Volume 97, Issue 5-6

Following the ideas of Helen Megaw, we describe the changes in the conformation of the tetrahedral framework of feldspars in terms of just four distinct tilt systems of rigid tetrahedra. These systems are based on the four allowed tilts of a ring of four corner-linked tetrahedra with point symmetry 2. Of the four tilt systems, only two result in significant volume change of the unit cell. We show that all of the essential features of the structures, unit-cell parameters and volumes of the AlSi 3 feldspars, and their expansion and compression induced by changes in pressure, temperature, and composition at crustal pressures, are generated by the simultaneous application of these two tilts to an un-tilted framework of regular tetrahedra. In combination these two tilts impose significant anisotropy upon the expansion of the unit cell of the feldspar with the majority of the expansion accommodated by the expansion of d(100). This demonstrates that the fundamental reason for the anisotropy of feldspars lies in the topology of the tetrahedral framework. A comparison of the actual tilts observed in alkali feldspars with the model shows that the tilts maximize the shortest O-O distances in the structure, and therefore O-O repulsions along with the volume requirement control the values of the tetrahedral tilts in alkali feldspars and thus the anisotropy of the structure. Therefore, the bonding requirements of the bridging O atoms and the directionality of the bonding to the extraframework cations only play a secondary role in modifying this basic pattern of anisotropy, which is intrinsic to the common topology of the framework of all feldspars.

Raman spectroscopy can measure water concentrations of hydrous silicate glasses with several advantages such as: (1) highspatial resolution of 1-2 μm 2 ; (2) nondestructive character; and (3) easy access, without any specific sample preparation or mounting techniques. The latter reasons render Raman highly suitable for studying natural products, such as volcanic pumice and scoriae fragments. Two spectral regions can be distinguished in Raman spectra of hydrated silicate glasses: a lowwavenumber region (15-1500 cm -1 ), which corresponds to vibrations of the silicate network, and a highwavenumber region (3100-3750 cm -1 ), corresponding to the OH stretching vibrations of H 2 O molecules and OH groups. Behrens et al. (2006) have published empirical equations relating the area ratio between these two regions and the water content. However, the proposed internal calibrations depend on chemical composition of the glasses. In this paper, we reinvestigated the previous procedures to improve the background subtraction. Our results allow us to present a more general and linear calibration. Water concentrations up to 13 wt% can be measured for a broad range of natural silicate melts, from basalts to rhyolite (40 up to 80 wt% SiO 2 ), using a single calibration curve with an absolute error of 0.2 wt%.

Electronic structure calculations are carried out to estimate the enthalpies of formation of rareearth orthophosphates from their oxides. The calculated enthalpies of formation are systematically less exothermic than the measured values. The discrepancy is almost entirely in the electronic total energy calculated from density functional theory, and appears to be intrinsic to the generalized-gradient exchange-correlation functional used. However, comparison with electronic structure calculation of the enthalpies of formation of alkaline earth oxyacid carbonates, silicates, and sulfates suggests that near chemical accuracy can be obtained for the enthalpies of formation of most of the compounds in the phosphate system by applying a scaling factor obtained from the simpler alkaline earth oxyacids. The increasingly exothermic ΔH f ox with increasing ionic radius (i.e., LaPO 4 is more exothermic than ScPO 4 ) results from the higher charge localization on the oxide anion (O 2- ) relative to the phosphate anion (PO 4 3- ), making it more favorable, in a relative sense, to pair the smaller cation with the oxide anion than with the phosphate anion. This effect is also manifested in ΔH f ox of the other oxyacids, such as carbonates, silicates, and sulfates.

Garnet has long been recognized as an important mineral in metamorphic petrology thanks to its widespread distribution and successful use in the reconstruction of P-T-t paths. However, the potential of garnet to solve geological problems may have been underestimated. In this paper, we make use of the role of garnet as a safe container of U-rich inclusions that, despite being metamict, are screened from Pb loss-causing processes and can be dated by isotope dilution-thermal ionization mass spectrometry. Garnet itself is not a true U-Pb geochronometer, as it normally contains no uranium, but it represents a vessel that protects U-rich inclusions from later disturbances. Garnet is virtually free of common Pb, it is clean and transparent, and provides a good control for the selection of suitable grains. In this study, inclusions such as uranitite in garnet were successfully used to obtain a precise U-Pb age of 318.36 ± 0.32 Ma of a pegmatite vein (Évora, Portugal), which otherwise would have been undatable due to the absence and/or strong alteration of common geochronometers such as zircon.

Wassonite, ideally stoichiometric TiS, is a titanium monosulfide not previously observed in nature, that was discovered within the Yamato 691 EH3 enstatite chondrite. Twelve Ti-S phase grains were identified in a rare barred olivine (BO) chondrule; three of the grains were extracted by the focused ion beam technique. Because of the submicrometer size of the wassonite grains, it was not possible to determine conventional macroscopic properties. However, the chemical composition and crystal structure were well constrained by extensive quantitative energy-dispersive X-ray analysis and electron diffraction using transmission electron microscopy (TEM). The crystal system for wassonite is rhombohedral (a = 3.42 ± 0.07, c = 26.50 ± 0.53 Å) with space group: R3̅m, cell volume: 268.4 ± 0.53 Å 3 , Z = 9, density (calculated): 4.452 g/cm 3 , empirical formula: (Ti 0.93 ,Fe 0.06 ,Cr 0.01 )S. The wassonite grains crystallized from the chondrule melt that was itself formed in the solar nebula, not on the parent asteroid. The other crystalline phases in the BO chondrule include forsterite, enstatite, troilite, metallic Fe-Ni, and osbornite (as well as the new Ti-S-bearing minerals and schollhornite) are highly reduced and indicate formation at low-oxygen fugacities.

The valence states of Mn in mixed-valent layer and tunnel structure manganese dioxides (MnO 2 ), usually referred to as phyllomanganates and tectomanganates, can be measured by X-ray absorption near-edge structure (XANES) spectroscopy with a precision and accuracy that are difficult to estimate owing to the paucity of well-characterized standards. A compilation of the Mn K-edge XANES spectra of most naturally occurring manganates, synthetic analogs of known structure and chemical composition, and pure-valence phase species is presented and made available as an open source. We intend this compilation to serve as a basis for the spectroscopic determination of the fractions of the Mn 2+, 3+, and 4+ valences in mixed-valent manganates and phase mixtures. The XANES derivatives of tectomanganates and phyllomanganates with no or little Mn 3+ in the MnO 2 layer exhibit intensities, shapes, and relative energy positions of the main features characteristics of a particular valence composition. For these compounds, valence fractions can be derived using linear combination fitting analysis. Best quantitative results are obtained when the unknown spectrum is fit to a weighted sum of all reference spectra in the database with the fractions of species constrained to be non-negative (Combo method). The accuracy of the average valence is estimated to 0.04 v.u. in the range of 3+ to 4+, and decreases when the proportion of divalent Mn is higher than 15%. The accuracy of the method is also lower in (layer Mn 3+ , Mn 4+ ) manganates, because the XANES features are affected non-additively by the amount and distribution of the Jahn-Teller Mn 3+ cations. The merit of the Combo method for the determination of manganese valence sums relative to the methods based on calibration curves is discussed.

Fluid-assisted mass transport reactions by dissolution-precipitation, where a precursor mineral reacts with a fluid, play an important role in metamorphism and metasomatism. We investigated titanite growth on rutile in time series experiments between one and 107 days at constant P-T conditions of 600 °C and 400 MPa in the system TiO 2- CaO-SiO 2- Na 2 O-HCl-H 2 O. A two-capsule assemblage allows for transport of Ca and Si from dissolving wollastonite to dissolving rutile, the Ti-source, in a NaCl-bearing aqueous fluid, according to the general reaction CaSiO 3 + TiO 2 = CaTiSiO 5 . Complete overgrowth of rutile by titanite occurred after just one day of experiment. Fine-grained lozenge-shaped titanite crystals of short-time runs (up to 14 days) reorganize to larger predominantly prismatic crystals after >14 days. After investigation by scanning electron microscopy, the titanite overgrowth was removed from the rutile by hydrofluoric acid, to provide a three-dimensional view of the dissolution-reaction front on the rutile surface. The morphology of the rutile surface is dominated by humps or ridges beneath the central region of a titanite crystal and valleys at the grain boundaries between adjacent titanite crystals. The dissolution pattern on the rutile surface mimics the titanite overgrowth and changes with changing grain size and shape of the titanite with longer run times. The preferred dissolution of rutile in the valleys is clearly linked to the position of the titanite grain boundaries, which served as pathways for fluidassisted element transport. Rutile-titanite and titanite-titanite boundaries show a significant porosity in transmission electron microscopy images of foils prepared by focused ion beam milling. The large-scale dissolution pattern on the rutile surface is independent of the crystallographic orientation of the rutile and entirely dominated by the arrangement of titanite crystals in the overgrowth. Dissolution features on a scale smaller than ~1 μm are dominated by stepwise dissolution and etchpits following the crystallographic orientation of the rutile. Similar observations were made in experiments with an additional Al-source, although these experiments result in a different overgrowth pattern; i.e., an exposed rutile surface is always present and solitary titanite crystals are accompanied by partial overgrowths. Quantitative characterization of the surface morphology by whitelight interference microscopy demonstrates that, with increasing grain size of titanite, dissolution of rutile is strongly enhanced at the titanite grain boundaries. Natural examples of titanite overgrowths on rutile show the same relations between element pathways, arrangement of titanite crystals and 3D dissolution pattern on rutile as in the experimental systems. We conclude that the transport of Ti away from the rutile and of Ca + Si into the reaction rim occurred in a grain boundary fluid, the composition of which must have been strongly different from the composition of the bulk fluid in the experiment, as well as in the natural system. The reaction progress depends on the availability of a fluid, and relicts of rutile in titanite indicate restricted availability of fluid in the natural system (e.g., a fluid pulse that was consumed by the reactions). The reaction examined here can serve as a proxy for other reactions of the conversion of oxide minerals (e.g., spinel or corundum) into silicates.

We report the discovery of oriented needles of rutile and ilmenite in garnet crystals from granulite facies metapelitic rocks of the Merrimack synclinorium. Connecticut, and present a precipitation model for their origin. The rocks were strongly metamorphosed and deformed during the Devonian Acadian orogeny. The needles are primarily elongated parallel to <111> in garnet. Rutile has anomalous extinction angles as great as ~35° (cf. Griffin et al. 1971). Rutile and ilmenite needles are typically a few hundred nanometers to several micrometers in diameter and are several tens of micrometers to nearly a millimeter long. Other oxide inclusions that may be present include submicrometer- to micrometer-scale twinned rutile bicrystals, as well as srilankite and a crichtonite group mineral. Some garnet cores have unusual, box-shaped quartz inclusions, which coexist with Ti±Fe oxide needles and commonly contain micrometer-scale rods of F-OH-C1 apatite. Negative garnet ciystal "pores” are also widespread. Ti±Fe oxide needles are restricted to garnet core regions; rims have a distinctly different inclusion population dominated by granulite facies minerals including sillimanite, spinel, cordierite, and K-feldspar. Consequently, the garnet core regions represent an earlier, distinct period of growth relative to the rims. Garnet cores contain ~25-35% pyrope. and a host of minor and trace constituents including TiO 2 (0.07-0.6 wt%). Cr 2 0 3 (0.01-0.10 wt%). Na 2 O (0.01-0.03 wt%). P 2 O 5 (0.01-0.09 wt%). and ZrO 2 (up to -150 ppm). Na 2 O and ZrO 2 correlate positively with TiO 2 .Titanium zoning is preserved in some garnets; zoning profiles and two-dimensional chemical mapping show' that Ti and. to a lesser degree. Cr are depleted around Ti±Fe oxide inclusions. Therefore, we conclude that the needles are precipitates that formed from Ti-bearing garnet during exhumation and cooling. Garnet contained sufficient Ti to form precipitates; no Ti source external to garnet was necessary. Titanium­bearing garnets that contain oriented Ti±Fe oxide needles are known primarily from ultrahigh-pressure metamorphic rocks, mantle peridotites and pyroxenites, and high-pressure granulites. Thus, the presence of needle-bearing garnets in Connecticut strongly suggests that a previously unrecognized domain of extreme pressure and/or temperature metamorphism exists in the Acadian orogen.

Sapphirine, a typical ultrahigh-temperature metamorphic mineral, is rarely found in mantle xenoliths. Here we report the occurrence and characteristics of sapphirine in a mantle-derived xenolith from the Cenozoic basalts of Hannuoba in the North China Craton. The xenolith consists of a clinopyroxene, spinel, and sapphirine assemblage, with the sapphirine occurring as the reaction rim surrounding spinel. The mineral compositions of this sample are all characterized by high Mg contents, similar to those of minerals from other sapphirine-bearing rocks reported from high-Mg-Al granulites elsewhere in the world. Clinopyroxene is relatively rich in Al and Ca in comparison to pyroxene in peridotite and pyroxenite xenoliths in the Hannuoba basalts, as well as in global mafic and felsic granulites in other terranes, a feature that is consistent with the bulk composition. The P-T compilations from both experimental and natural rock data show a restricted stability field for the coexisting clinopyroxene + spinel + sapphirine assemblage of around 8-15 kbar and 800-900 °C. The rare occurrence of sapphirine in a mantle-derived xenolith therefore suggests specific bulk composition, restricted P-T range and possible melt-peridotite interaction. Such conditions are best satisfied in a tectonic setting with basaltic magma underplating and interaction between the infiltrating melts and the wall-rock peridotite.

Interactions between metallic iron and clay minerals have been extensively studied under lowtemperature anoxic conditions owing to their potential impact on the long-term safety of high-level nuclear waste disposal in deep geological repositories. To complement the studies investigating the destabilization mechanisms and rates of various initial clay minerals, the prediction of the storage long-term performance requires a comprehensive characterization of the reaction products. The Ferich 1:1 phyllosilicates resulting from interactions at 80 °C and in the absence of O 2 between metallic iron and smectites with contrasting compositions are thus characterized chemically and structurally using various experimental techniques (X-ray and electron diffractions, infrared, energy-dispersive, and electron energy loss spectroscopies, and high-resolution electron microscopy). Cronstedtite and odinite are the two Fe-rich 1:1 phyllosilicates formed under the experimental conditions investigated, both species differing from their relative contents and the average valence state of structural Fe. No parental link has been evidenced between the two minerals despite their contrasting crystal morphologies and thermodynamical predictions. The formation of the 1:1 phyllosilicates apparently results from the destabilization of the initial smectite through the formation of an intermediate gel.

The high-pressure elastic behavior and the P-induced structure evolution of a natural cancrinite from Cameroun {Na 6.59 Ca 0.93 [Si 6 Al 6 O 24 ](CO 3 ) 1.04 F 0.41 ·2H 2 O, a = 12.5976(6) Å, c = 5 .1168(2) Å, space group: P6 3 } were investigated by in situ single-crystal X-ray diffraction under hydrostatic conditions up to 6.63(2) GPa with a diamond-anvil cell. The P-V data were fitted with an isothermal Birch-Murnaghan type equation of state (BM EoS) truncated to the third order. Weighted fit (by the uncertainty in P and V) gave the following elastic parameters: V 0 = 702.0(7) Å 3 , K V0 = 51(2) GPa, and K´ V = 2.9(4). A linearized BM EoS was used to fit the a-P and c-P data, giving the following refined parameters: a 0 = 12.593(5) Å, K a0 = 64(4) GPa, K a ´ = 4.5(9), for the a-axis, and c 0 = 5.112(3) Å, K c0 = 36(1) GPa, K c ´ = 1.9(3) for the c-axis (elastic anisotropy: K a0 :K c0 = 1.78:1). A subtle change of the elastic behavior appears to occur at P > 4.62 GPa, and so the elastic behavior was also described on the basis of BM EOS valid between 0.0001-4.62 and 5.00-6.63 GPa, respectively. The high-pressure structure refinements allowed the description of the main deformation mechanisms responsible for the anisotropic compression of cancrinite on (0001) and along [0001]. A comparative analysis of the structure evolution in response of the applied pressure and temperature of isotypic materials with cancrinite-like topology is carried out.

Birnessite is a widespread, naturally occurring layered manganese oxide that exerts significant influence over the geochemical cycling of environmentally relevant cations due to its high-interlayer adsorption capacity. Triclinic sodium birnessite was used as a synthetic analog to gain a better understanding of the nature of cation adsorption in this important phyllomanganate. Drawing from previous work correlating observed 23 Na NMR shifts in manganese oxides with local environment and Mn oxidation state, the 23 Na NMR spectra of metastable buserite and two birnessite samples, NaBi-H-I and NaBi-II, were analyzed to determine the nature of the bound interlayer sodium ions in these materials. The small 23 Na chemical shift of buserite shows that its interlayer sodium is fully hydrated. X-ray diffraction indicates that NaBi-H-I is a disordered birnessite while NaBi-II is highly crystalline. High-field (14.1 T) fast MAS NMR spectra of NaBi-H-I and NaBi-II supports these observations, resolving multiple sodium environments for NaBi-H-I and only two sodium environments for NaBi-II. The observed hyperfine shifts were less than expected for sodium environments with manganate layers composed of 2/3 Mn 4+ and 1/3 Mn 3+ ions, and the 23 Na line shapes indicated that the Na+ ions are in distorted environments. Both these factors suggest that the sodium ions are offset in the interlayers toward a single oxide layer and located near Mn 3+ -rich environments within the layer.

Among the polymorphs of the compound As 4 S 4 , realgar and β-As 4 S 4 exhibit an interesting phenomenon of light-induced alteration that eventually leads to the transformation to pararealgar and arsenolite through the structural modification of the As 4 S 4 molecule. The mechanism generally invoked to explain the transformation assumes reaction with oxygen, subsequent modification of the molecule through an insertion of a sulfur atom and the eventual production of arsenolite according to the reaction 5As 4 S 4 + 3O 2 → 4As 4 S 5 + 2As 2 O 3 . Early studies showed that the light-induced transition from realgar to pararealgar is reversible through heat and that implies a transition through the χ-phase, even though the presence of arsenolite was not observed. To further assess the action of the oxygen during the process, we carried out experiments of light-induced alteration of β-As 4 S 4 under ambient air and under isopropyl alcohol. The material was investigated by means of X-ray powder diffraction (XRPD) using quantitative phase analysis (QPA) and the Rietveld method. The further study of the heat-induced transformation of the products showed that β-As 4 S 4 exhibits a dual behavior: if the light-induced alteration occurs under air, arsenolite plus an amorphous phase is produced and the transformation is not reversible, if the alteration occurs without any contact to air none of such phases is produced and the transformation is reversible. These new experimental evidences suggest that the production of arsenolite is not strictly required for the transformation of the β-As 4 S 4 into pararealgar and that the current model invoked to explain the mechanism of alteration should be modified to take into account the dual behavior of the β-As 4 S 4 altered by light.

Despite its important geophysical implications, direct probing of the local electronic structure of mantle minerals, such as MgSiO 3 perovskite and post-perovskite is experimentally challenging. Recent advances in ab initio calculations have allowed us to explore the details of the local electronic bonding structure around oxygen in MgSiO 3 polymorphs in Earth’s interior. Here, we calculate the O K-edge energy-loss near-edge structure (ELNES) spectra for SiO 2 and MgSiO 3 polymorphs (i.e., α-quartz, stishovite, enstatite, ilmenite-type MgSiO 3 , MgSiO 3 perovskite, and post-perovskite) using ab initio calculations based on the full-potential linearized planewave (FP-LAPW) method. The calculated O K-edge ELNES spectra for SiO 2 and MgSiO 3 polymorphs show characteristic oxygen K-edge features caused by distinctive local atomic configurations and topology around oxygen, and are in good agreement with previous experimental O K-edge X-ray Raman scattering (XRS) results. The O K-edge ELNES spectra for α-quartz and enstatite show similar edge features at ~538 eV, which is characteristic of corner-sharing oxygen sites ( [4] Si-O- [4] Si). The spectra for stishovite and ilmenite-type MgSiO 3 show edge features with double peaks at ~537-538 and ~541-543 eV due to an electronic excitation from an oxygen in edge-sharing topology. The spectrum for MgSiO 3 perovskite shows a broad peak spanning from ~538 to ~543 eV, which results from corner-sharing oxygen with two six-coordinated silicon ( [6] Si-O- [6] Si). The calculated O K-edge ELNES spectrum for MgSiO 3 post-perovskite shows a predicted main feature at ~543-545 eV, approximately 3 eV higher than that of MgSiO 3 perovskite. These O K-edge features systematically shift to higher energy with increasing degree of densification in atomic arrangement in the polymorphs (from enstatite, ilmenite, perovskite, to post-perovskite), indicating an increase in the energy of unoccupied oxygen 2p-state with pressure. The calculated O K-edge spectra also show the effect of densification on the changes in the edge features for the crystallographically distinct oxygen sites: the features for the corner-sharing oxygen move to higher energy from enstatite, perovskite, to post-perovskite. A drastic peak shift for edge-sharing O atoms in ilmenite-MgSiO 3 and post-perovskite is also observed. These results confirm that the oxygen K-edge features at ~540-550 eV for MgSiO 3 glass at pressures above ~20 GPa can be due to densification of the atomic configurations around oxygen in melt networks associated with enhanced proximity between oxygen atoms. The current methods also shed light on a unique opportunity to probe the pressure-induced electronic bonding transitions and topology in diverse simple and complex oxides in Earth’s interior using ab initio calculations of O K-edge ELNES spectra.

(Al 1-x Cr x ) 3+ Nb 5+ O 4 (with 0 ≤ x ≤ 0.5) compounds have been investigated through the combination of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In spite of the natural occurrence of AlTaO 4 , the lack of a mineral with composition AlNbO 4 contrasts with the strong geochemical affinity between Nb and Ta elements. Rietveld refinements of XRPD data showed that the effective coordination numbers of the two non-equivalent octahedral sites (M1 and M2) in the AlNbO 4 structure are much lower than expected, especially the one mainly occupied by Nb. This is in agreement with the very low crystal field strength values (10Dq) found by EAS for Cr 3+ replacing Al at site M2. These findings imply that an unfavorable bonding situation occurs for Nb, Al, and Cr ions in the AlNbO 4 structure, which can be regarded as substantially strained compared to AlTaO 4 , thus explaining the lack of a natural AlNbO 4 isomorph. The observed long local Cr-O distances (low 10Dq) reveal that the AlNbO 4 lattice is not relaxed as a consequence of the Cr-Al substitution (the relaxation coefficient ε is close to zero) and the AlNbO 4 structure appears to follow the Vegard’s law. This is due to the fact that the Cr 3+ for Al 3+ substitution, for the limited range of solid solution (up to 0.2 apfu at site M2), does not induce any additional octahedral strain in a lattice already significantly strained.

Europium L III -edge XAS spectra were recorded for silicate glasses of different compositions, quenched from melts equilibrated at different oxygen fugacity (f O₂ ). The Eu XANES spectra vary systematically with glass composition and with f O₂ (-log f O₂ ~0 to ~11.9) indicating changes in the Eu oxidation state. The intensity of the main peaks on the absorption edges were quantified and used to determine the Eu 2+ /(Eu 2+ +Eu 3+ ) ratio. All the Eu-bearing glasses synthesized in air show the prevalent presence of Eu 3+ but also, unexpectedly, the presence of a small amount of Eu 2+ in the basaltic glasses and up to 20% of Eu 2+ in the haplogranitic sample. Moreover, XANES analyses of the samples synthesized at reducing conditions (from FMQ to IW-2) show that europium in haplogranitic glasses is always more reduced than in basaltic glasses. No relationship has been found between Eu valence and alkali content in the studied glasses. The structural environment of Eu in the glasses was determined by EXAFS analyses, demonstrating the different Eu behavior as function of the f O₂ . In fact, in air, Eu 3+ both for basaltic and haplogranitic compositions, is bonded to six O atoms in a regular octahedron (CN = [6 ± 0.5]) with similar <Eu-O> distances of about 2.30 ± 0.02 Å. On the other hand, the almost purely divalent samples have Eu2+ in a higher coordination (CN = [9 ± 1]) and longer <Eu-O> distances (2.68 ± 0.02 Å). This work clearly demonstrates that, in addition to oxygen fugacity, melt composition also plays a strong role in affecting Eu oxidation state. Moreover, for the first time, experimentally derived structural data of Eu 2+ in silicate glasses of geological interest are presented.

A multi-cycle heating and cooling thermogravimetric (TG) method was used to study the kinetic behavior of partially dehydroxylated illite, aluminoceladonite, and dioctahedral smectite samples. The method consists of consecutive heating cycles separated by intervals of cooling to room temperature, with the maximum cycle temperatures (MCTs) set incrementally higher in each consecutive cycle. In the studied samples, dehydroxylation of each portion of the initial OH groups follows the kinetics of a homogeneous zero-order reaction in each heating cycle. The activation energies (E a ) were calculated in terms of this model for separate heating cycles of each sample with regression coefficients R 2 ≥ 0.999. A zero-order reaction determined at each heating cycle indicates that at each stage of partial dehydroxylation, there is no formation of an intermediate phase and the reaction is probably the direct transformation of the original layers into completely dehydroxylated layers. The Wyoming montmorillonite and illite consisting of cis-vacant (cv) layers had the highest E a values (53-55 kcal/mol). In the samples consisting of trans-vacant (tv) layers and having almost the same octahedral cation composition the maximum E a values varied from 45 to 30 kcal/mol and the E a of each sample in this group are similar over a wide range of the D T . For the samples consisting of a mixture of cv and tv illite structures, a bimodal distribution of the E a values exists with increasing MCT and D T . The maximum E a values for dehydroxylation of the tv and cv illite structures are different. The activation energies from the tv aluminoceladonite and Otay tv montmorillonite samples have similar maximum E a values (39.4 to 41.8 kcal/mol), but the variation in E a with D T has a skewed bell-like distribution that is probably related to a heterogeneous octahedral cation composition of the 2:1 layers. The E a values calculated for the mineral structures in this study are compared with those published and the main factors controlling the E a variation at different stages of the partial dehydroxylation are discussed.

Iron is a common constituent in minerals from the Earth’s crust and upper mantle and often occurs in minerals as mixtures of two valence states, Fe 3+ or Fe 2+ . Quantification of the values of Fe 3+ /Fe Total , where Fe Total = Fe 3+ +Fe 2+ , in minerals may be necessary to accurately apply certain mineral equilibria to determine equilibrium values of important variables such as temperature (T), pressure (P), and oxygen fugacity (ƒ O₂ ). Most useful would be an analytical technique that permits determination of values of Fe 3+ /Fe Total within a single mineral grain that is contained within a standard petrographic thin section, and the excellent spatial resolution and relative accessibility of the electron microprobe (EMP) have resulted in various attempts to use this instrument to determine values of Fe 3+ /Fe Total . These efforts have typically involved quantifying characteristics of the FeLα and/or FeLβ peaks. In this paper, we employ the method of Fialin et al. (2001), who have shown that the location of the FeLα peak changes as a function of Fe content and values of Fe 3+ / Fe Total , to determine values of Fe 3+ /Fe Total in amphiboles. We have characterized the FeLα peak in several amphiboles with known values of Fe 3+ /Fe Total using the electron microprobe at Texas A&M University. Initial analyses employed a beam current of 20 nA in an effort to avoid Fe-oxidation due to electron beam generated H-loss (Wagner et al. 2008). Subsequent analyses were conducted at 100 nA, and the results are consistent with the 20 nA data only when relatively short duration analytical times were used. The position of the FeLα peak was determined for three suites of amphiboles that have been experimentally treated such that grains in any one of these mineral suites are chemically identical except for differences in the values of Fe 3+ /Fe Total . A linear relation between the FeLα peak location and value of Fe 3+ /Fe Total was observed for each of these three amphibole suites. These three lines differ from one another in both their slope and intercept and these differences vary as a function of Fe content. Thus, these amphiboles served as the basis for the derivation of a relation between Fe content and FeLα peak location, both measured with the EMP, and the value of Fe 3+ /Fe Total as originally determined with 57 Fe Mössbauer spectroscopy. The relation between the relative peak position (RPP = hematite standard FeLα peak position - amphibole FeLα peak position), Fe content, and Fe 3+ /Fe Total is Fe 3+ /Fe Total = RPP - RPP(0)/RPP(1) - RPP(0), where RPP(0) = -1.37 × FeO 2 + 19.59 × FeO - 3.85, RPP(1) = -1.25 × FeO 2 + 21.39 × FeO + 13.05, and FeO refers to the wt%FeO. This relation reproduces the measured values of Fe 3+ /Fe Total to within ±0.07 and, therefore, should permit determination values of Fe 3+ /Fe Total in amphiboles with Fe contents from 7 to 13 wt% FeO with similar precision. The amphiboles that were used in this study were kaersutites, Ti-bearing pargasites, and pargasitic hornblendes. The calibration presented here should, at the very least, be applicable to amphiboles with similar compositions, and although further verification is necessary, this calibration may be useful for determining values of Fe 3+ /Fe Total in amphiboles with distinctly different compositions and may even be more universally applicable.

Ammoniumillite (NH 4 -I) is one of the alteration products present in a breccia structure in the fossil hydrothermal system from Harghita Bãi (East Carpathians), Romania. A series from smectite (S) via ordered interstratified structures to NH 4 -I (40 to 5%S) was characterized by Xray diffraction (XRD), Fourier transforminfrared spectroscopy (FTIR), scanning and transmission electron microscopy (SEM and TEM), and chemical analyses. Calculation of one-dimensional Xray patterns was simulated with the NEWMOD code. Transition from two- to one-water smectite interlayer was identified by XRD. Selected samples were saturated with K + , Mg 2+ , and Li + cations to differentiate low- to high-charge smectite or beidelite layers. X-ray patterns of random powders of K + saturated samples, heated at 300 °C show a transition from 1Md to cis- and trans- vacant 1M polytype. The cell parameters of the cisvacant and transvacant 1M polytype were calculated by oblique texture electron diffraction. The vibration frequencies at 1430 cm -1 of the NH bond were identified in the samples analyzed. Scanning and transmission electron microscopy images show morphological changes from flaky to lathlike shapes. The mean shape ratio of lath crystals ranges from 6 to 5.42 nm and the mean area from 7.8 to 24 × 10 4 nm 2 . The mean thickness of the NH 4 -I layers ranges from 4.62 to 7.89 nm. The calculated structural formula of end-member NH 4 -I (5%S) is: [(NH 4 + ) 0.66 K + 0.10 Na + 0.01 Sr 2+ 0.02 ] 0.81 (Al 3+ 1.85 Fe 3+ 0.01 Mg 2+ 0.15 ) 2.01 (Si 4+ 3.30 Al 3+ 0.70 ) 4.00 O 10 (OH) 2 . The fixed NH 4 + content quantified ranges from 0.39 to 0.66 atoms per half unit cell [O 10 (OH) 2 ]. Tetrahedral and octahedral substitutions took place as the %S decreases. The NH 4 -I-S series formed via direct precipitation from solution at different temperatures.

Allanite-(Nd), ideally CaNdAl 2 Fe 2+ (SiO 4 )(Si 2 O 7 )O(OH), the Nd-analog of allanite-(Ce), occurs in the Åskagen pegmatite, central Sweden. It forms fine-grained aggregates within altered thalénite-(Y). The other associated minerals include: iimoriite-(Y), keiviite-(Y), allanite-(Y), and tengerite-(Y). Allanite-(Nd) is biaxial (-); with refractive indices α = 1.723(5), β = 1.754(7), γ = 1.772(5), and measured 2V = 82°(±3°). Larger fragments of allanite-(Nd) are moderately pleochroic (α = pale grayishbrown, γ = grayish-brown); small fragments are colorless. Allanite-(Nd) is monoclinic, space group P2 1 /m, with the following unit-cell parameters: a = 8.8897(5), b = 5.7308(2), c = 10.1010(6) Å, β = 115.166(7)°, V = 465.75(4) Å 3 , Z = 2. The strongest five peaks from the X-ray powder diffraction patterns [d (Å)(I)(hkl)] are: 3.51(46)(2̅11), 2.89(100)(1̄13), 2.87(45)(020), 2.70(60)(120), 2.61(60)(3̅11). Allanite-(Nd) has a density of 3.98 g/cm 3 , vitreous luster, grayish-brown streak, and the Mohs hardness is ca. 6. It is brittle, with imperfect cleavage, and conchoidal fracture. The Mössbauer spectroscopy revealed Fe 3+ /Fe tot ratio of 0.27 in the type sample. The refined empirical formula of the crystal used for the structure determination is: A1 (Ca 0.957 Mn 0.043 ) Σ1.000 A2 (Nd 0.337 Ca 0.154 Sm 0.131 Ce 0.125 Y 0.112 Gd 0.062 Pr 0.051 Dy 0.016 La 0.007 Er 0.005 ) Σ1.000 M1 (Al 0.860 Fe 3+ 0.140 ) Σ1.000 M2 Al M3 (Fe 2+ 0.760 Al 0.112 Fe 3+ 0.070 Mg 0.058 ) Σ1.000 T1,2,3 Si 3 O 11.978 (OH) 1.032 . The allanite-(Nd) was approved by CNMNC (IMA 2010-060).

Tsilaisite, NaMn 3 Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 OH, is a long-expected new mineral of the tourmaline supergroup. It occurs in an aplitic dike of a LCT-type pegmatite body from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, and schorl. Crystals are greenish yellow with a vitreous luster, a white streak, and show no fluorescence. Tsilaisite has a Mohs hardness of approximately 7; it is brittle with a sub-conchoidal fracture, and has a calculated density of 3.133 g/cm 3 . In plane-polarized light, tsilaisite is pleochroic, O = pale greenish yellow, E =very ale greenish yellow; it is uniaxial negative, ω = 1.645(5), Ɛ = 1.625(5). Tsilaisite is rhombohedral, space group R3m, a = 15.9461(5), c = 7.1380(3) Å, V = 1571.9(1) Å 3 , Z = 3. The strongest eight ‑raydiffraction lines in the powder pattern [d in Å(I)(hkl)] are: 3.974(100)(220), 2.942(94)(122), 2.570(79) (051), 2.034(49)(152), 4.205(41)(211), 6.329(22)(101), 2.377(21)(003), and 1.592(21)(550). Analysis by a combination of electron microprobe, secondary ion mass spectrometry, and optical absorption spectroscopy gives SiO 2 = 36.10(3), TiO 2 = 0.32(4), Al 2 O 3 = 37.10(5), MnO = 9.60(10), CaO = 0.09(4), Na 2 O = 2.11(7), K 2 O = 0.03(1), F = 0.79(3), B 2 O 3 = 10.2(6), Li 2 O = 0.8(1), H 2 O = 3.1(2), sum 99.95 wt%. The unit formula is X(Na 0.67 ⃞ 0.30 Ca 0.02 K 0.01 ) Y (Mn 2+ 1.34 Al 1.14 Li 0.54 Ti 0.04 ) Z Al 6 T (Si 5.94 Al 0.06 )B 2.91 O 27 V (OH) 3 W (OH 0.39 F 0.41 O 0.20 ). The structure, refined also taking into account the positional disorder of the O1 and O2 anions, converged to statistical indices R1 for all reflections of about 2%. The resulting site populations indicate that the Z site is occupied by Al and that the Y site is dominated by Mn 2+ . Aluminum is incorporated at Y through two types of substitutions: Y Al+ W O 2- → Y Mn 2+ + W OH, which has the result of replacing OH at the W site by O 2- , and Y (Al+Li)+ W F → 2 Y Mn 2+ + W OH, which relates fluor-elbaite to the tsilaisite component. Infrared absorption spectra measured in the principal OH-stretching region were interpreted on the basis of local arrangements consistent with the short-range bond-valence requirements. A compositional trend from fluor-elbaite to tsilaisite is observed in samples from Elba Island. The occurrence of tsilaisite is very rare in nature, as a consequence of both the requirement of extraordinary petrogenetic conditions and limited structural stability.

In situ observations of the growth of the anhydrite (100) surface in contact with supersaturated aqueous solutions under conditions within the stability field of this mineral (60-120 °C) were conducted using a hydrothermal atomic force microscope (HAFM). Advancement rates were measured for [001] steps, the most stable ones on the anhydrite (100) surface. Isothermal data fit well to linear correlations between step advancement rate and supersaturation; the activation energy for step advancement is 73 ± 5 kJ/mol. This is not significantly higher than activation energies reported for the growth of gypsum (60-70 kJ/mol) and does not support that slow dehydration rates of aqueous calcium is responsible for the well-known difficulty to precipitate anhydrite crystals from supersaturated aqueous solutions at temperatures well above the anhydrite-gypsum equilibrium temperature. The role of structural factors that could inhibit the growth of anhydrite is discussed.

The influence of Mn content on the stability of the high-pressure CaCO 3 phases CaCO 3 -I, CaCO 3 - II, and CaCO 3 -III at 300 K has been investigated up to 40 mol% MnCO 3 using Raman spectroscopy recorded in situ with a diamond-anvil cell at pressures up to 14 GPa. Beyond about 5 mol% MnCO 3 , there is a progressive linear upward shift in the pressure of the CaCO 3 -I → CaCO 3 -II and CaCO 3 -II → CaCO 3 -III transitions, and expansion of the field of the CaCO 3 -II phase, with increase in MnCO 3 content. The shifts in transition pressure are 0.19 GPa/mol% for I → II and 0.26 GPa/mol% for II → III over the 5 to 40 mol% MnCO 3 composition interval, results fully consistent with elevation of transition pressure by the introduction of a smaller cation. However, minor and trace amounts of Mn appear to have a relatively insignificant influence on the pressure of these transitions.

Vorlanite, cubic CaUO 4 heated at temperatures above 750 °C, transforms irreversibly into rhombohedral CaUO 4 , proving that the latter crystallized as a precursor of vorlanite in high-temperature skarns, Vorlanite most probably originated due to pseudomorphic transformation of rhombohedral CaUO 4 , caused by disordering of cations and uranyl-bonds that resulted from α-decay events of uranium. The ease of the transition from rhombohedral CaUO 4 to vorlanite and the fast reversal transition during heating can be explained by the similarity of both structures. Formation of vorlanite prevents CaUO 4 from undergoing metamictization.