Volume 95, Issue 7

An aluminous gneissic rock associated with high-pressure mafic and felsic granulites in the Palghat- Cauvery Shear Zone of southern India provides a classic example of quartz-absent muscovite melting. The anatectic gneiss shows a conspicuous migmatitic structure defined by closely spaced centimeter to decimeter sized, corundum-bearing leucosomes developed in a weakly foliated mesosome of plagioclase (An 21 Ab 77 Or 2 ) and biotite (4.9 wt% TiO 2 , X Mg = 0.51-0.47). The boundaries between leucosome and mesosome domains are sharp, and no melanosome selvages are developed at the interface. Corundum occurs as euhedral crystals up to 2 cm in diameter, typically centered in the leucosome matrix of coarse-grained perthitic alkali feldspar (integrated composition: An2Ab35Or63), minor relict biotite (4.2-5.1 wt% TiO 2 , X Mg = 0.48-0.46) and plagioclase (An 21 Ab 78 Or 1 ). In some domains, the mesosomes also contain elongate clusters of similarly oriented smaller corundum plates that are intergrown with perthitic alkali feldspar, presumably replacing former kyanite blades. The textural and mineralogical characteristics and petrogenetic grid considerations indicate breakdown of muscovite through two successive dehydration-melting reactions: (1) formation of corundum+K-feldspar-clusters via the reaction muscovite+aluminosilicate → corundum+liquid at the sites of kyanite/sillimanite, and (2) development of corundum-bearing leucosomes through the reaction muscovite → corundum+K-feldspar+liquid, focused around the sites of nucleation and growth of peritectic corundum. P-T pseudosection modeling in the Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 - SiO 2 -H 2 O-TiO 2 system locates the onset and completion of the muscovite-melting reaction 2 in the steep narrow quadrivariant field Ms+Bt+Pl+Kfs+Crn+Liq, which extends from ~6 kbar, 720 °C to higher pressures. Biotite remained stable and was not involved in the melting reactions. Two-feldspar thermometry gives peak-temperatures of 800 ± 50 °C. Combined with P-T estimates for metapelitic granulites in the area, these P-T constraints appear to be consistent with a clockwise P-T evolution of the eastern Palghat Cauvery shear zone with peak P-T conditions not exceeding ca. 800 °C and 10-12 kbar. The timing of partial melting and HT-metamorphism is constrained at ~529 Ma by U-Pb spot dating of oscillatory zoned individual grains and overgrowths on detrital zircon cores included in peritectic corundum of leucosome domains. The zircon cores indicate a Paleoproterozoic (2.5-2.0 Ga) provenance of the sedimentary protolith

The wavelength of the white mica (illite and muscovite) Al-OH absorption band near 2200 nm in 1036 samples from the Belt Supergroup and associated metasedimentary rocks was determined with a portable visible and near infrared reflectance spectrometer. The Al-OH wavelength decreases from 2225 nm in sub-biotite-zone samples to 2194 nm in sillimanite-zone samples; this decrease corresponds with an increase in total Al content of white mica from ~2.0 to ~2.8 atoms per 11 O atoms. These observations indicate that: (1) the frequency of the Al-OH vibration is controlled by the aluminoceladonite exchange reaction [ IV Si+ VI (Mg,Fe 2+ ) = IV Al+ VI Al], and (2) the reaction proceeds toward more Al-rich composition with increasing metamorphic grade. In these circumstances, Al-OH wavelength provides an indirect monitor of compositional variation and metamorphic grade. Metamorphic grade in most of the study area is in the biotite zone or lower, yet Al-OH wavelengths in low-grade rocks define systematic patterns that correlate with depth of burial and later structural displacements. In higher grade areas, wavelengths generally decrease from the garnet isograd through the sillimanite zone; however, anomalies occur locally, and it is not clear whether these result from differences in bulk composition and mineral assemblage or whether they point to actual metamorphic or structural discontinuities. These findings indicate that reflectance spectroscopy can yield valuable information on metamorphic intensity in rocks containing white mica, particularly in low-grade sequences where conventional indicators of metamorphic grade are lacking. Furthermore, this information can be obtained with field-portable spectrometers and the potential exists to obtain comparable results from airborne and spaceborne imaging spectrometers

Synchrotron radiation X-ray fluorescence (SR-XRF) was used to characterize As speciation within natural fluid inclusions from three deposits with different hydrogeochemical and geological settings. The studied samples represent different compositions of Au-bearing fluids: typical orogenic Au deposit (low-salinity, ~6 mol% CO 2 ± CH 4 ; Brusson, Western Italian Alps); brines from a Proterozoic (Fe)- Cu-Au deposit (Starra, Queensland, Australia); and an As-rich magmatic fluid with a bulk composition similar to that typical of orogenic gold (Muiane pegmatite, Mozambique). Arsenic K-edge X-ray absorption spectra (XAS) were obtained from fluid inclusions at temperatures ranging from 25 to 200°C, and compared with spectra of aqueous As(III) and As(V) solutions and minerals. X-ray absorption near edge structure (XANES) data show that initially the fluid inclusions from all three regions contain some As in reduced form [As(III) at Brusson and Muiane; As-sulfide or possibly As(0) at Starra]. However, this reduced As is readily oxidized under the beam to As(V). Therefore, extended X-ray absorption fine structure (EXAFS) spectra for the As(III) aqueous complex could be collected only on the sample from the Muiane pegmatite containing large fluid inclusions with high As concentrations (>>1000 ppm). Analysis of these EXAFS data shows that As(OH) 3(aq) (coordination number of 3.0 ± 0.2 atoms, bond length of 1.76 ± 0.01 Å) is the dominant arsenic aqueous species in the Muiane fluid inclusions at 100 °C, in accordance with predictions based on studies conducted using autoclaves. The As(V) complex resulting from photooxidation in the Muiane inclusions was characterized at 200 °C; the As-O bond distance (1.711 ± 0.025 Å) corresponds to that found in the arsenate group in minerals, and to that measured for the (HAsO 4 )2- complex at room temperature (1.700 ± 0.023 Å). The extent of the XAS information that could be obtained for As in this study was limited by the rapid photooxidation that occurred in all inclusions, despite the relatively low photon flux density used (~4.4 × 10 6 photons/s/μm 2 ). Photosensitivity was not observed in autoclave experiments and is the result of a complex interaction between redox-sensitive complexes in solution and the products of water radiolysis generated by the beam. Even under such challenging experimental conditions, the information gathered provides some precious information about As chemistry in ore-forming fluids

New discoveries of kuksite, Pb 3 Zn 3 Te 6+ P 2 O 14 , from the Black Pine mine, Montana, and Blue Bell claims, California, have enabled a detailed crystal-chemical study of the mineral to be undertaken. Single-crystal X-ray structure refinements of the structure indicate that it is isostructural with dugganite, Pb 3 Zn 3 Te 6+ As 2 O 14 , and joëlbruggerite, Pb 3 Zn 3 (Sb 5+ ,Te 6+ )As 2 O 13 (OH,O). Kuksite from the Black Pine mine crystallizes in space group P321, with unit-cell dimensions a = 8.392(1), c = 5.204(1) Å, V = 317.39(8) Å 3 , and Z = 1 (R 1 = 2.91% for 588 reflections [F o > 4σF] and 3.27% for all 624 reflections), while Blue Bell kuksite has the unit cell a = 8.3942(5), c = 5.1847(4) Å, and V = 316.38(4) Å 3 (R1 = 3.33% for 443 reflections [F o > 4σF] and 3.73% for all 483 reflections). Chemical analyses indicate that solid-solution series exist between kuksite, dugganite, and joëlbruggerite. Raman spectroscopic and powder X-ray diffraction data are also presented for samples from both occurrences. The crystal structure of the chemically related species yafsoanite, (Ca,Pb) 3 Te 2 6+ Zn 3 O 12 , from the type locality (Delbe orebody, Kuranakh Au Deposit, Aldan Shield, Saha Republic, Russia), has been refined to R 1 = 2.41% for 135 reflections [F o > 4σF] and 3.68% for all 193 reflections. A garnet-type structure has been confirmed and significantly improves upon the results of an earlier structure determination

Chabazite-Mg, (Mg 0.67 K 0.52 Ca 0.48 Na 0.08 Sr 0.03 ) Σ1.78 [(Al 3.16 Si 8.89 ) Σ12.05 O 24 ]·9.68H 2 O, is a new zeolite species of the chabazite series, occurring in basalts of the Karikás-tető area of Prága Hill, Veszprém County, Balaton Highland, Transdanubia, West Hungary. It crystallizes as single, colorless rhombohedra up to 0.4 mm in size. The streak is white and the luster is strong vitreous. Mohs’ hardness is about 4. The observed density is 1.98(1) g/cm3 and the calculated density is 1.964(7) g/cm 3 . Chabazite-Mg is anisotropic, uniaxial (+), ω = 1.465(5), ε = 1.469(5) (546 nm). In its chemical composition, a predominance of Mg is observed among the extraframework cations. However, K and Ca are also very abundant, while Na and Sr levels are very low. The ratio Si/(Si+Al) is among the highest found in chabazite of hydrothermal genesis. Chabazite-Mg is rhombohedral, R̅3̅m space group, a = 9.3433(5) Å, α = 94.894(4)°. The six strongest X-ray lines measured in the powder pattern [d in Å (I) (hkl)] are: 9.306 (60) (100), 5.537 (37) (11̅ 1̅), 4.958 (25) (111), 4.315 (100) (201̅), 2.924 (78) (31̅ 1̅), 2.869 (41) (310). Single-crystal structure refinement of chabazite-Mg indicated that the extraframework occupation is distinct from other chabazite-series minerals. In particular, the Mg site (C3a) and one water site (W6a) are displaced from the threefold axis parallel to [111]

Solution calorimetric measurements have been made on 17 synthetic fluorapatite-chlorapatite crystalline solutions at 50°C in 20.0 wt% HCl under isoperibolic conditions. Analysis of the calorimetric data indicates that heats of mixing across the series may reach values as high as 8.3 kJ/mol. Normally such a high degree of thermodynamic nonideality would be associated with immiscibility, yet no such miscibility gap is indicated by either synthetic or natural fluor-chlorapatite specimens. Based on full chemical analyses, all Cl-rich samples (X Cl > 0.65) of this study have halogen deficiencies that imply the presence of 4-11 mol% vacancies in the anion sites, which are interpreted to be associated with oxyapatite substitution. Separate analysis of data for the vacancy-free samples produces a linear fit for enthalpy of solution vs. composition, which yields an alternative interpretation of thermodynamic ideality. Together these models define the limits of enthalpy behavior for the fluor-chlorapatite system

Pertsevite-(OH), end-member formula Mg 2 (BO 3 )(OH), is a new mineral found in a ludwigite-kotoite magnesian skarn from the Snezhnoye deposit in Sakha-Yakutia Republic, Russia. The Commission on New Minerals, Nomenclature and Classification, IMA (IMA 2008-060) has approved the mineral and the mineral name. Moreover, the Chairman of the CNMNC agreed to renaming pertsevite to pertsevite-(F). The two minerals constitute the pertsevite series with the general formula Mg 2 (BO 3 ) 1-x (SiO 4 ) x (OH,F) 1-x , where x = 0.1-0.3. Pertsevite-(OH) is biaxial: 2V z = 55-65°(meas), 2V z = 75°(calc); α = 1.611(1), β = 1.623(1), γ = 1.644(1)°; X = b, Y = c, Z = a. It is associated with kotoite, szaibelyite, ludwigite-azoproite, clinohumite, hydroxylclinohumite, forsterite, chondrodite, calcite, REE-bearing sakhaite, sphalerite, and goethite. Ion microprobe analyses of pertsevite-(OH) using secondary ion mass spectrometry gave B, F, and H contents consistent with B and F contents determined by electron microprobe analyses (EMPA), and with OH contents calculated from EMPA data, confirming that OH/F ratio > 1 in pertsevite-(OH). Pertsevite-(OH) is orthorhombic, with space group Pnma. Cell dimensions refined from X-ray powder diffraction data are as follows: a = 20.499(1), b = 11.900(1), c = 4.589(1) Å, and V = 1119.4(3) Å 3 . The strongest lines of the X-ray diffraction pattern are [(hkl), d-spacing in Å, (I)]: (331) 2.7480 (61), (141) 2.4788 (42), (711) 2.4197 (35), (441) 2.2455 (86), (801) 2.2408 (45), (442) 1.7124 (100), (802) 1.7074 (47), and (12.4.0) 1.4817 (51). The main bands in the FTIR spectrum of pertsevite-(OH) are 1354, 1261, 1178, 1022, 975, 929, 888, 745, 555 and OH-specific 3696, 3562, 3530 cm -1

Two new antimonian garnets-bitikleite-(SnAl) Ca 3 SbSnAl 3 O 12 and bitikleite-(ZrFe) Ca 3 SbZrFe 3 O 12 -have been found as accessory minerals in the cuspidine zone of high-temperature skarns in a carbonate-silicate xenolith at the contact with ignimbrites within the Upper Chegem structure in the Northern Caucasus, Kabardino-Balkaria, Russia. The bitikleite series forms a solid solution with garnets of the kimzeyite-schorlomite and toturite type. Antimony-garnets form crystals up to 50 μm across containing kimzeyite cores and thin subsequent zones of complex lakargiite-tazheranitekimzeyite pseudomorphs after zircon. Bitikleite-(SnAl) has a = 12.5240(2) Å, V = 1964.40(3) Å 3 and bitikleite-(ZrFe) has a = 12.49 Å, V = 1948.4 Å 3 (Ia3d, Z = 8). The strongest powder diffraction lines of bitikleite-(SnAl) are [d, Å (hkl)]: 4.407 (220), 3.118 (440), 2.789 (420), 2.546 (422), 1.973 (620), 1.732 (640), 1.668 (642), and 1.396 (840). The strongest calculated powder diffraction lines of bitikleite-(ZrFe) are [d, Å (hkl)]: 4.416 (220), 3.123 (440), 2.793 (420), 2.550 (422), 1.975 (620), 1.732 (640), 1.669 (642), and 1.396 (840). The Raman spectra of bitikleite garnets are similar to the spectra of kimzeyite and toturite. Larnite, rondorfite, wadalite, magnesioferrite, tazheranite, lakargiite, kimzeyite, and toturite associated with bitikleite garnets are typical of high-temperature (>800 °C) formation

The structural environment of silicon dissolved in rutile and α-PbO 2 -structured TiO 2 (TiO 2 -II) was probed using 29 Si MAS NMR on 29 Si-enriched samples. At 1 atm, about 0.01 wt% SiO 2 is incorporated into TiO 2 as IV Si, presumably in interstitial sites. Rutile recovered from 6 GPa, 1600 °C contains about 0.6 wt% SiO 2 , incorporated both as VI Si (~90%) and IV Si (~10%). TiO 2 -II, synthesized at 12 GPa, 1200°C, contains only VI Si. The chemical shift for VISi in TiO 2 -II is slightly less negative than that for rutile, and the peak is split, suggesting either a more complex mechanism of substitution or a different response to quenching or decompression in the lower-symmetry structure. Future thermodynamic studies of the TiO 2 -SiO 2 solid solution will have to take into account the mixed coordination environment of the Si in TiO 2 , at low pressures

The orientation of spins in a natural sample of Ti-bearing hematite (Fe 2 O 3 ) has been measured from 2-300 K using time-of-flight neutron powder diffraction. It is shown that the antiferromagnetic alignment vector is tilted out of the basal plane by an average angle of 30°, independent of temperature, contrary to the normal expectation that all spins lie in the basal plane due to the suppression of the Morin transition by Ti. This unusual result is related to the non-uniform spatial distribution of Ti in this sample, which takes the form of ~1 nm exsolution lamellae of ilmenite (FeTiO 3 ), observed using transmission electron microscopy. It is suggested that the exsolution lamellae lead to a localization of Fe 2+ species within the lamellar interfaces, which cause tilting of some spins toward the crystallographic c axis. The presence of an out-of-plane component of spin at room temperature reconciles experimental and computational attempts to explain the phenomenon of “giant exchange bias” that appears when this sample is zero-field cooled below the ilmenite Néel temperature

Density functional perturbation theory is used to calculate the Raman spectrum of forsterite (Mg 2 SiO 1 ). In addition to the fundamental mode frequencies and atomic displacements, the scattering intensities are computed from first principles for the first time. Six independent single-crystal Raman spectra are measured for synthetic forsterite, and good agreement is found between calculation and experiment over a range of nearly six orders of magnitude of scattered intensity. Calculated atomic displacements of these fundamental Raman modes generally agree closely with the results of previous lattice dynamics studies. Modes with frequencies above 500 cm -1 consist primarily of motions internal to the SiO 4 tetrahedra, while those below 500 cm -1 are dominated by Mg2 displacements mixed with SiO 4 translations and rotations. The considerably larger Raman amplitudes for modes above 500 cm -1 appear to be due to displacements within the highly polarizable oxygen environments surrounded by covalently bonded Si 4+ and ionically bonded Mg 2+ . With regard to calculated frequencies, the theory underestimates frequencies by as much as 8 cm -1 for modes over 500 cm -1 , while it generally overestimates frequencies by as much as 17 cm -1 for modes below 500 cm -1 . An equivalent set of Raman spectra were measured for the Fe end-member of the olivine solid-solution series, fayalite (Fe 2 SiO 4 ), and compared to the results for forsterite

Solubility and solution mechanism(s) of reduced (N+H)- and H-containing N-O-H volatile components in Na 2 O-SiO 2 composition melts in equilibrium with NH 3 +H 2 +N 2 and H 2 O+H 2 fluid and H- and N-isotope concentrations in these melts were determined experimentally at 1.5 GPa and 1400 °C as a function of hydrogen fugacity, f H₂ , and melt polymerization (composition), NBO/Si (NBO/Si = 0.4-1.18). This NBO/Si-range is similar to that between dacite and olivine tholeiite melt (NBO/Si ~0.4-1). The f H ₂ was controlled between that of the iron-wüstite + H 2 O [log f H₂ (IW) ~3.42 (bar)] and that of the magnetite-hematite + H 2 O [log f H₂ (MH) ~ -0.91 (bar)] buffer. The N solubility decreases from 0.98 to 0.28 wt% in the melt NBO/Si-range from 0.4 to 1.18 at fH₂(IW) and decreases by about 50% between f H₂ (IW) and f H₂ (MH). The H solubility at f H₂ (IW) is insensitive to NBO/Si and averages 0.76±0.28 wt% and 0.48±0.07 wt% H in (N+H)-saturated and in N-free and H-saturated melts, respectively. The H solubility in the melts decreases by at least ~70% between f H₂ (IW) and f H₂ (MH). Their N and H isotope ratios are systematic functions of the abundance ratio of structurally bound N and H (as NH 2 -- and OH--groups bonded to Si 4+ ) relative to molecular H 2 , N 2 , and NH 3 in the melts. Molecular H 2 O plays a subordinate role in these melts, the bulk H 2 O content of which is <5 wt%. The NH 2 -/NH 3 and OH-/H 2 abundance ratios vary by ~55 and ~500% between NBO/Si = 1.18 and 0.4 relative to the values at NBO/Si = 0.4. In this same NH 2 - /NH 3 abundance ratio range, the δ 15 N of (N+H)-saturated melts, relative to that of melts with NBO/Si = 0.4, varies by ~2‰, whereas the δD varies by ~87‰. In N-free melts, the δD varies by ~12‰. Changing abundance of volatiles dissolved in silicate melts in molecular form and as structural complexes that form bonds with the silicate melt structure is an important factor that can affect stable isotope fractionation during melting and crystallization at high pressure and temperature

Compressional (P) and shear (S) wave velocities across the olivine-spinel transformation in Fe 2 SiO 4 were investigated in situ using combined synchrotron X-ray diffraction, X-ray imaging, and ultrasonic interferometry up to 5.5 GPa along the 1173 K isotherm. The onset of the spinel to olivine transformation at 4.5 GPa and olivine to spinel transition for Fe 2 SiO 4 at 4.8 GPa was concurrently observed from X-ray diffraction, the amplitude of the ultrasonic signals, the calculated velocities, and the ratio of P and S wave velocities (v P /v S ). No velocity softening was observed prior to the fayalite to spinel transition. The velocity contrasts across the Fe 2 SiO 4 spinel to fayalite phase transition are derived directly from the measured velocities, which are 13 and 12% for P and S waves, respectively, together with a density contrast of 9.4%. A comparison with literature data indicates that the changes in compressional-wave velocity and density across the olivine-spinel transformation in Fe 2 SiO 4 are comparable to those with different iron concentrations in the (Mg,Fe) 2 SiO 4 solid solution, whereas the shear wave velocity contrast decreases slightly with increasing iron concentration

In this paper, the term “equilibrium assemblage diagrams” refers to diagrams strictly based on assemblages predicted by Gibbs free energy minimization. The presented Theriak/Domino software uses a unique algorithm of scanning and bookkeeping, which allows to compute completely and automatically a great variety of diagrams: phase diagrams, pseudo-binary, pseudo-ternary, isopleths, modal amounts, molar properties of single phases or bulk-rock properties like total ΔG, volume of solids, etc. The speed and easiness of use makes thermodynamic modeling accessible to any student of Earth sciences and offers a powerful tool to check the consistency of thermodynamic databases, develop new solution models, plan experimental work, and to understand natural systems. The examples described in this paper demonstrate the capacity of the software, but also to show the usefulness and limitations of computed equilibrium assemblage diagrams. For most illustrations, a metapelite (TN205) from the eastern Lepontine Alps is used. The applications include the interpretation of complex diagrams, mineral reactions, the effect of Al content on the equilibrium assemblages, the interpretation of Si per formula unit in white mica, understanding some features of garnet growth, dehydration and isothermal compressibility, a broadening of the concept of AFM diagrams, combining equilibrium assemblage diagram information with thermobarometry, and comparing the results produced with different databases. Equilibrium assemblage diagrams do not always provide straightforward answers, but mostly stimulate further thought

A solid solution can exist of magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ), which is commonly referred to as nonstoichiometric or partially oxidized magnetite. The degree of stoichiometry in magnetite is quantitatively measured by determining the ratio of Fe 2+ to Fe 3+ . Magnetite stoichiometry (x = Fe 2+ /Fe 3+ ) strongly influences several physical properties, including the coercitivity, sorption capacity, reduction potential, and crystalline structure. Magnetite stoichiometry has been extensively studied, although very little work exists examining the stoichiometry of nanoparticulate samples (<<100 nm); when the stoichiometry was measured for nanoparticulate samples, it was not validated with a secondary technique. Here, we review the three most common techniques to determine magnetite stoichiometry: (1) acidic dissolution; (2) Mössbauer spectroscopy; and (3) powder X-ray diffraction (pXRD), specifically with nanoparticulate samples in mind. Eight samples of nonstoichiometric magnetite were synthesized with x ranging from 0 to 0.50 and with the particle size kept as similar as possible (BET specific surface area = 63 ± 7 m 2 /g; particle size ≈ 20 nm). Our measurements indicate excellent agreement between stoichiometries determined from Mössbauer spectra and by acidic dissolution, suggesting that Mössbauer spectroscopy may be a useful means for estimating magnetite stoichiometry in nanoparticulate, multi-phases samples, such as those found in the environment. A significant linear correlation was also observed between the unit-cell length (a) of magnetite measured by pXRD and magnetite stoichiometry, indicating that pXRD may also be useful for determining particle stoichiometry, especially for mixed phased samples

The low-temperature behavior of a natural kalsilite (ideal formula KAlSiO 4 ) with P31c symmetry has been investigated by in situ single-crystal diffraction. A series of intensity data collections and structural refinements have been performed at 298, 250, 200, 150, and 100 K on decreasing temperature, and 175, 225, and 275 K on increasing T. The variations of the unit-cell parameters of kalsilite as a function of T are continuous, and show no evidence of any phase transitions or thermo-elastic anomalies in this temperature range. An expansion is observed along [0001] with decreasing temperature. The axial and volume thermal expansion coefficients (α j = l j -1 ⋅∂l j /∂T, α V = V -1 ⋅∂V/∂T) between 298 and 100 K, calculated by weighted linear regression through the data points, are α a = α b = 1.30(6)·10 -5 , α c = -1.5(1)·10 -5 , α V = 1.1(2)·10 -5 K -1 . The main structural change on decreasing temperature is a cooperative anti-rotation of tetrahedra forming the six-membered rings lying parallel to (0001). This tetrahedral rotation is coupled with a change in the distances between the extra-framework cations and the framework O atoms. A small decrease in the tetrahedral tilts perpendicular to [0001] is responsible for the negative thermal expansion along [0001]; the implications of these mechanisms for thermal expansion in nephelines and kalsilites are discussed

Three {n10}/[001] tilt grain boundaries (n = 2, 3, and 4) of MgO were studied as a function of pressure from first principles within density functional theory. Our results show that the physical properties of the grain boundary are very distinct from the bulk properties. The predicted symmetric boundary containing well-defined dislocation pipes at zero pressure transforms to asymmetric boundaries with denser structures at higher pressures. The asymmetric boundary structure stable at 50 GPa can be associated with a shear in the boundary plane, whereas the asymmetric boundary structure stable at 100 GPa can be associated with an additional shear in the direction perpendicular to the boundary plane. Unlike in the bulk, several nonequivalent sites exist for vacancy formation in the boundary regions, and the calculated Schottky defect formation enthalpy varies among different boundaries with the {310} and {410} boundary values at zero pressure being similar to the bulk value. Pressure increasingly stabilizes the boundary vacancies relative to the bulk thereby causing an enhancement in the vacancy concentration, which is further enhanced due to high-binding energy for cation-anion vacancy pairs in the interfacial regions. Also, the grain boundary was shown to induce electronic states in the band gap below the conduction band, which can trap the electrons inside the free space at the interface. Additional states with strong electron localization character appear at the boundary in the presence of the vacancies. Our results are expected to be useful to understand how grain boundaries can serve as primary storage sites for defects and high-diffusion pathways

The role of the oxo-component on the compressibility of amphibole was studied by means of highpressure in situ single-crystal X-ray diffraction on two natural kaersutite megacrysts (samples DL5 and FR12) from alkaline basalts. The oxo-component varies significantly (1.1 and 1.9 apfu in DL5 and FR12, respectively), whereas the cation composition is very similar, apart from the Fe 3+ /(Fe 2+ +Fe 3+ ), which is 0.33 in DL5 and ~1 in FR12. The larger oxo-component of FR12 is attributed to the Fe 2+ + OH- = Fe 3+ + O 2 - + ½H 2 substitution. Unit-cell parameters were collected at different pressures up to about 8 GPa. Structural refinements of both samples were performed with data collected at different P up to 6 GPa. Fitting the P-V data to a third-order Birch Murnaghan EoS yielded the following parameters: K 0 = 94(1) GPa, K′ = 6.3(4), and V 0 = 903.6(2) Å 3 for FR12 and K0 = 91(2) GPa, K′ = 6.2(4), and V 0 = 914.1(2) Å 3 for DL5. The axial moduli of the two amphibole samples were: K0a = 86(3) GPa, K′ a = 7(1), and a 0 = 9.815(2) Å; K 0b = 115(3) GPa, K′ b = 4.8(8), and b 0 = 18.012(2) Å; K 0c = 112(5) GPa, K″ c = 7(1), and c0 = 5.300(1) Å for sample FR12 and K 0a = 85(3) GPa, K′ a = 5(1), and a 0 = 9.8660(9) Å; K 0b = 113(2), K′ b = 4.4(6), and b 0 = 18.0548(6) Å; K 0c = 107(3) GPa, K′ c = 7(1), and c 0 = 5.3185(5) Å for sample DL5. This suggests that the compressibility of kaersutite decreases with increasing oxo-component. Structural refinements show that the polyhedral compressibility follows the order A = M4 > M2 > M3 > M1 for DL5 and A = M4 > M2 > M1 > M3 for FR12. The most evident geometrical effect induced by P is the decrease in the bending of the double tetrahedral chain, when adjacent I-beams are pushed against each other. This effect is largest for DL5, which has a larger concavity of the A site, (O7-O7′ changes from 3.03 to 2.82 Å) compared to the one of FR12, (O7-O7′ changes from 2.92 to 2.79 Å). This mechanism is confirmed by the evolution of T1-O7-T1 angle (from 135.4° to 132.5° in FR12 and from 136.6° to 132.2° in DL5).

The change with temperature of IR absorption bands in OH in muscovite was studied using unpolarized in-situ high-temperature infrared microspectroscopy. The molar absorption coefficient ε at 3628 cm-1 for OH in muscovite at room temperature (20 °C) has been determined to be 127 ± 6 L/mol·cm. Using the orientation factor γ = 0.47 for the angle between OH vector and c* axis (75 ± 5°), the true molar absorption coefficient ε at 3628 cm -1 is determined to be 270 ± 10 L/mol·cm at 20 °C. Integral molar absorptivities are also determined from 20 to 650 °C showing no weight loss. The value decreases from 7060 ± 190 L/mol·cm 2 at 20 °C to 5190 ± 270 L/mol·cm 2 at 650 °C (26% decrease). OH orientation of muscovite sample measured by polarized IR microspectroscopy at 20 °C showed that the angle between the projection of OH vector to (001) plane (the OH′ vector) and b axis is 30.5° at 20 °C and did not change greatly at higher temperatures until 650 °C. The tilting of OH dipoles toward the c* axis from 75° to about 43° while keeping the same angles along b axis would explain the observed decrease in integral molar absorptivities at higher temperatures, but further studies are needed to clarify the OH behavior at high temperatures

Following the recent thermal expansion work of Hovis et al. (2008) on AlSi 3 feldspars, we have investigated the thermal expansion of plagioclase, Ba-K, and Ca-K feldspar crystalline solutions. X-ray powder diffraction data were collected between room temperature and 925 °C on six natural plagioclase specimens ranging in composition from anorthite to oligoclase (and as well on their Kexchanged equivalents) and five synthetic Ba-K feldspars with compositions from 25 to 99 mol% BaAl 2 Si 2 O 8 . The resulting thermal expansion coefficients for volume (αV) have been combined with earlier results for end-member Na- and K-feldspars. Unlike AlSi 3 feldspars, Al 2 Si 2 feldspars, including anorthite and celsian from the present study, and Sr- and Pb-feldspar from other workers, show essentially constant, and relatively limited, thermal expansion. In the context of structures where the Lowenstein rule requires Al and Si to alternate among tetrahedra, the proximity of bridging Al-O-Si O atoms to divalent neighbors (ranging from 0 to 2) results in Ca-O (or Ba-O) bonds that are especially short. It is suggested that short bonds such as these have a partly covalent character resulting from the requirement for local charge balance. This, in turn, stiffens the structure. For feldspar series with coupled substitution, the change away from a purely divalent M-site occupant gives the substituting (less strongly bonded) monovalent cations increasingly greater influence on thermal expansion. Overall, thermal expansion in the feldspar system is well represented on a plot of αV against room-temperature volume (V RT ), where one sees a quadrilateral bounded by data for: (1) AlSi 3 feldspars whose expansion behavior is governed largely by the size of the monovalent alkali-site occupant; (2) Al 2 Si 2 feldspars whose expansion is uniformly limited by divalent cations having stronger, partially covalent, bonds to bridging Al-O-Si O atoms; and (3,4) plagioclase and Ba-K feldspars where expansion behavior across the series transitions from one control to the other. Overall, the coefficient of thermal expansion in any binary feldspar series is a linear function of V RT between the pertinent end-members. This makes it possible to predict the thermal expansion behavior of any feldspar simply from knowledge of its chemical system and room-temperature volume

An experimental apparatus is described that allows for optical in situ observation of samples at 1 bar and temperatures up to 1250 °C for durations up to several days. The apparatus resembles a Bassett-type externally heated diamond cell, where the diamond anvils are replaced by cylinders of synthetic moissanite (SiC). The sample is placed inside a gasket of glassy carbon between the two moissanite windows. The moissanite cell allows for the direct and continuous observation of the sample with an optical resolution in the range of a few micrometers. Partially molten systems can be investigated during cooling-heating cycles within a single experimental run. Texture evolution can be observed continuously through time. The technique also allows for direct measurements of crystal growth and dissolution rates at defined cooling and heating rates. The method was tested by observing the crystallization of KNO 3 -LiNO 3 and NaCl-KCl salt melts as well as of a basaltic-trachyandesitic melt. Dendrite formation, static grain growth, and dissolution were observed in the salt melts. During textural coarsening (static grain growth) of rock salt in the presence of melt, grain coalescence was observed. Both nitrate and rock salt grains growing in contact with melt generally showed curved boundaries and roundish shapes, rather than facets. In one experiment, the final texture did not contain any traces of the previous textural evolution anymore. In a basaltic trachyandesite at temperatures near the liquidus several phenomena, in particular the interactions between crystals, magmatic flow and bubbles during nucleation, growth and dissolution of crystals, and crystal-clusters were observed simultaneously. Crystal growth rates and dissolution rates were also measured

Polezhaevaite-(Ce) NaSrCeF 6 is a new member of the gagarinite mineral group [hexagonal, P6 3 /m, a = 6.207(7) Å, c = 3.801(9) Å, V = 126.8(2) Å 3 , Z = 1]. It is found in a natrolitized microcline-aegirinesodalite lens within apatite-rich urtite at Mt. Koashva (Khibiny massif, Kola Peninsula, Russia) in association with аegirine, albite, arfvedsonite, astrophyllite, burbankite, catapleiite, chlorbartonite, djerfisherite, elpasolite, fluorapatite, fluorite, galena, hydroxylapatite, ilmenite, lamprophyllite, lorenzenite, leucophanite, microcline, natrolite, nepheline, orickite, pectolite, pyrochlore, sodalite, sphalerite, strontiofluorite, tainiolite, titanite, vinogradovite, and villiaumite. Polezhaevaite-(Ce) occurs as parallel and sheaf-like aggregates of extremely thin fibers (up to 1 mm long and <1 μm thick), which fill leaching voids within burbankite crystals in natrolite and mantles around partially dissolved burbankite crystals in intimate association with strontiofluorite. Polezhaevaite-(Ce) is translucent (transparent in separate fibers), snowy-white, with a silky luster (in aggregates) and a white streak. Cleavage is not observed; fracture is splintery (in aggregates). The Mohs hardness of individual crystals could not be determined and approaches 3 in aggregates. In transmitted light, the mineral is colorless, uniaxial positive: ε = 1.497(5), ω = 1.490(5) (for λ = 589 nm). D calc = 4.646 g/cm 3 , D meas = >4.2 g/cm -3 . The mean chemical composition determined by electron microprobe is (wt%): Na 5.27, Ca 3.08, Sr 29.72, Ba 0.48, La 11.76, Ce 14.12, Pr 0.49, Nd 3.09, F 31.95, total 99.96. Empirical formula calculated on the basis of F = 6 apfu is: (Na 0.82 Ca 0.18 ) Σ=1.00 (Sr 1.21 Ce 0.36 La 0.30 Ca 0.09 Nd 0.08 Ba 0.01 ) Σ=2.06 F 6 (charge imbalance is +0.05). Its simplified formula is NaSrCeF 6 . The strongest X-ray powder-diffraction lines [d in Å, (I), (hkl)] are: 5.416(40)(100), 3.120(100)(101, 110), 2.198(70) (201), 1.796(90)(121, 211, 300, 102), 1.554(30)(220), 1.173(70)(321, 231, 140, 410,132, 312, 113). The mineral is named in honor of Lyudmila Ivanovna Polezhaeva (b. 1935), a Russian expert in electron microprobe analysis of minerals for her contribution to the mineralogy of alkaline rocks

We recorded angle-dependent XANES (AXANES) spectra at the potassium K-edge for three compositionally intermediate polylithionite-siderophyllite trioctahedral 1M-micas using polarized synchrotron radiation. We evaluated the experimental spectra for both their in-plane and out-of-plane component fractions of the electric dipole contribution using the analytical formulae of Brouder (1990), referring to theory to extract the origin of their multiple-scattering pathways of Natoli et al. (2003). This analysis was extended to a fourth lithian mica studied previously and allowed characterization of the local environment and ordering around the potassium atoms in the interlayer of the entire set of micas. The AXANES in-plane components are notably similar to the XANES spectra recorded on randomly oriented powders, provided these are oriented at the “magic angle” (Pettifer et al. 1992). Most observed contributions arise from multiple-scattering interactions of the photoelectron ejected from the potassium absorber colliding with atoms located in the interlayer itself. Note that this includes not only interactions with other coplanar potassium and/or alkali atoms distributed along the interlayer plane, but also with their near- and next-nearest neighboring oxygen atoms which lie on the basal planes of the tetrahedral sheets facing the interlayer. By contrast, the AXANES out-of-plane component suggests that several multiple-scattering pathways cross the energetic and structural barrier represented by the tetrahedral sheets. They reach not only the X anions that are located on the upper level of the octahedral sheets, at the center of the open cavity in the tetrahedral sheet, but also the metal cations centering the octahedral sheet itself. Therefore, the out-of-plane components provide indirect information on the number of independent octahedral sites, the cation oxidation state, and the trans- vs. cis-orientation of the anionic sites

Despite the ubiquity of arsenic in hydrothermal-magmatic environments, its abundance, distribution, and chemical and structural status in natural silicate melts and glasses remain poorly known. Here we report the first in situ measurements of the redox state and molecular structure of As, using X-ray absorption fine structure (XAFS) spectroscopy, in a rhyolitic peraluminous glass from Macusani (SE Peru) that is representative of anatectic melts derived from metasedimentary crustal protoliths. Arsenic abundances as well as the concentrations of other trace elements were measured in the glass using a femtosecond laser ablation-inductively coupled plasma-quadrupole mass spectrometry (LA-ICP-QMS). The glass shows enrichments, by factors of 10 to 100, in comparison with the mean continental crust values, for As and other incompatible trace elements (e.g., Be, B, Rb, Sn, Sb, and Ta), and by factors of 100 to 200 for Li, Cd, and Cs. Arsenic is present in the peraluminous glass in trivalent state in the form of oxy-hydroxide complexes like AsO(OH) 2 - and/or As(OH) 3 , similar to those dominant in the aqueous fluid vapor phases at hydrothermal-magmatic conditions. The similar As chemical speciation between the fluid and the melt is consistent with As fluid/melt partitioning coefficients close to one, as observed in experiments on rhyolite-water systems. The depolymerized melt structure caused by elevated H 2 O, F, and P contents is likely to allow accomodation of high concentrations of metalloid hydroxide/hydrated complexes. Consequently, hydrous silicate melts may be important transporting media in shallow magmatic-hydrothermal settings for As and similar elements like B and Sb due to their high affinity to water and hydroxide ligands

Recent advances in the dating of monazite using the electron microprobe indicate that ThSiO 4 - and/ or CaTh(PO 4 ) 2 -enriched intergrowths in monazite can give ages younger than the original monazite. The morphology of the intergrowths suggests that the original monazite grain has been partly altered by a fluid in which Th, Si, and Ca are mobile. This hypothesis has been tested in the piston-cylinder apparatus at 1000 MPa and 900 °C utilizing a natural, unzoned, Th- and Pb-bearing monazite-(Ce) and Na 2 Si 2 O 5 +H 2 O. During the experiment, a subset of the monazite grains acquired ThSiO 4 -enriched areas with sharp compositional boundaries devoid of Pb and show evidence of being a pseudomorphic partial replacement of the monazite rather than an overgrowth of the monazite. These experiments support the hypothesis that similar Th-enriched or Th-depleted patches with sharp compositional boundaries observed in natural monazite could be the result of fluid-induced alteration via coupled dissolutionreprecipitation. If so, such altered regions would yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of single or multiple metasomatic events assuming that all pre-existing radiogenic Pb is removed during alteration

The existence or otherwise of the hydronium cation within the mineral alunite has proved difficult to verify based on experimental data alone. In the present study, we employ first-principles quantum mechanical techniques to determine the nature of the structure of hydronium alunite. A new model for the hydrogen atom disorder is predicted that differs from a proposed arrangement in the case of the analogous stoichiometric hydronium jarosite. Instead of occupying one of two orientations that respect the threefold symmetry axis passing through the oxygen, the cation is found to occupy a tilted orientation leading to a greater degree of disorder. Dynamical simulations indicate that the reorientation of the hydronium ions is rapid and exhibits no correlation between the cation sites. The higher level of disorder in the hydrogen positions offers an explanation as to why their location has proved elusive thus far

The dense hydrous-magnesium silicate phase D, which has the ideal formula MgSi 2 H 2 O 6 , may be an important link in a chain of hydrous phases that carry H 2 O in the ultramafic portions of subducting lithosphere, into the Earth’s lower mantle. We have synthesized a new Al-rich form of phase D, containing up to 50 wt% Al 2 O 3 , using a multi-anvil device at ~1300 °C and 25 GPa. The phase, with the formula Mg 0.2 Fe 0.15 Al 1.8 H 1.8 SiO 6 , was initially produced in a bulk composition designed to synthesize Al- and Fe-rich magnesium silicate perovskite with a composition similar to that produced in experiments on mid-ocean ridge basalt bulk compositions at lower mantle conditions. Further experiments using a starting mixture based on the composition of this Al-rich phase resulted in the synthesis of 60-70 μm long single crystals at similar conditions. The recovered crystals were slightly richer in H 2 O (Mg 0.2 Fe 0.12 Al 1.5 Si 0.92 H 3.1 O 6 ) and their unit-cell parameters were similar to those of MgSi 2 H 2 O 6 phase D. A refinement of the crystal structure was carried out in the P3̅1m space group and revealed a more disordered cation distribution than magnesium silicate phase D. All cation-oxygen distances are similar, suggesting a high degree of Si/Al disorder. Although the stability field of this new variant of phase D is yet to be determined, this phase may be an important host for H 2 O within portions of subducted oceanic crust in the lower mantle

Based on systematic ab initio exploration, we reported last year that two new interlayer translations for kaolinite, -a/3 and (a+b)/3, allowed a new family of kaolin polytypes under moderate pressure. Both translations place each silicon atom of a kaolin layer on top of a hydroxyl group from the kaolin layer below, resulting in a triangular dipyramidal fivefold coordination to all silicon atoms. The predicted -a/3 translation has since been independently observed experimentally at ~7 GPa, as kaolinite III phase, by compression of a natural kaolinite (Keokuk, Iowa). Encouraged by that verification, we extend here to the entire kaolin system both translations we had predicted for kaolinite. Based on calculated enthalpies and cell volumes for models optimized with ab initio density functional theory (DFT) calculations under pressure, we report three main results. First, we predict the existence of a kaolinite IV phase at a pressure not higher than 60 GPa and its likely crystal structure. Second, we predict three novel high-pressure crystal-structure models for nacrite, ranked by their enthalpy value, one of which is likely to be observed at about 10 GPa. Finally, three other novel and ranked highpressure crystal-structure models are reported for dickite. Our results from interpretation of ab initio DFT calculations should guide experimental studies and facilitate their interpretation