Volume 87, Issue 7

Compressibility of solids generally drops as compression proceeds. Bonds shorten and stiffen with density increase in the same structure. However for the B1-B2 phase transition in many measured alkali halides, bond lengths increase in going to the denser phase. And IR mode frequencies decrease (Hofmeister 1997), leading to the counter-intuitive expectation that compressibility should increase in going to the denser B2 phase. Past volume measurements have been equivocal at best in recognizing any such compressibility increase. New thermal equations of state for B1 and B2 KCl from in-situ X-ray diffraction measurements of phase volumes on Station 16.4 of the CLRC Daresbury Laboratory are no less equivocal about the issue of whether compressibility increases or decreases across the transition. In contrast, the new volume measurements show an easily resolved thermal expansion increase in going from B1 to B2 KCl. The product a·K, which is better known than either α or K, increases from 0.0195 ± 0.0005 kbar/°C in B1 to 0.0275 ± 0.0009 kbar/°C in B2 KCl. Yagi (1978) demonstrated a similar increase for KF, also supported mainly by increases in a. This increase can also be seen in RbCl (Walker et al. 2001). Bond weakening indicated by the thermal expansion increase is consistent with the elusive compressibility increase that is expected across the B1-B2 transition but which is not resolved from volume measurements. The thermal effects are more visible than the compressional effects on α·K across the transition. Bond tightening upon decompression reduces α, increases solid viscosity, and hence decreases the Rayleigh number. An upwelling of material undergoing a decompression phase change with decrease of coordination number may have its convective friskiness damped at such a transition

We have determined infrared molar absorptivities for water absorption bands in Fe-bearing and Fe-free andesitic glasses. Water dissolves in andesitic glasses as both hydroxyl groups and molecular water as observed in other silicate glasses. Concentrations of molecular water and hydroxyl species are a strong function of total water content. IR molar absorptivities for Fe-bearing andesite are ε 3570 = 62.32 ± 0.42 L/mol·cm, ε 4500 = 0.79 ± 0.07 L/mol·cm, ε 5200 = 1.07 ± 0.07 L/mol·cm, and ε 1630 = 42.34 ± 2.77 L/mol·cm. Molar absorptivities for Fe-free andesite are 69.21 ± 0.52 L/mol·cm for e3570, 0.89 ± 0.07 L/mol·cm for e4500, 1.46 ± 0.07 L/mol·cm for e5200, and 52.05 ± 2.85 L/mol·cm for ε 1630 . Molar absorptivities show significant compositional dependencies that can be predicted based on tetrahedral cation (Si +4 , Al +3 )/total cation fraction

We present experimental results constraining water activity in H 2 O-N 2 fluids containing 40-90 mol% water at pressures of 6-13 kbar and temperatures of 680-840 °C. In the experiments, the displacement of the brucite dehydration equilibrium was used as a gauge of water activity. The experiments were performed in a conventional piston-cylinder apparatus, with NaCl pressure medium and silver azide, AgN 3 , as a source of nitrogen. Reversals of the dehydration reaction were used to bracket the equilibrium fluid compositions within 3 mol% H 2 O. Water activities were computed from the equilibrium brucite dehydration conditions in pure H 2 O as determined by Aranovich and Newton (1996) using thermodynamic data of Holland and Powell (1998). The experimentally derived activities were fit to a van Laar-type equation that reproduces our compositional data with a standard error of 1.6 mol% H 2 O: RT lnγ 1 = (X 2 ) 2 W{V 1 0 (V 2 0 ) 2 /[(V 1 0 + V 2 0 )(X 1 V 1 0 + X 2 V 2 0 ) 2 ]} where γ 1 is the activity coefficient of H 2 O, Xi is the mole fraction of end-member i (1 = H 2 O and 2 = N2), V i 0 is the molar volume of the pure end-member at the pressure (P) and temperature (T) of interest, and W is analogous to a regular solution parameter. The parameter W was fit as a function of pressure and temperature by the expression W = (A - BT)[1 - exp(-20P)] + C·P 0.3 T, with A = 40005 J, B = 51.735 J/K, C = 14.848 J/(K·kbar 0.3 ), P in kbar and T in K. With these expressions, activityconcentration relations in H 2 O-N 2 fluids can be reconstructed in a broad P-T-X range using any equation of state (EOS) for pure H 2 O and N 2 . The activity-concentration relations are similar to the semi-empirical EOS of Duan et al. (2000) and the theoretical EOS of Churakov and Gottschalk (2002a), although the former somewhat underestimates activities within the experimental pressure range whereas the latter appears to overestimate activities of the components at pressure above 20 kbar

The kinetics of Fe oxidation-reduction in two hydrous rhyolitic melts, one metaluminous and the other peralkaline, have been studied at 800°C, 2kb, for melt water contents from ~5 wt% to saturation and f O2 between NNO-2 and NNO + 3 (NNO = nickel-nickel oxide redox buffer). The metaluminous melt (~1 wt% FeO T ) reached redox equilibrium after 10 hours and the peralkaline one (~3 wt% FeOT) after 3 hours. The kinetics of Fe oxidation and reduction are similar and unaffected by the presence or absence of a hydrous fluid phase. No redox front is observable in the glass as the Fe 3+ /Fe 2+ ratio evolves, implying that the Fe redox kinetics in hydrous silicic melts is rate-limited neither by the diffusion of H 2 nor by the mobilities of divalent cations, as observed for anhydrous basaltic melts. We propose a two-step reaction mechanism that involves: (1) virtually instantaneous diffusion of H 2 in the sample, followed by (2) slower structural/chemical reorganizations around Fe atoms. The overall redox process involving iron and hydrogen in Fe-poor, H 2 O-rich melts is thus reaction-limited and obeys a first-order logarithmic rate law. The relatively slow kinetics of oxidation/ reduction explains why melt Fe 3+ /Fe 2+ can be readily quenched in laboratory experiments. Simulation of oxidation of magmas due to H 2 exchange with wall rocks is performed using these new kinetics laws and two D H2 values extracted from the literature. We demonstrate that the metaluminous composition is not significantly modified whereas the peralkaline composition undergoes important and fast changes of Fe 3+ /Fe 2+ during short processes such as ascent prior to Plinian-style eruptions

The time and temperature evolution of intracrystalline Mg-Al exchange in synthetic MgAl 2 O 4 spinel was studied by single-crystal X-ray diffraction of quenched samples, with the aim of contributing to rock cooling-rate estimation. Flux-grown, homogeneous crystals (0.1 mm in size) were annealed at 1000°C and then were isothermally ordered at 900, 800, and 700°C for 10 seconds to 40 days. The cation ordering process was investigated by measuring the spinel O atom positional parameter u, which had been previously demonstrated to be closely related to the inversion x at equilibrium by the linear equation (R 2 = 0.995): x = 21.396 - 80.714 u. From x = 0.27 after annealing runs, the inversion decreased with time for all of the three ordering runs, and equilibrium was reached after about 4 min at T = 900°C (x = 0.25), about 55 min at T = 800°C (x = 0.23), and about 700 min at T = 700°C (x = 0.21). The Mueller kinetic model was satisfactorily applied to the experimental data. Solution of Mueller’s integral gave the kinetic ordering constants K 900 = 1.12 ± 0.57, K 800 = 0.112 ± 0.047, and K 700 = 0.0171 ± 0.0045 min -1 , corresponding to t 1/2 of 0.6, 6.2, and 40.5 min, respectively. The linear dependence of K with temperature (R 2 = 0.99) was observed, and may be expressed by the Arrhenius equation: ln K = 20.189 - 23722/T (K) An activation energy of 197 ± 22 kJ/mol was obtained for the intracrystalline Mg-Al ordering reaction, which compares well with the value of 217 kJ/mol reported in the recent literature for Mg- Fe 3+ exchange in spinels

An unusually Mn-rich ilvaite sample from the Perda Niedda mine in Sardinia, Italy, was studied in order to clarify the Mn 2+ distribution among the different structural sites, and to observe the structural response of the mineral upon thermally induced oxidation-dehydrogenation. The crystal structure and the chemical composition of one crystal [a = 13.014(5), b = 8.867(3), c = 5.838(4) Å, b = 90.02(4)°] were investigated. X-ray crystal-structure refinement, performed in the Pnam space group, and electron microprobe analyses yielded the formula (Ca 0.98 Mn 2+ 0.02 )(Fe 3+ Fe 2+ )(Mn 0.72 Fe 2+ 0.28 ) (Si 2 O 7 )O(OH). Crystal chemical details, compared to structural data from literature, led to the assumption that Mn 2+ replaces Fe 2+ , mainly at the M2 site. Annealing experiments and structure refinements were performed in the temperature range 400-690°C. No phase transition was observed over the entire temperature range. Oxidation of Fe 2+ at the M1 site, with concomitant dehydrogenation, was deduced from examination of the structural adjustments occurring as the temperature was increased. A useful model to evaluate a possible OH ↔ O 2- substitution in ilvaite was obtained

Inclusions of coesite pseudomorphs in garnets have been found in type I eclogites interlayered with mafic blueschists and carbonate eclogites, and quartz exsolution lamellae in omphacites have been recognized in type II eclogites with pillow structure in western Tianshan, China. Based on mineralogy and petrology, the metamorphic evolution of western Tianshan eclogites can be divided into three stages: (1) pre-peak, ultrahigh-pressure (UHP), eclogite-facies stage (356-443 °C, 8-10 kb); (2) UHP eclogite-facies stage (496-598 ∞C, 25.7-26.7 ± 1 kbar); and (3) retrograde epidote blueschist-facies stage (500-530 °C, 10-12 kbar). Consequently, the eclogites from the western Tianshan region of China have undergone unambiguous UHP metamorphism

Carbonates of ultrahigh-pressure metamorphic (UHPM) origin were discovered in eclogites from western Tianshan, China. In the eclogites, relict magnesite (X Mg = 0.79) occurs as rounded to subidiomorphic inclusions (0.01-0.1 mm) within matrix dolomite, and also as rounded inclusions with thin reaction rims of dolomite in glaucophane. Based on the textural evidence and calculated phase relationship, these eclogites record an early UHPM assemblage overprinted by a blueschist assemblage, which can be explained by the reaction: Using THERMOCALC (v2.75) and the updated internally consistent mineral thermodynamic database of Holland and Powell (1998), we have calculated the P-T grid involving glaucophane (gl), grossular (gr), jadeite (jd), magnesite (mag), dolomite (dol), clinozoisite (cz), coesite (coe), CO 2 , and H 2 O in a 7-component system of NCMASCH (Na 2 O-CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 -H 2 O). The calculated P-T slope of the above reaction is nearly parallel to the temperature axis and suggests that it can be used as an excellent geobarometer. Using the analyzed compositions and mineral-thermodynamic database of Holland and Powell (1998), the above retrograde metamorphic reaction in Tianshan eclogites occurred at 27-28 kbar and 525-607°C calculated from Grt-Cpx geothermometers

Transmission electron microscopy (TEM) reveals that some enstatites from garnet-pyroxenites in the Chinese Dabie-Sulu ultrahigh-pressure (UHP) terrane consist of intimate intergrowths of orthoenstatite (OREN) and clinoenstatite (CLEN), and some grains of pyroxene (MgFe) 2 Si 2 O 6 exhibit inclined extinction. These garnet pyroxenites are enclosed in coesite-bearing eclogite or interlayered with harzburgite and omphacitite. They contain garnet (Prp 54 to Prp 73 ), “enstatite” (En85 to En92), magnesite, and titanomagnetite (Sulu sample) or clinohumite (Dabie sample), and recrystallized at about 4.0-6.5 ± 0.2 GPa, ~750 ± 50°C. All clinoenstatite lamellae in the host orthoenstatite have even numbers of 9 Å (100) fringes without twins, and are oriented parallel to (100) of the host. The lamellae and host phases have nearly identical (MgFe) 2 Si 2 O 6 compositions. The thickest observed CLEN lamellae are ~0.4 mm. Clinoenstatites from Sulu have cell parameters: a = 9.67(7) Å, b = 8.88(6) Å, c = 5.22(3) Å, b = 107.7(2)∞, and V = 427(8) Å 3 , and those from Dabie: a = 9.53(6) Å, b = 8.88(6) Å, c = 5.24(3) Å, b = 107.0(2)°, and V = 424(8) Å 3 . The TEM electron-diffraction patterns indicate that both clinoenstatites possess P2 1 /c structures and are the low-P polymorph of clinoenstatite. The host orthoenstatites have cell dimension: a = 18.32(12) Å, b = 8.88(6) Å, c = 5.20(3) Å for the Sulu sample, and a = 18.24 (12) Å, b = 8.88(6) Å, c = 5.18(3) Å for the Dabie sample. Clinoenstatites in garnet pyroxenites from the Dabie-Sulu UHP terrane may have formed either by inversion from OREN, or by the displacive transformation from high-P clinoenstatite during decompression

Titanites previously reported from high-pressure rocks commonly contain high Al and F contents. This finding has led previous workers to conclude that high Al and F in titanite can expand its stability field to high-pressure and ultrahigh-pressure (UHP) conditions. In this study, a coesite inclusion was identified in titanite of aragonite- and jadeite-bearing gneiss from the Dabieshan UHP metamorphic terrane of eastern China. We also found UHP titanite in carbonate-bearing garnet clinopyroxenite from the Sulu UHP terrane. Interestingly, both types of the UHP titanite mentioned above contain low Al and F contents. Therefore, our data indicate that the high-Al and F contents are not necessary criteria for stabilizing titanite to UHP conditions. It is inferred that the UHP titanite formed via the reaction TiO 2 + CaCO 3 + SiO 2 = CaTiSiO 5 + CO 2 , and that the presence of the UHP assemblage aragonite + rutile + clinopyroxene + coesite in the host rock and a very low X CO₂ value in the coexisting fluid are crucial conditions for the formation of these UHP titanites

Radiation damage and the recrystallization of natural titanite (CaTiSiO 5 ) were studied using infrared spectroscopy in the spectral range from 50 to 7500 cm -1 . The results show that radiation damage leads to systematic changes in spectral features: decreasing absorption and reflectivity, line broadening, and loss of orientational dependence. Strongly damaged titanite shows hydroxylate bands between 2200 and 3500 cm -1 . The band most affected by radiation damage is the Ti-O stretching band near 670 cm -1 . It shifts to 710 cm -1 in the most damaged samples, possibly indicating the presence of TiO 5 complexes in metamict titanite. Titanite glasses (quenched melts of CaTiSiO 5 ) show spectral features different from those of radiation-damaged titanite, especially in the Ti-O and Si-O stretching regions. Annealing radiation-damaged titanites at high temperatures results in the recovery of damaged crystalline regions. Recrystallization near 900 K is characterized by an increase in reflectivity, integrated absorbance, and line sharpening. Different infrared bands show recovery at different temperatures. The restoration of the Ti-O stretching band near 670 cm-1 and an infrared band near 285 cm -1 took place at temperatures of 1200-1400 K. Temperature-induced changes of the OH-absorption bands could be responsible for the previously reported differences in the temperature evolution of infrared spectra of OH species between in situ and quench experiments

Fine structure in the principal OH-stretching bands of amphiboles of the tremolite-ferro-actinolite series have been examined. In samples with partly filled A sites, a broad (composite) band is observed at 3725~3680 cm -1 and is assigned to two types of configurations: (M1M1M3)-OH- A ( Na,K): T1 Si T1 Al in which Al occurs at the T1 site, and (M1M1M3)-OH- A (Na,K)- O3 (O 2 -,F-,Cl-); the component of (M1M1M3)-OH- A (Na,K): T1 Si T1 Si configuration is small, because Na and K at the A site are locally associated with Al at an adjacent T1 site. In tremolite, manganoan tremolite, and Fe 2+ -poor actinolite, a weak shoulder on the principal A band at ~3669 cm -1 is assigned to the configuration M4 Ca M4 (Mg,Fe 2+ ,Mn 2+ ,Na):(MgMgMg)-OH- A ◻: T1 Si T1 Si (◻ = vacancy). Fine structure in the principal bands B (B' and B'') and C (C' and C'') are also observed: the higher-frequency band B'' is assigned to M1 Fe 2+ M1 Mg M3 Mg-OH- A ◻, and the lower-frequency band B' to M1 Mg M1 Mg M3 Fe 2+ -OH A ◻; the higher-frequency band C' is assigned to M1 Fe 2+ M1 Fe 2+ M3 Mg-OH- A ◻ and the lower-frequency band C'' to M1 Mg M1 Fe 2+ M3 Fe 2+ -OH- A ◻. Some broad OH-stretching bands attributed to (M1M1M3)- OH- A ◻: T1 Si T1 Al are observed at 3640~3580 cm-1. In amphiboles of the tremolite-ferro-actinolite series that show a substantial B [(MgMgFe 2+ )-OH] band, a new OH-stretching band (at around 3641 cm -1 ), E, appears near the principal C band (at around 3643 cm -1 ) on heat treatment. The shape of band E is similar to that of the original band B, and its local configuration is O 2- -(MgMgFe 3+ )-OH-. A weak and broad band A** appears at ~3690 cm -1 on heat treatment of some Na-bearing actinolites, and is ascribed to the (MgMgMg)-OH-ANa-O 2 - configuration

The concentration of structural OH in diopside was determined for four granulite facies siliceous marble samples from the Adirondack Mountains, New York, using FTIR spectroscopy. Single-crystal polarized IR spectra were measured on (100) and (010) sections of diopside. The relative intensities of four OH bands in the 3700-3200 cm -1 region vary among the samples, with the 3645 cm -1 band dominating the spectra of diopside from a xenolith at Cascade Slide. Total OH content in the diopsides ranges from 55 to 138 ppm H 2 O by weight. The OH concentration in diopside increases monotonically with increasing f H2O for the sample, as estimated using oxygen isotope systematics for diffusion rates in these samples from Edwards and Valley (1998). There is no significant variation in OH content within a single diopside grain or among diopside grains from the same hand sample. Charge-coupled substitution with M 3+ and Ti 4+ in the crystal structure may have allowed retention of OH in the diopside structure during and after peak metamorphism (~750 °C, 7-8 kbar). The Cascade Slide diopsides have an Fe 3+ /Fe 2+ of 0.98, compared to Fe 3+ /Fe 2+ (0 to 0.05) for the other samples, implying that some loss of hydrogen through oxidation of Fe was possible in this sample. This is the first study we know of which shows that the OH content in anhydrous minerals from natural samples correlates to the rate of oxygen isotope diffusion

The Fe 3+ -Al substitution in synthetic zoisite was studied in the system CFASH at 2.0 GPa and 750°C (compositional range: 0.0-0.14 X ps ). The samples were characterized by powder X-ray diffraction, FTIR, and electron microprobe. Discontinuities in refined lattice parameters at ~0.05 X ps are attributed to two distinct and hitherto unknown modifications, zoisite I (<0.05 Xps) and zoisite II (>0.05 X ps ). The following lattice parameters were derived In both modifications, substitution of Fe 3+ expands the M3 octahedron, resulting in opposed rotations of the corner-linked T1 and T2 tetrahedra of the Si 2 O 7 group. The extent of rotation is limited and controls the maximum Fe 3+ content in zoisite I and II. With increasing Fe 3+ content, zoisite I transforms to zoisite II and zoisite II to clinozoisite. The transformation from zoisite I to II can be classified as a substitutionally induced isosymmetric displacive phase transition. Four significant IR bands were observed at ~3250, ~3195, ~3155, and ~2170 cm -1 . The first three bands are attributed to the configurations Al 2 [M1,2] -O10-H···O2-Al 2 [M1,2] (Al,Fe 3+[M3] , Al 2 [M1,2] -O10- H···O4-Al 2 [M1,2] Fe 3+[M3] , and Al 2 [M1,2] -O10-H···O4-Al 2 [M1,2] Al [M3] . O10-H···O2 is bifurcated between the two symmetrically arranged O2 and O2' atoms. The band at ~2170 cm-1 is interpreted as the first overtone of the bending vibration of O10-H···O2. In analogy with the results from powder XRD the IR bands show discontinuities at ~0.05 X ps , confirming the two modifications of zoisite

To determine the reaction boundary dolomite = aragonite + magnesite, in situ X-ray experiments on dolomite decomposition and synthesis were carried out in the temperature range 900 to 1300 K. Dolomite decomposition experiments were conducted with increasing pressure at constant temperature, and the boundary was determined to be 5.3-5.9 GPa in the temperature range 800-1200 K and 5.9-6.3 GPa at 1300 K. Dolomite synthesis experiments were carried out with decreasing pressure at constant temperature or with increasing temperature at constant press load. The dolomite synthesis boundary was determined to be 6.7-6.9 GPa at 1300 K, 3.7-4.4 GPa at 1100 K, and 2-3 GPa at 800 K. Except at 1300K, the synthesis boundary is much lower in pressure and has a steeper dP/dT slope than the decomposition boundary. The difference in the reaction boundary reflects the different kinetics between decomposition and synthesis reactions, and the former may be closer to the equilibrium phase boundary. The experimental results show that the phase boundary between dolomite and aragonite + magnesite is located at ◻6.4 GPa at 1300 K and has a dP/dT slope of 0.001 ± 0.001 GPa/ K in the temperature range 900 to 1200 K.

The crystal structure of a natural F-bearing chondrodite [Mg 4.64 Fe 0.28 Mn 0.014 Ti 0.023 (Si 1.01 O 4 ) 2 F 1.16 OH 0.84 ; a = 4.7328(1), b = 10.2749(2), c = 7.8756(1) Å, a = 109.065(1)∞; space group P2 1 /b (a unique)] from the Tilly Foster mine (Brewster, New York) and the structure of a deuterated synthetic analogue [Mg 5 (SiO 4 ) 2 F 1.10 OD 0.90 ] were refined at high pressure using single-crystal X-ray and time-of-flight neutron powder diffraction data, respectively. The isothermal equation of state was determined from single-crystal high-precision X-ray diffraction measurements of unit-cell parameters as a function of pressure up to 9.63(1) GPa. A third-order Birch-Murnaghan equation-of-state fit to pressurevolume data results in V 0 = 362.00(2) Å 3 , K 0,T = 117.0(4) GPa and K T ' = 5.6(1). The larger value of K 0,T relative to that determined for the OH end-member confirms that F makes the chondrodite structure less compressible. The axial compressibilities behave anisotropically with fitted parameters a 0 = 4.73282(9) Å, K(a) 0 = 458(3) GPa, and K(a)' = 28.9(9); b 0 = 10.2759(3) Å, K(b) 0 = 298(1) GPa, and K(b)' = 13.4(4); c 0 = 7.8759(2) Å, K(c) 0 = 316(2) GPa, and K(c)' = 16.7(5). X-ray structural investigations at 0.0001, 4.952(7), and 9.550(7) GPa in a diamond-anvil cell reveal a complex compression mechanism. The greatest changes with pressure occur in the M2 octahedra, which are located at the flexion points of the octahedral chain. These octahedra have the longest bonds at ambient conditions due to cation-cation repulsion, which displaces the cation from the centroid of the octahedron. The application of pressure overcomes the effect of cation-cation repulsion inducing a strong bond-length compression. The neutron powder data were collected at 0.0001, 1.26(1), 2.89(2), 3.87(1), 5.27(2), and 7.04(2) GPa in the Paris-Edinburgh cell using a new gasket design and a fluid pressure medium. Changes in the O-D···O/F hydrogen bond geometry with pressure can be interpreted in terms of a rotation of the OD vector and a reduction in the degree of static disorder associated with the hydrogen bond acceptor/donor site. These new crystallographic data are discussed in terms of corresponding spectroscopic measurements reported recently in the literature

Hydrous ferric oxides (HFO) play a tremendous role in the environment, and as a result, they have often been analyzed for their bulk chemical and mineralogical composition, but rarely at the particle level. This study used a combination of transmission electron microscopy (TEM), parallel electron energy loss spectroscopy (TEM-PEELS), and selected area electron diffraction (TEMSAED), to determine the morphology, stoichiometry, and mineralogy of synthetic biotic HFO and assess the effect of specimen preparation (i.e., dehydration) on their overall chemical composition. A mixture of synthetic lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) was used as reference material for all TEM analyses. HFO were formed by the oxidation of Fe 2+ (pH = 8) in the presence of Bacillus subtilis (bacteria without extracellular polymers) and Bacillus licheniformis cells (with extracellular polymers). Biotic Fe-oxides occurred as small particles (5-10 nm diameter) on the bacterial cell walls and associated exopolymers and were identified as 2-line ferrihydrite by TEMSAED. Quantitative TEM-PEELS results showed that (1) freeze-dried abiotic samples had an Fe/O atomic ratio of 0.54 ± 0.03, whereas the Fe/O ratio of the biotic ferrihydrite formed in the presence of B. licheniformis and B. subtilis was 0.48 ± 0.06 and 0.54 ± 0.08, respectively; (2) air-dried biotic ferrihydrite possessed an Fe/O ratio close to 0.30, whereas the ratio of the abiotic air-dried HFO samples remained unchanged (i.e., 0.51 ± 0.04). These results indicate that quantitative TEM-PEELS can be used for the chemical characterization of Fe-oxides, since the measured Fe/O atomic ratios of the reference HFO are in agreement with their known stoichiometry (i.e., 0.50 for FeOOH). Our results also suggest that ferrihydrite dehydrates during freeze-drying and that specimen preparation is very important when assessing chemical composition

Apatites in a fluorine-free chemical system have been synthesized hydrothermally at 650 ∞C and 1.5 kbar along the hydroxylapatite-britholite-(Y) join [i.e., Ca 10 (PO 4 ) 6 (OH) 2 - Ca 4 Y 6 (SiO 4 ) 6 (OH) 2 join], from oxide mixtures at nominal Si-contents of 0, 0.5, 1, 2, 3, 4, 5.5, and 6 pfu. The hexagonal apatite unit-cell volume decreases by 1.5% from the phosphate to the silicate end-member. A single 31 P MAS NMR resonance is recognized at 2.8 ppm in hydroxylapatite. An additional broad line centered around 1.5 ppm is present for Y-rich compositions. Although the 31 P MAS NMR spectra could not be fully assigned, the 2.8 ppm resonance that persists even in Si-rich compositions must include the contribution of P atoms involved in P-O-Y bonds. 1 H NMR spectroscopy shows that the H content decreases by around 50% from the phosphate to the silicate end-member although all the compounds were synthesized hydrothermally at 650°C. In addition to the expected YSiCa -1 P -1 substitution, a second substitution vector, Y◻Ca -1 H -1 , is inferred. It is proposed that the resulting proton vacancies influence neighbor H atoms to give rise to a 1 H line at 4.9 ppm. Beside the 4.9 ppm resonance, four other resonances at 0.2, 1.2, 1.5, and 2.0 ppm, are encountered along the series. In the apatite columns, OH groups are bonded to three cations from the Ca2 site and form (Ca2) 3 OH groups. In the proposed assignment model, the 0.2 ppm line, the only resonance present in the hydroxylapatite spectrum, is readily attributed to protons from (Ca)3OH groups. The 1.2 and 1.5 ppm resonances are assigned to (Ca 2 Y)OH groups whereas the 2.0 ppm line represents the contribution of protons from both (CaY 2 )OH and (Y) 3 OH groups

The structure of dioctahedral 2:1 phyllosilicates with different interlayer charge was studied theoretically using ab initio calculations. The standard Kohn-Sham self-consistent density functional method was used in the generalized gradient approximation (GGA), with numerical atomic orbitals as the basis set, by means of the SIESTA program. Once the method had been checked satisfactorily, the theoretical study was extended beyond the systems for which there are experimental information, and structural characteristics were predicted. The SIESTA program was shown to be a useful tool in studying the crystallographic properties of 2:1-dioctahedral phyllosilicates. The crystal structures of pyrophyllite, beidellite, and other smectites and illites were simulated. The experimental crystallographic properties of phyllosilicates with high, medium, and low interlayer charge were reproduced. The calculated structures agree with the main experimental structural features of the crystal lattice of these minerals. The effect of cation substitutions in the octahedral and tetrahedral sheets was also studied. The calculated effects are consistent with experimental results. The Mg 2+ cations were found to have a tendency to be distributed in the octahedral sheet, in contrast to Fe 3+ ions that tend to be clustered

The planar disorder of Ca-montmorillonite (Fuller’s earth) has been investigated using structural simulations of X-ray powder patterns. A standard sample was fully characterized using chemical, microscopic, and diffraction methods. Earlier models of disorder taken from the literature and newly formulated combined models were used to generate simulated powder patterns to be compared with the experimental spectrum. A new model of disorder with random shifts of -a/3 and ±b/3, with a total density of defects of 75%, gives the best fit to the observed data. Thus, the sample cannot be classified as a turbostratic structure (fully disordered) and consequently turbostratic disorder does not invariably apply to all smectite samples. These findings open a debate on the nature and application of turbostratic disorder: is it possible for smectite samples to have intermediate degrees of disorder between a fully disordered stacking (turbostratic) and a highly faulted but well-defined stacking or is the result obtained for the Ca-montmorillonite just an exception? This model of disorder is useful for the quantitative phase analysis by X-ray powder diffraction based on the Rietveld method, which can now benefit from a more reliable initial structure model for Ca-montmorillonite and which will improve the accuracy of the weight-fraction estimates

Ferripedrizite, ideally A Na B Li 2 C (Fe 3+ 2 Mg 2 Li)TSi 8 O 22 X (OH) 2 , is a new amphibole end-member found in episyenites formed after cordierite-bearing porphyritic granites in the Eastern Pedriza Massif (Central System, Spain). It contains the maximum amount of Li that can be incorporated in the amphibole structure. The name was approved by the IMA-CNMMN together with restriction of the use of the prefix sodic in the pedrizite series to compositions with B Na > 0.5 apfu; its use for compositions with Na tot > 0.5 apfu has been maintained in the rest of the Mg-Fe-Mn-Li group. Complete solid solution between ferripedrizite and leakeite [ideally, A Na B Na 2 C (Fe 3+ 2 Mg 2 Li) T Si 8 O 22 X (OH) 2 ] has been found in the Pedriza Massif. According to the present nomenclature rules, this join bridges three different amphibole groups. Samples with B (Mg+Fe+Mn+Li) ≥ 1.0 apfu and 0. 0 £ B Na £ 0.50 apfu belong to the Mg-Fe-Mn-Li group and are termed ferripedrizite; samples with B (Mg + Fe + Mn + Li) ≥1.0 apfu and 0.50 < B Na £ 0.99 apfu belong to the Mg-Fe-Mn-Li group and are called sodicferripedrizite; samples with B (Mg + Fe + Mn + Li) < 1.0 apfu and B Na ≥ 1.50 apfu belong to the sodic group and are named leakeite; samples with B (Mg+Fe+Mn+Li) < 1.0 apfu and 1.0 £ B Na < 1.50 apfu belong to the sodic-calcic group (albeit Ca is negligible) and deserve a new root name. The ferripedrizite sample from Pedriza is black, vitreous, translucent, non-fluorescent, and brittle, and has gray streak, H = 6, uneven fracture, perfect {110} cleavage, D meas = 3.15, D calc = 3.19 g/cm 3 . It is strongly pleochroic, X = yellow green, Y = green blue, Z = bluish green (Y = Z >> X), Z = b, Y ^ c = 15(6)°, X ^ a = 3°. It is biaxial positive: a = 1.695(1), b = 1.700(2), and g = 1.702(1); 2VZ = 125(17)°, dispersion r > v. It is monoclinic, space group C2/m, a = 9.501(1), b = 17.866(2), c = 5.292(1) Å, b = 102.17(2)°, V = 878.1(2) Å 3 . The ten strongest lines in the X-ray powder-diffraction pattern [d in Å,(I),(hkl)] are: 8.251(3)(110), 4.466(2)(040), 3.411(2)(131), 3.050(10)(310), 2.747(3)(330), 2.711(4)(151), 2.495(2)(2-02), 2.161(2)(261), 1.642(4)(461), 1.394(3)(6-61). Structure refinement and electron- and ion-microprobe analysis of a crystal with composition A(K 0.04 Na 0.52 ) B (Na 0.25 Ca 0.05 Li 1.70 ) C (Li 0.64 Fe 3+ 1.64 Mg 1.49 Fe 2+ 0.85 Al 0.21 Ti 0.09 Mn 0.07 Zn 0.01 ) T Si 8 O 22 X (OH 1.31 F 0.69 ) are provided, together with some discussion on cation ordering

Vajdakite, a new mineral from Jáchymov, NW Bohemia, Czech Republic, forms minute acicular, gray-green crystals associated with arsenolite, scorodite, parascorodite, kanˇkite, annabergite, köttigite, pyrite, marcasite, nickelskutterudite, and löllingite. Microprobe analysis gave (in wt%): As = 27.72, Mo = 35.39, O = 36.66, total = 99.77. The simplified chemical formula is [(Mo 6+ O 2 ) 2 (H 2 O) 2 As 3+ 2 O 5 ]·H 2 O. The mineral is monoclinic, P2 1 /c, a = 7.0515(6), b = 12.0908(9), c = 12.2190(14) Å, b = 101.268(9)°, V = 1021.7(2) Å 3 , Z = 4, D meas = 3.50(2) g/cm 3 , and D calc = 3.509 g/cm 3 . The strongest lines in the powder X-ray diffraction pattern d(I)(hkl) are: 6.046 (100)(020), 3.324 (59)(023), 6.915 (26)(100), 2.264 (19)(310), 3.457 (16)(200), 2.624 (15)(230), and 3.819 (10)(031). Vajdakite is optically positive, with X || b and Z . c =12; elongation is positive. Its birefrigence is 0.28, with 2V calc = 35.1° n α = 1.757(2), n β = 1.778(2), and n γ = 2.04(1). The pleochroic scheme is X ~ Y = light greenish gray, and Z = yellowish gray. Crystal size varies between 0.1 to 0.5 mm. TG curve and IR spectra show that vajdakite contains two distinct types of water molecules. The crystal structure was solved by direct methods (MoKα radiation) and refined using 1787 unique reflections to R = 0.0455, Rw = 0.1143. There are double chains built up by two individual chains with a sequence -O-As-OMo- interconnected by oxygen atoms from two triangular AsO 3 groups and two structurally nonequivalent MoO 5 (H 2 O) octahedra. The two vertex-sharing, triangular AsO 3 groups form an (As 2 O 5 ) 4 - diarsenite group. The first type of water molecule is not included in the coordination, but the second one is in octahedral coordination around Mo. The water molecules are linked by a complicated net of interlayer and intralayer hydrogen bonds

I comment on some aspects of fission-track observational biases and orientation effects arising in recent work by Donelick, Ketcham, and Carlson, and their relation to earlier work by myself and colleagues. Isotropic and anisotropic random line segment models provide a sound basis for understanding variation in fission-track measurements, and some useful mathematical formulae are available. The effects of orientation bias can be avoided, without introducing arbitrary assumptions, by modeling confined track length conditional on angle to the c axis

Synchrotron Radiation X-ray Photoelectron Spectroscopy (SRXPS) and conventional X-ray Photoelectron Spectroscopy (XPS) of pristine (unreacted) loellingite fracture surfaces reveal two distinct contributions to the As 3d spectrum. One is derived from As atoms of the bulk phase (fully coordinated As atoms) and the other is derived from polymeric As species located on fracture surfaces. Surface polymerization reactions probably proceed in response to fracture whereby As dimers at the surface react to produce surface tetramers and possibly surface heptamers. Trimeric and pentameric species may form where As-As bond scission occurs during fracture. The (001) surface of loellingite is autocompensated and unlikely to undergo reconstruction. The (010) and (100) surfaces are, however, arsenic-terminated and uncompensated, and surface reconstruction is expected. The short “c” dimension of the loellingite unit cell is critically important to surface polymerization because arsenic dimers on (100) and (010) surfaces are separated by 2.88 Å (normal to c) which is only 16% greater than the As-As bond length. Because of the short distance, adjacent surface dimers likely tilt toward each other and bond to produce surface tetramers. This should introduce minimal strain and eliminate high-energy dangling bonds thus stabilizing the surface. The polymerization reaction also explains the absence of surface (core-level-shifted) As dimeric and monomeric signals such as observed on pyrite surfaces

The yield strength of moissanite was investigated at pressures up to 18.3 GPa and temperatures up to 1200 °C by analyzing the peak shapes of diffraction lines from a powder sample. At room temperature, the moissanite crystal behaves elastically with increasing pressure up to 13.7 GPa. At higher pressures applied, the sample is yielded; the yield strength of moissanite is determined to be 13.6 GPa. Upon heating at 18.3 GPa, significant stress relaxation is observed at temperatures above 400°C, and the yield strength of moissanite decreases rapidly from 12.8 GPa at 400°C to 4.2 GPa at 1200°C. Such behavior will place severe limitations on the use of moissanite as an anvil material when external heating is desired for high pressure and temperature experiments

Crystallization of diamond and graphite from the carbon component of magnesite, upon its decarbonation in reactions with coesite and enstatite at pressures of 6-7 GPa and temperatures of 1350-1800°C has been accomplished experimentally. In a series of experiments, diamond was obtained in association with enstatite, coesite, and magnesite, as well as with forsterite, enstatite, and magnesite. Octahedral diamond crystals with sizes up to 450 mm were studied by FTIR spectroscopy and were found to contain nitrogen and hydrogen, which are known as the most abundant impurities in natural type Ia diamonds. We found that growth of diamond on the cubic faces of seed crystals proceeds with formation of a cellular surface structure, which is similar to natural fibrous diamonds. The isotopic composition of synthesized diamonds (δ 13 C = -1.27‰) was determined to be close to that of the initial magnesite (δ 13 C = -0.2‰)

Marked variations of biotite Fe-Mg compositional profiles on approach to interfaces with garnet as a function of biotite crystallographic orientation are found in a natural high-temperature metamorphic rock. For biotite with (001) planes nearly parallel to the interface, Mg increases and Fe decreases steeply toward the interface. If oblique, the profile is less steep, and if nearly perpendicular, the profile is flat. These variations are evidences for anisotropic Fe-Mg intradiffusion within biotite during cooling. For precise application of geothermobarometry and geospeedometry, the crystallographic orientation and compositional gradient in biotite must be considered

Recently, there has been substantial interest in post-stishovite high-pressure polymorphs of SiO 2 , discovered as extraterrestrial minerals, or synthesized in the laboratory. Previous investigators reported the presence of “α-PbO 2 -like” and “baddeleyite-like” SiO 2 in the Martian meteorite Shergotty, and also the synthesis of an a-PbO 2 -like phase at pressures of 60-80 GPa in a laser-heated diamond anvil cell. To provide definitive information about the nature of the natural “a-PbO 2 phase,” we recovered a small sample from the Shergotty meteorite, obtained powder X-ray diffraction patterns, and performed a Rietveld refinement of the structure. The resulting cell parameters and space group are a = 4.097(1) Å, b = 5.0462(9) Å, c = 4.4946(8) Å, and Pbcn. The structure refinement confirms that this sample does have the α-PbO 2 structure