 14.1 T. The 27 Al 3QMAS NMR spectrum of CaTs has revealed various local environments around octahedral and tetrahedral Al, both ordered and disordered, symmetrical, and distorted. Rietveld refinement of powder X‑ray diffraction data has confirmed the high-temperature, long-range disordered C2/c structure for this sample. The 27 Al MAS NMR spectra of CaTs look broadly similar at 14.1 and 21.1 T. Both spectra exhibit two distinct peaks at the octahedral site and an irregularly shaped, broad tetrahedral site. The line widths are significantly broader at lower field and the 27 Al 3QMAS NMR spectrum at 21.1 T exhibits several additional peaks. At least three peaks were resolved at the octahedral site (in both the MAS spectrum along F2 and the 3Q spectrum along F1), whereas two peaks are clearly resolved at the tetrahedral site at 21.1 T. One tetrahedral peak, observed at both fields, is broad in F1 and narrow in F2, spread along the isotropic shift diagonal, indicating a highly disordered but relatively symmetric environment. The second peak, newly observed at 21.1 T, is narrow in F1 but very broad in F2, indicating an ordered but highly distorted environment. This peak has not been observed at lower magnetic field strengths. Octahedral peak assignments have been made according to the number of Al atoms in the six tetrahedral sites around M1, where an increasing number of NNN tetrahedral Al (2Al, 3Al, and 4Al) is correlated with displacement of the chemical shift to higher frequency. Assigned sites are similar to the local tetrahedral environments around M1 in ordered P2 1 /n or C2 structures, suggesting the presence of ordered domains in this otherwise disordered C2/c structure. To aid in the interpretation of these tetrahedral Al environments, density functional theory (DFT) calculations have been performed using two approximations of the CaTs crystal structure: fully ordered tetrahedral chains or fully disordered tetrahedral chains. These calculations suggest that the tetrahedral Al site is a sensitive indicator of orderdisorder in CaTs. Tetrahedral Al sites in ordered tetrahedral chains (Si-O-Al-O-Si…) are predicted to have only large C Q , whereas tetrahedral Al sites in disordered systems (Al-O-Al-O-Al…) are predicted to have only small C Q . Both environments appear to exist in the synthetic CaTs sample in this study. Cation order-disorder has implications for thermobarometry based on CaTs-containing pyroxenes. The discovery of the new highly distorted tetrahedral site at ultrahigh magnetic field suggests that highly distorted Al sites in silicate minerals may be NMR-invisible in 27 Al 3QMAS NMR spectra acquired at lower fields, and these will have been systematically overlooked. This underscores the necessity to collect 27 Al NMR spectra of silicates at the highest available magnetic field strength.

Six Fe-bearing mica samples with different Fe ordering, Fe 2+ /(Fe 2+ +Fe 3+ ) ratio, octahedral, and tetrahedral composition were studied. Four micas belong to the phlogopite-annite join (space group C2/m), two are Mg-rich annite and two are Fe-rich phlogopite, one is a tetra-ferriphlogopite (space group C2/m) and one is Li-rich siderophyllite (space group C2). Thus these samples had a different environment around the Fe cations and layer symmetry. These micas were characterized by chemical analyses, single-crystal X‑ray diffraction, X‑ray absorption spectroscopy, and magnetic measurements. In samples with Fe mostly in octahedral coordination, dominant magnetic interactions among Fe atoms are ferromagnetic, which reach a maximum at higher Fe 2+ /(Fe 2+ +Fe 3+ ) ratios. Samples with higher Fe 2+ /(Fe 2+ +Fe 3+ ) ratio are also characterized by higher values of the Curie-Weiss θ constant. Where Fe 2+ /(Fe 2+ +Fe 3+ ) ratios decrease, θ values also decrease. The Fe 3+ -rich phlogopite shows predominant Fe 3+ in tetrahedral coordination and shows anti-ferromagnetic interactions with a negative value of the Curie-Weiss θ constant (i.e., θ = -25 K). Fe ordering in octahedral trans- and in one of the two cis-sites accounts for a greater θ value in Li-rich siderophyllite when compared to other samples showing similar octahedral Fe content. Our data suggest that Fe 3+ cations and other non-ferromagnetic cations hinder long range magnetic ordering. This observation may produce for the different role of octahedral Fe magnetic interactions that can in principle develop along long Fe-rich octahedral chains, when compared to tetrahedral-octahedral interactions that are confined within the layer by the non-ferromagnetic cations of the interlayer. Spin glass behavior is indicated by the dependency of the temperature to produce maxima in the susceptibility curve. These maxima are related to the frequency of the applied AC magnetic field.

We measured the bulk modulus and phase stability of a natural Mn-dolomite, kutnohorite, to 19 GPa. At room temperature, kutnohorite is stable in the rhombohedral dolomite phase up to 19 GPa, with an isothermal bulk modulus of 85(6) GPa (Kʹ = 4). The compressibility of kutnohorite is found to match well with both single and double carbonate trends with respect to bulk modulus and unit-cell volume. The thermoelastic properties measured in this study show that the Mn dolomite end-member fits well with the systematic of all the rhombohedral carbonates, both calcite (single carbonate) and dolomite (double carbonate) type.

Spin transitions in (Mg,Fe)SiO 3 bridgmanite have important implications for the chemistry and dynamics of Earth’s lower mantle, but have been complex to characterize in experiments. We examine the spin state of Fe in highly Fe-enriched bridgmanite synthesized from enstatites with measured compositions (Mg 0.61 Fe 0.38 Ca 0.01 )SiO 3 and (Mg 0.25 Fe 0.74 Ca 0.01 )SiO 3 . Bridgmanite was synthesized at 78-88 GPa and 1800-2400 K and X‑ray emission spectra were measured on decompression to 1 bar (both compositions) and compression to 126 GPa [(Mg 0.61 Fe 0.38 Ca 0.01 )SiO 3 only] without additional laser heating. Observed spectra confirm that Fe in these bridgmanites is dominantly high spin in the lower mantle. However, the total spin moment begins to decrease at ~50 GPa in the 74% FeSiO 3 composition. These results support density functional theory predictions of a lower spin transition pressure in highly Fe-enriched bridgmanite and potentially explain the high solubility of FeSiO 3 in bridgmanite at pressures corresponding to Earth’s deep lower mantle.

This study investigates distinctive oriented, needle-shaped multiphase inclusions found within cores of garnets from felsic granulites of the Upper Member of the Bigelow Brook Formation and the Brimfield Schist (northeastern Connecticut, U.S.A.). The rocks crop out in the southern end of the Central Maine Terrane and thus are a part of the Acadian/Neoacadian Orogen. The typical mineral assemblage is garnet + sillimanite + K-feldspar + plagioclase + quartz + cordierite + biotite ± spinel. The sillimanite is commonly pseudomorphous after kyanite. The multiphase needle inclusions and associated exsolved needles of rutile and ilmenite are mostly oriented parallel to <111> of garnet. The multiphase needles contain various combinations of quartz, micas, chlorite, rutile, graphite, a siliceous compositionally variable phase (“Phase-CV”), Zn-spinel, apatite, zircon, and rare ilmenite. We hypothesize that they represent inclusions of fluid that adhered to exsolving Ti±Fe oxide needles (mostly rutile) or extended along zones of weakness in garnet. This requires that multiphase needle formation occurred in response to cooling and/or decompression. The needles ultimately decrepitated during retrogression. We note that micaceous needle-shaped multiphase inclusions are rarely described; the closest analogs of which we are aware are found in UHP garnets of the Erzgebirge (Perchuk 2008). The Brimfield Schist in this area underwent ultrahigh-temperature metamorphism (UHTM) of ~1000 °C at a minimum pressure of ~1 GPa (Ague et al. 2013). Here we provide new temperature estimates for the adjacent Upper Member of the Bigelow Brook Formation. Ternary feldspar reintegration using the activity model of Benisek et al. (2004) and Zr-in-rutile thermometry (Tomkins et al. 2007) give average temperatures of ~990 and ~1010 °C, respectively, at 1 GPa for this unit. Therefore, the recently discovered UHT zone in the Brimfield Schist of northeastern Connecticut extends to the east to include the Upper Member of the Bigelow Brook Formation. Consequently, the first confirmed regional UHT locality in the United States is larger than initially recognized, and is at least 25 km long and 5-10 km wide. The oriented, elongate multiphase inclusions and petrographically obvious oriented Ti±Fe oxide needles may be useful indicators of extreme temperature and/or pressure rocks in other field areas.

The solubility and solution behavior of F and Cl in peralkaline aluminosilicate compositions in the systems Na 2 O-Al 2 O 3 -SiO 2 and K 2 O-Al 2 O 3 -SiO 2 have been determined for glasses quenched from melts equilibrated at 1400 and 1600 °C in the 1.0-2.5 GPa pressure range. With Al/(Al+Si) increasing from 0 to 0.33 in sodium aluminosilicate melts, F solubility (saturation concentration) increases from 3.3 to 7.4 mol%, whereas Cl solubility decreases from 5.7 to 2.5 mol%. There is no difference in F solubility in sodium or potassium aluminosilicate melts. However, the Cl solubility in potassic aluminosilicate melts is 40-60% lower than in sodic melts with the same Al/(Al+Si) and Na or K mole fraction. Fluorine depolymerizes the silicate melt structure and forms Si-F, Al-F, and Na-F complexes, the proportion of which depends on the melt Al/(Al+Si) ratio. Dissolution of Cl results in a small degree of depolymerization of Al-free silicate melt, whereas Cl has a polymerizing effect in aluminosilicate melts. In both cases, formation of Na-Cl complexes appears to be the driving solution mechanism. The differences in F and Cl solution mechanisms result in contrasting depolymerizing effects that become more pronounced with increasing degree of magmatic differentiation. Through such modifications of melt structure, F and Cl have significant effects on magma properties such as viscosity, compressibility, and element partitioning.

The solubility of H 2 O- and CO 2 -bearing fluids in trachytic and trachybasaltic melts from erupted magmas of the Campi Flegrei Volcanic District has been investigated experimentally at 1100 and 1200 °C, respectively, and at 100, 200, 300, 400, and 500 MPa. The solubility of H 2 O in the investigated melts varies between 3.48 ± 0.07 wt% at 100 MPa to 10.76 ± 0.12 wt% at 500 MPa in trachytic melts and from 3.49 ± 0.07 wt% at 100 MPa to 9.10 ± 0.11 wt% at 500 MPa in trachybasaltic melts. The content of dissolved CO 2 in melts coexisting with the most CO 2 -rich fluid phase increases from 281 ± 24 ppm at 100 MPa to 2710 ± 99 ppm at 500 MPa in trachyte, and from 727 ± 102 ppm at 100 MPa to 3565 ± 111 ppm at 500 MPa in trachybasalt. Natural samples from the Campanian Ignimbrite eruption (trachyte) and from the Solchiaro eruption (trachybasalt) were collected around the city of Naples and on Procida Island. Deuterium/hydrogen (D/H) ratios were analyzed in natural pumices pre-heated at different temperatures to remove water adsorbed and/or imprinted by glass alteration processes. It has been determined that heating of the glass to 350 °C efficiently removes most of secondary water and the remaining concentrations represent primary magmatic water preserved in the erupted material. Hydrogen isotope composition (with δD values ranging between -70‰ and -110‰) and its correlation with bulk water content in selected pumice samples of the Campanian Ignimbrite eruption are consistent with isotopic fractionation between magmatic fluid and melt during degassing of erupting magma. Hence, the H 2 O and CO 2 contents in natural glasses from pumice samples are considered as minimum estimates on volatile concentrations in the melt just prior to the eruption or at the fragmentation event. The water contents in natural glasses vary from 0.83 ± 0.07 to 3.74 ± 0.06 wt% for trachytes from the Campanian Ignimbrite eruption and from 1.96 ± 0.06 to 3.47 ± 0.07 wt% for trachybasalts from the Solchiaro eruption. The CO 2 contents vary from 78 ± 120 ppm CO 2 to 1743 ± 274 ppm for trachytes from the Campanian Ignimbrite eruption and from 240 ± 293 to 1213 ± 250 ppm for trachybasalts from the Solchiaro eruption. A combination of natural and experimental data provides minimum pressure estimates for the storage and ascent conditions of magmas. The Campanian Ignimbrite magma could have been stored or ponded during its rising path at two different levels: a deeper one corresponding to depth of about 8 to 15 km and a shallower one at about 1 to 8 km. Trachybasalts from Solchiaro erupted from the deepest level of about 11 km with a storage or ponding level at around 2 to 8 km depth. Although an uncertainty of at least a kilometer has to be considered in estimating storage or ponding depths, these estimates point to significantly deeper magmatic sources for both eruptions as those considered previously.

In the pressure range in the Earth’s mantle where many basaltic magmas are generated (1 to 3 GPa) (Stolper et al. 1981), increases in the coordination numbers of the network-forming cations in aluminosilicate melts have generally been considered to be minor, although effects on silicon and particularly on aluminum coordination in non-bridging oxygen-rich glasses from the higher, 5 to 12 GPa range, are now well known. Most high-precision measurements of network cation coordination in such samples have been made by spectroscopy (notably 27 Al and 29 Si NMR) on glasses quenched from high-temperature, high-pressure melts synthesized in solid-media apparatuses and decompressed to room temperature and 1 bar pressure. There are several effects that could lead to the underestimation of the extent of actual structural (and density) changes in high-pressure/temperature melts from such data. For non-bridging oxygen-rich sodium and calcium aluminosilicate compositions in the 1 to 3 GPa range, we show here that glasses annealed near to their glass transition temperatures systematically record higher recovered increases in aluminum coordination and in density than samples quenched from high-temperature melts. In the piston-cylinder apparatus used, rates of cooling through the glass transition are measured as very similar for both higher and lower initial temperatures, indicating that fictive temperature effects are not the likely explanation of these differences. Instead, transient decreases in melt pressure during thermal quenching, which may be especially large for high initial run temperatures, of as much as 0.5 to 1 GPa, may be responsible. As a result, the equilibrium proportion of high-coordinated Al in this pressure range may be 50 to 90% greater than previously estimated, reaching mean coordination numbers (e.g., 4.5) that are probably high enough to significantly affect melt properties. New data on jadeite (NaAlSi 2 O 6 ) glass confirm that aluminum coordination increase with pressure is inhibited in compositions low in non-bridging O atoms.

To constrain the crystallization of alkaline and volatile-rich lavas present in intraplate settings, we studied the petrological features and the geochemical composition of major, trace, and volatile elements of mineral and bulk-rock of two sodalite-bearing phonolites: (1) haüyne-plagioclase-bearing Si-K-rich phonolite from Laacher See (Germany) and (2) nosean-nepheline-bearing Si-poor phonolite from Saghro (Morocco). In haüyne-bearing phonolites (55-59 wt% SiO 2 , K > Na, Na+K/Al = 0.96-1.08), we found that the low silica and low sodium activity promoted the early crystallization of S-rich haüyne (13.7-13.9 wt% SO 3 , 0.4 wt% Cl) + S-rich apatite (0.7-0.9 wt% SO 3 ), titanite, and rare pyrrhotite followed by clinopyroxene-plagioclase-sanidine at relatively low pressure and temperature (P = 250 MPa and T = 850 °C) and oxidized condition (ΔNNO-NNO+1, where NNO is nickel-nickel oxide buffer). The crystallization of haüyne occurred at fluid-undersaturated conditions from a silicate melt with 6 wt% H 2 O, 0.17-0.23 wt% Cl, 0.11-0.4 wt% S, and 0.07-0.14 wt% F. Nosean-bearing phonolites from Saghro are silica-poor and peralkaline (52-54 wt% SiO 2 , Na > K, Na+K/Al = 1.2) and crystallized at higher P and T (300 MPa and 950 °C) and more reduced conditions (NNO) compared to haüyne-bearing phonolites. The incongruent reaction to form nosean requires high silica and Na 2 O activity. The mineral assemblage and composition suggest early crystallization of nepheline followed by nosean (7.8-8.8 wt% SO 3 ; 1-1.1 wt% Cl). The sequence of crystallization is: clinopyroxene + nepheline + S-poor apatite (<0.04 wt% SO 3 ) + pyrrhotite followed by nosean and titanite. Noseanbearing magmas are fluid-undersaturated with relatively low volatile content (4 wt% H 2 O, <0.25 wt% Cl, <0.056 wt% S, 0.08-0.1 wt% F), although Cl may have exsolved during ascent and formed a fluid phase (NaCl-bearing). Both haüyne- and nosean-bearing phonolites are last equilibrated at relatively low pressure and high temperature. Haüyne and nosean crystallized at oxidized and volatile-rich pre-eruptive conditions. They record the volatile concentrations at depth and may be used as oxybarometer. The incongruent reactions involved to form haüyne and nosean suggest that phonolitic magmas became more oxidized during crystallization. The initial volatile concentrations in basanite/nephelinite magmas, from partial melting of volatile-bearing K 2 O-rich mantle rock, should have been one important factor influencing the crystallization of haüyne-bearing Si-K-rich phonolite and nosean-bearing Si-poor phonolite in intracontinental setting.

The equation of state of aragonite was determined by in situ synchrotron X‑ray diffraction experiments on a hot-pressed polycrystalline specimen of aragonite within its stability field up to 5.8 GPa and 1173 K. As a complement to this experimental study, first-principle density functional theory calculations were performed up to 20 GPa at 0 K. Thermoelastic parameters for aragonite (CaCO 3 ) were derived by a least-squares fit of the experimental P-V-T data to the third-order Birch-Murnaghan equation of state (EOS), yielding the bulk modulus and its pressure and temperature derivatives K 0 = 65.24 ± 0.24 GPa, Kʹ 0 = 4.95 ± 0.12, (∂K T /∂ T ) P = -0.024 ± 0.002 GPa/K and volume thermal expansion α 300 = (6.1 ± 0.7) × 10 -5 K -1 . The analyses of the axial compressibility at ambient temperature show that the c-axis is much more compressible than the a- and b-axes. Based on first-principle calculations, the anisotropic compression behavior of aragonite structure is explained by the heterogeneous shortening of <Ca-O> and <C-O> bond lengths and the rotation of <O-C-O> angles along the a-, b-, and c-axes, whereas the unit-cell volume change of aragonite under compression is accommodated by comparable compression rate of the CaO 9 polyhedra and the voids in the crystal lattice. The results attained from this study provide important thermoelastic parameters for understanding the thermodynamic behavior and chemical reactions involving aragonite at subduction zone conditions.

A new Al-bearing hydrous Mg-silicate that we named as 23 Å phase was synthesized at 10 GPa and 1000 °C, while also coexisting with diaspore and pyrope in the following system: phase A [Mg 7 Si 2 O 8 (OH) 6 ] + Al 2 O 3 + H 2 O. The chemical composition of this new 23 Å phase is Mg 11 Al 2 Si 4 O 16 (OH) 12 , and it contains about 12.1 wt% water. Powder X‑ray diffraction and electron diffraction patterns show that this new 23 Å phase has a hexagonal structure, with a = 5.1972(2), c = 22.991(4) Å, and V = 537.8(2) Å 3 , and the possible space group is P6̅c2, P6 3 cm, or P6 3 /mcm. The calculated density is 2.761 g/cm 3 accordingly, which was determined by assuming that the formula unit per cell (Z) is 1. This crystal structure is quite unique among mantle minerals in having an extraordinarily long c axis. Several experiments revealed that its stability region is very similar to that of phase A. We further confirmed that this new 23 Å phase was stable in the chlorite composition at 10 GPa and 1000 °C. The present results indicate that this new 23 Å hydrous phase will form in an Al-bearing subducting slab, and transport water together with Al into the deep upper mantle or even into the upper part of the transition zone.

Single crystals of (Mg 0.89 Fe 0.11 ) 2 SiO 4 wadsleyite with dimensions up to ~1 mm were synthesized by solid-state recrystallization under high pressure. Synthesis experiments of the wadsleyite single crystals were performed at 16 GPa and 1870 K for 1-3 h using a Kawai-type multi-anvil apparatus. The wadsleyite crystals are virtually free of inclusions and cracks. Their chemical compositions are homogeneous with Fe/(Mg + Fe) of 0.112(2). Unpolarized infrared spectra indicate that the synthesized sample contains 0.15-0.30 wt% H 2 O. The method of synthesizing large, high-quality single crystals of wadsleyite will facilitate future measurements of physical properties including elasticity and elastic anisotropy, electrical and thermal conductivities, atomic diffusivity, and creep strength, which will improve models of the composition and dynamics of the mantle transition zone.

Decagonite is the second natural quasicrystal, after icosahedrite (Al 63 Cu 24 Fe 13 ), and the first to exhibit the crystallographically forbidden decagonal symmetry. It was found as rare fragments up to ~60 mm across in one of the grains (labeled number 126) of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The meteoritic grain contains evidence of a heterogeneous distribution of pressures and temperatures that occurred during impact shock, in which some portions of the meteorite reached at least 5 GPa and 1200 °C. Decagonite is associated with Al-bearing trevorite, diopside, forsterite, ahrensite, clinoenstatite, nepheline, coesite, pentlandite, Cu-bearing troilite, icosahedrite, khatyrkite, taenite, Al-bearing taenite, and steinhardtite. Given the exceedingly small size of decagonite, it was not possible to determine most of the physical properties for the mineral. A mean of seven electron microprobe analyses (obtained from three different fragments) gave the formula Al 70.2(3) Ni 24.5(4) Fe 5.3(2) , on the basis of 100 atoms. A combined TEM and single-crystal X‑ray diffraction study revealed the unmistakable signature of a decagonal quasicrystal: a pattern of sharp peaks arranged in straight lines with 10-fold symmetry together with periodic patterns taken perpendicular to the 10-fold direction. For quasicrystals, by definition, the structure is not reducible to a single three-dimensional unit cell, so neither cell parameters nor Z can be given. The likely space group is P10 5 /mmc, as is the case for synthetic Al 71 Ni 24 Fe 5 . The five strongest powder-diffraction lines [d in Å (I/I 0 )] are: 2.024 (100), 3.765 (50), 2.051 (45), 3.405 (40), 1.9799 (40). The new mineral has been approved by the IMA-NMNC Commission (IMA2015-017) and named decagonite for the 10-fold symmetry of its structure. The finding of a second natural quasicrystal informs the longstanding debate about the stability and robustness of quasicrystals among condensed matter physicists and demonstrates that mineralogy can continue to surprise us and have a strong impact on other disciplines.

Recent studies of mineral diversity and distribution lead to the prediction of >1563 mineral species on Earth today that have yet to be described-approximately one fourth of the 6394 estimated total mineralogical diversity. The distribution of these “missing” minerals is not uniform with respect to their essential chemical elements. Of 15 geochemically diverse elements (Al, B, C, Cr, Cu, Mg, Na, Ni, P, S, Si, Ta, Te, U, and V), we predict that approximately 25% of the minerals of Al, B, C, Cr, P, Si, and Ta remain to be described-a percentage similar to that predicted for all minerals. Almost 35% of the minerals of Na are predicted to be undiscovered, a situation resulting from more than 50% of Na minerals being white, poorly crystallized, and/or water soluble, and thus easily overlooked. In contrast, we predict that fewer than 20% of the minerals of Cu, Mg, Ni, S, Te, U, and V remain to be discovered. In addition to the economic value of most of these elements, their minerals tend to be brightly colored and/or well crystallized, and thus likely to draw attention and interest. These disparities in percentages of undiscovered minerals reflect not only natural processes, but also sociological factors in the search, discovery, and description of mineral species.

A unique symplectitic intergrowth of magnetite + rutile is hosted by ilmenite in the gabbro of the Xinjie Fe-Ti oxide-bearing, mafic-ultramafic layered intrusion. The crystallization of rutile in the symplectite is probably formed by oxidation of ilmenite-hematite solid solution (Ilm-Hem ss ). Segregation of Fe 3+ in the Ilm-Hem ss at the rutile-host interfaces triggered the crystallization of magnetite along the margin of the growing rutile, and shaped the vermicular morphology of the rutile. The crystallization of magnetite can also locally release Ti 4+ to enhance the progressive growth and subsequent nucleation of rutile in the symplectite. The growth of the symplectite ceased when the temperature decreased to that of the miscibility gap of Ilm-Hemss and Fe 3+ began to exsolve to form hematite lamellae in the ilmenite.