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Boron nitride (BN) is a commonly used pressure-transmitting material in experimental petrology. It is often considered to be as inert as MgO or Al2O3, and its redox potential is seldomly discussed. It is generally implied that, when used as a capsule sleeve, BN may impose relatively reduced conditions, similar to the effect of the fayalite-magnetite-quartz (FMQ) buffer. However, sediment melting experiments performed at 1050 °C and 3 GPa with BN as the capsule sleeve, produced a hydrous rhyolitic melt with dissolved H2S and CH4 (Li et al. 2021). The resulting fO2 estimate is significantly more reduced than that for the magnetite-wüstite (MW)-buffered experiment where H2S and CH4 were undetected (Li et al. 2021), possibly to the extent of the quartz-iron-fayalite (QIF) buffered conditions produced when BN is used as a capsule or crucible (Wendlandt et al. 1982). To establish an explanation for such a discrepancy, we have conducted further investigation to better constrain the fO2 imposed by BN, when used as a capsule sleeve. Here we report results on analyses of Fe content in Au capsules, a comparative experiment using a QIF buffer and an experiment with an Fe-(Mg,Fe) O sensor for direct analysis of fO2. The calibration of the equilibrium between FeO in melt and Fe in the Au capsule, from Ratajeski and Sisson (1999) appears to be inadequate in constraining fO2 for our experiments. However, we were able to obtain Fe diffusion coefficients in Au from the Fe diffusion profiles observed in the capsule of the Fe-(Mg,Fe)O sensor experiment, and both the inner and outer capsules of the MW-buffered experiment, with resulting values of 1 × 10–13 m2/s, 3 × 10–14 m2/s, and 5 × 10–14 m2/s, respectively. The QIF-buffered and Fe-(Mg,Fe)O sensor experiments provide several lines of evidence supporting the observation that BN imposes QIF-like redox conditions. First, the Fe-(Mg,Fe)O sensor returned an fO2 value of QIF. Second, the "apparent" partition coefficients between FeO content in melt and Fe in the Au capsules are similar between the BN experiment and the QIF-buffered experiment. Third, we observe CH4 and H2O peaks with similar intensities in the Raman spectra of melts from these two experiments, suggesting similar H2 and thus O2 fugacity. As our experiments were performed on a cubic press with the experimental assembly encased in a pyrophyllite cube, we interpret that the significantly reduced conditions imposed by BN are likely due to high H2O activity maintained by dehydration of pyrophyllite, which can be explained using the reaction 2BN + 3H2O = B2O3 + N2 + 3H2. Lower H2O activity will reduce or inhibit the oxidation of BN and its fO2 buffering ability. If heat-treated, BN acts as a highly efficient H2 barrier, as shown by Truckenbrodt et al. (1997). Through our efforts to determine the fO2 imposed by using BN as a capsule sleeve in our experimental assembly, we are able to demonstrate the reducing ability of BN as an assembly component and, furthermore, shed light on the process by which BN imposes such reduced fO2. We hereby present what we have learned during the course of this investigation in the hope that the effect of BN on fO2 control is both recognized and further exploited in future experimental studies. [ABSTRACT FROM AUTHOR]

The concentration profiles of 13 trace elements (Li, Na, Al, P, Ca, So, Ti, V, Cr, Mn, Co, Ni, and Y) were determined in a natural mantle olivine xenocryst included in a hybridized hydrous dioritic magma during cooling from ~1000[degrees]C, which allow trace-element chemical diffusion coefficients to be evaluated relative to Mg-Fe interdiffusion under conditions that are difficult to access in the laboratory. The effective diffusion coefficients of many elements (Li, Ca, Sc, Mn, Co, Ni, and Y) fell within a factor of three of the corresponding Mg-Fe interdiffusion coefficient, in agreement with results from laboratory experiments at higher temperatures. By contrast, the concentration profiles for Na, Ti, and V implied much faster diffusion rates, while P showed no discernible diffusion. The Al and Cr profiles, which were well correlated with each other, were complex and variable on a small length scale due to local precipitation of spinel. These data show that the diffusion coefficients of cations in olivine are not simple functions of ionic charge or ionic radius, but they are likely controlled by the availability of suitable diffusion pathways. doi: 10.1130/G30788.1

There are four ways by which OH commonly substitutes into olivine, namely those associated with (1) Si vacancies, (2) Mg vacancies, (3) Ti, or (4) trivalent cations. The four mechanisms, which we label [Si], [Mg], [Ti], and [triv], respectively, may each be fingerprinted by their characteristic O-H stretching modes in the infrared spectrum. We show by comparing the integrated intensities of these characteristic infrared peaks against total water content analyzed by secondary ion mass spectrometry, obtained for a suite of synthetic olivines plus one natural olivine, that the different substitution mechanisms require different absorption coefficients (k). For [Ti], we find k = 0.18 [+ or -] 0.07, identical to the value previously obtained from natural olivines in which the water was mainly associated with [Ti] defects. Values of k for [Si] and [triv] are 0.57 [+ or -] 0.04 and 0.18 [+ or -] 0.05, respectively; that for [Mg] is too small to be accurately determined (0.03 [+ or -] 0.03). The values of k for [Ti] and [Si] defects differ by a factor of three even though their average wavenumbers are virtually the same. The [Ti] and [triv] defects, on the other hand, have very similar absorption coefficients at significantly different wavenumbers. This highlights the inadequacy of using wavenumber-dependent calibrations for olivine and presumably for NAMs in general. Different substitution mechanisms have their own crystallographic environments that determine their absorption coefficients. The large variation in absorption coefficients within a single mineral emphasizes the importance of distinguishing the substitution mechanism if meaningful quantitative results are to be obtained from infrared spectroscopy. Keywords: Nominally anhydrous minerals, olivine, infrared spectroscopy, secondary ion mass spectrometry, extinction coefficients DOI: 10.2138/am.2010.3313

The predictions of the theory of light propagation in weakly absorbing anisotropic minerals are tested against systematic measurements of the infrared absorbance spectra ofcalcite, olivine, and topaz oriented in both principal and random sections, using both polarized and unpolarized light. We show that if the linear polarized maximum absorbance is smaller than -0.3, or if the ratio of maximum and minimum absorbance is close to unity, then (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are, to a good approximation, linearly proportional to thickness, regardless of the direction of the incident light; (2) the angular variation of polarized light absorption is indistinguishable from the theoretical predictions within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the polarized maximum and minimum absorbance; and (4) the average unpolarized absorbance of randomly oriented grains is one third of the Total Absorbance (defined as the sure of the three principal absorbances). Therefore, calibrations relating Total Absorbance to absorber concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be applied to measurements with unpolarized light on a population of randomly oriented sections. We show that 10 such measurements are sufficient to achieve a petrologically useful accuracy. The method enables water concentrations in nominally anhydrous minerals to be determined from samples where the preparation of oriented specimens is not feasible, such as high-pressure experimental runs and fine-grained mantle xenoliths. The method may also be used for obtaining quantitative measurements on low-symmetry minerals. Keywords: Infrared spectroscopy, absorbance spectroscopy, unpolarized light, olivine, calcite, topaz, nominally anhydrous minerals

A new approach to the use of spectroscopic absorbance measurements for anisotropic crystals allows results to be extracted using unpolarized light incident on random crystal orientations. The theory of light propagation in anisotropic absorbing crystals is developed from Maxwell's equations to devise an expression for the transmittance of linearly polarized light traveling in an arbitrary direction in weakly absorbing media. This theory predicts the distribution of transmittance and absorbance as a function of direction and polarization angle of incident light. It is shown how a previously deduced empirical expression, commonly used in infrared spectroscopy, is a good approximation to the full theory under a wide range of conditions. The new theory shows that principal polarized absorbances correspond to the eigenvalues of an absorbance ellipsoid. An expression is derived for the unpolarized absorbance as a function of the angles describing incident light direction, [A.sub.unpol]([PHI], [PSI]), and the principal polarized absorbances, [A.sub.a], [A.sub.b], [A.sub.c] in an anisotropic crystal [A.sub.unpol]([PHI],[PSI]) = 1/2[[A.sub.a]([cos.sup.2][PHI][cos.sup.2][PSI} + [sin.sup.2][PSI]) + [A.sub.b]([cos.sup.2][PHI][sin.sup.2][PSI] + [cos.sup.2][PSI]) + [A.sub.c][sin.sup.2][PHI]]. Integration of this expression over all incident angles leads to a simple relationship between total measured unpolarized absorbance and the three principal polarized absorbances. Using this theory, a procedure is proposed for estimating both total ([A.sub.a] + [A.sub.b] + [A.sub.c]) and principal absorbances from spectroscopic measurements of absorbance using unpolarized light on a set of randomly oriented crystals. Keywords: Absorption index theory, anisotropic media, unpolarized light, spectroscopy

Chromium K-edge X-ray absorption near-edge structure (XANES) spectra were recorded at room temperature for 27 CaO-MgO-[Al.sub.2][O.sub.3]-Si[O.sub.2] (CMAS) glass compositions quenched from melts equilibrated at various oxygen fugacities ([f.sub.[O.sub.2]]) at 1400 [degrees]C. Values of [Cr.sup.2+]+/ [summation]Cr were determined from the intensity of a shoulder on the main absorption edge, attributed to the 1s [right arrow] 4s transition, which is characteristic of [Cr.sup.2+] in these glasses. For each composition, [Cr.sup.2+]/[summation]Cr could be quantified as a function of [f.sub.[O.sub.2]], using a theoretical expression, from as few as three samples ([Cr.sup.2+]/[summation]Cr [approximately equal to] 0, 0.5, and 1). This allowed log K', or the reduction potential of the [Cr.sup.3+/2+] half-reaction, and hence the relative change in the ratio [[gamma].sup.melt.sub.[Cr.sup.3+][O.sub.0.15]]/ [[gamma].sup.melt.sub.[Cr.sup.2+] O], to be determined for each composition. At constant [f.sub.[O.sub.2]], log[[Cr.sup.2+]+/[Cr.sup.3+]] was found to decrease linearly with increasing optical basicity. The variation in log K' with composition is controlled by [[gamma].sup.melt.sub.[Cr.sup.3+][O.sub.0.15]], corresponding to the capacity of the melt to stabilize both the charge and the preferred solvation site of [Cr.sup.3+]. The method was then applied to spectra recorded in situ at 1400 [degrees]C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing [Cr.sup.2+]+/[summation]Cr to be quantified in a Fe-bearing melt for the first time. [Cr.sup.2+]+/[summation]Cr was found to vary from ~0.45 at the nickel-nickel oxide (NNO) [f.sub.[O.sub.2]] buffer to ~0.90 at iron-wustite (IW). This indicates that [Cr.sup.2+] is likely to be the dominant oxidation state in terrestrial basaltic melts. Keywords: XANES spectroscopy, chromium oxidation states, silicate melts, MORB

The free energy of formation of zircon (ZrSi[O.sub.4]) from its oxides was determined between 1100 and 1300 K by an electrochemical method, in which values of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] defined by the two assemblages [Fe.sub.2]Si[O.sub.4]-Fe-Si[O.sub.2] (fayalite-iron-quartz) and [Fe.sub.2] Si[O.sub.4]-Fe-Zr[O.sub.4]-ZrSi[O.sub.4] were each measured using oxygen concentration cells with calcia-stabilized zirconia solid electrolytes. The difference in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] between these two assemblages corresponds to the reaction Zr[O.sub.2] + Si[O.sub.2](qz) = ZrSi[O.sub.4]. The results, when analyzed using calorimetric data for the entropies and high-temperature heat capacities of ZrSi[O.sub.4], Zr[O.sub.2], and Si[O.sub.2](quartz), yields [[DELTA].sub.f,ox][H.sup.0.sub.298K] = -24.0 [+ or -] 0.2 kJ/mol for ZrSi[O.sub.4], in good agreement with the calorimetric value of Ellison and Navrotsky (1992). ZrSi[O.sub.4] is predicted to decompose to Zr[O.sub.2] plus Si[O.sub.2] (cristobalite) at 1938 K, assuming a temperature of 1430 K for the martensitic phase transition between the tetragonal and monoclinic forms of Zr[O.sub.2] (baddeleyite), with an enthalpy of transition of 8.67 kJ/mol. The same experimental approach was used also to determine the free energy of formation of hafnon (HfSi[O.sub.4]). The entropy of hafnon ([S.sup.0.sub.298K] = 93.6 J/mol x K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic ([[DELTA].sub.f.ox]/[H.sup.0.sub.298K] = -25.0 [+ or -] 0.2 kJ/mol). The cells with either ZrSi[O.sub.4] + Zr[O.sub.2] or HfSi[O.sub.4] + Hf[O.sub.2] produce an anomalous excursion in EMF when the temperature of the [alpha]-[gamma] transition in Fe metal at 1184 K is traversed; this excursion takes >12 hours to decay back to the equilibrium value. This behavior is presumably related to strain caused by the volume change of the [alpha]-[gamma] transition. The redetermination of the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] of the [Fe.sub.2] Si[O.sub.4]-Fe-Si[O.sub.2](qz) equilibrium (the quartz-fayalite-iron or QFI oxygen buffer) carried out in the course of this study gave results in reasonable agreement with previous work, but with a different slope vs. temperature, implying a slightly higher value of [S.sup.0.sub.298K] for [Fe.sub.2]Si[O.sub.4] than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 [+ or -] 0.2 J/K x mol). Keywords: Zircon, hafnon, free energy of formation, fayalite, thermodynamic data

The effect of pressure on the [Fe.sup.3+]/[summation over Fe] ratio of an anhydrous andesitic melt was determined from 0.4 to 3.0 GPa at 1400 [degrees]C with oxygen fugacity controlled internally by the Ru + Ru[O.sub.2] buffer. Values of [Fe.sup.3+]/[summation over Fe] were determined by Mossbauer spectroscopy on quenched glasses with a precision of [+ or -] 0.01, one standard deviation. This precision was verified independently by XANES spectroscopy of the same samples. The XANES spectra show a systematic increase in energy and decrease in intensity of the 1s [right arrow] 3d transition with increasing pressure. The results to 2.0 GPa are in good agreement with predictions from density and compressibility measurements fitted to a Murnaghan equation of state, but the datum at 3.0 GPa has higher [Fe.sup.3+]/[summation over Fe] than predicted from the trend established by the lower-pressure data. This might be due to a coordination change in [Fe.sup.3+] at high pressure; although there is no evidence for this in the Mossbauer spectra, such a change could account for the change in intensity of the 1s [right arrow] 3d transition in the XANES spectra with pressure. Keywords: redox ratio, silicate melt, XANES, Mossbauer, experimental petrology

The magnetic susceptibility and Curie temperatures [T.sub.c] have been investigated for a series of synthetic samples with solid-solution compositions ranging from pure magnetite ([Fe.sub.3][O.sub.4]) to hercynite (Fe[Al.sub.2][O.sub.4]). The determined [T.sub.c] can be fitted by a straight line, which also fits the theoretical values for these end-members. With increasing hercynite concentration, susceptibility curves for one heating and cooling cycle become irreversible, indicating changes in the structural state of the samples during annealing. These changes occur in specific temperature ranges for each composition. For a sample of composition [Mag.sub.40][Hec.sub.60], irreversible changes occurring between about 200 and 300 [degrees]C are likely due to changes in the cation distribution, whereas above 300 [degrees]C, compositional fluctuations due to spinodal decomposition are evident. The exsolution mechanism has been investigated using energy-filtered transmission electron microscopy, which has allowed direct imaging of the compositional fluctuations consistent with the theoretical predictions of spinodal decomposition.

Cr K-edge X-ray absorption near-edge structure (XANES) spectra were recorded for silicate glasses of various compositions in the system CaO + MgO + [Al.sub.2][0.sub.3] + Si[O.sub.2] [+ or -] Ti[O.sub.2], quenched from melts equilibrated as a function of oxygen fugacity [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] at 1400[degrees]C. The spectra vary systematically with [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] indicating changes in the Cr oxidation state. The intensity of a shoulder on the absorption edge (attributed to the 1s [right arrow] 4s transition) was quantified using the corresponding peak in the XANES derivative spectrum and used to determine [Cr.sup.2+]/[SIGMA]Cr. The resulting [Cr.sup.2+]/[SIGMA]Cr values are in agreement with the theoretical dependence on [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] suggesting that the 1s [right arrow] 4s feature is diagnostic of [Cr.sup.2+] in a silicate glass and [SIGMA]([C.sup.r2+]/[SIGMA]Cr) = [+ or -] 0.015. The [Cr.sup.2+]/[summation of] Cr ratio for a given [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] changes with the glass composition which may reflect the ability of the melt to accommodate the Jahn-Teller distorted coordination geometry which stabilizes [Cr.sup.2+] [x] [Cr.sup.2+]/[SIGMA]Cr varies between ~ 0.3 and 0.8 over the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] range bounded by the Ni/NiO and Fe/FeO [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] buffers, suggesting that [Cr.sup.2+] may be important in natural melts even though this oxidation state has never been identified in a terrestrial material. The development of a methodology for determining [Cr.sup.2+]/[SIGMA] Cr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature.

Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% [sup.57][Fe.sub.2][O.sub.3] quenched from melts equilibrated over a range of oxygen fugacities at 1409 [degrees]C. The [Fe.sup.3+]/ [SIGMA]Fe ratios were determined previously by [sup.57]Fe Mossbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mossbauer results as a reference, various methods for extracting [Fe.sup.3+]/[SIGMA]Fe ratios from XANES spectra were investigated. The energy of the 1s [vector] 3d pre-edge transition centroid was round to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s [vector] 4s and crest (1s [vector] 4p) transitions. The [Fe.sup.3+]/ [SIGMA]Fe ratios determined from linear combinations of end-member spectra ([Fe.sup.3+]/[SIGMA]Fe ~0 and ~1) were round to deviate significantly from the Mossbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in [Fe.sup.2+] or [Fe.sup.3+] coordination with the [Fe.sup.3+]/[SIGMA]Fe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are hot applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that [Fe.sup.3+]/[SIGMA]Fe values may be obtained from XANES spectra, with an accuracy comparable to Mossbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.

Temperature-dependent cation order-disorder has been studied in many 2+ - 3+ oxide spinels but 4+ - 2+ spinels have been found to be either completely normal or completely inverse when examined at room temperature. Here we report the temperature dependence of the cation distribution in the 4-2 spinel synthetic qandilite ([Mg.sub.2]Ti [O.sub.4]) from in situ time-of-flight neutron powder diffraction experiments to 1416[degrees]C. At room temperature, [Mg.sub.2]Ti [O.sub.4] is confirmed to have completely inverse cation distribution, with Ti atoms occupying half the octahedrally coordinated cation sites. Cation disordering becomes observable above about 900[degrees]C, with 4% of the Ti occupying the tetrahedral site by 1416 [degrees]C. The rate of reordering on cooling is fast, such that high-temperature disorder is not preserved on cooling to room temperature. The thermodynamics of the change in cation distribution with temperature can be described by an enthalpy of Mg-Ti disorder of -46.1 [+ or -] 0.4 kJ/mol.

Thermodynamic data for magnesiochromite were obtained by analyzing the reaction MgCr2O4 + SiO2 = Cr2O3 + MgSiO3 in the pressure-time domain. The results were then compared with other published values of thermodynamic properties for orthoenstatite, beta-quartz and eskolaite to derive the entropy and enthalpy of formation of MgCr2O4. The results suggested that the available calorimetrically obtained figure for the entropy of MgCr2O4 was approximately 16 joules per kelvin per mole below the real value.

The measurement of the chemical potential of oxygen defined by the rhenium-oxygen equilibrium is verified by hydrothermal experiments. The chemical potential is measured in the large from 850 to 1250 kelvin by electrochemical methods utilizing the Ni and NiO or Cu and Cu2O as reference electrode and calcia-stabilized zirconia. The hydrothermal techniques used for verification use the double capsule process with NiO and Ni-Pd alloys as oxygen detector. The verification process reveals that the measurement of chemical potential can be accurately verified by the hydrothermal method.

The fO2 in metamorphic rocks and garnet peridotites of the mantle is investigated by the use of garnets having doubly ionized Fe and triply ionized Fe. The artificial preparation of Fe3Fe2Si3O12 garnet and another garnet along the almandine, which has the Al and triply ionized Fe mixture on its octahedral faces, enables the study of their properties. Only at pressures higher than 9.3 giga Pascals are the ferrous-ferric garnet's end members stable. The other end member, the skiagite, is unstable at higher pressures, less than 13 giga Pascals. The restriction of Fe2 and Fe3 to the dodecahedral and octahedral sites is indicated by Mossbauer spectroscopy.