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The behavior of structural OH in clinopyroxene (augite and omphacite) during successive heating has been investigated by in situ micro-FTIR measurements under temperatures ranging from room temperature to 500[degrees]C at 100[degrees]C increment. The first group of OH band (3620-3640 [cm.sup.-1]) exhibits a systematic decrease of peak position upon successive heating, while the other two groups (3520-3535 and 3450-3465 [cm.sup.-1]) show only little change. Both augite and omphacite display a decrease of integral absorbance of OH fundamental stretching vibration upon successive heating. The IR spectra of OH band are reversible when the temperature decreases from 500[degrees]C to room temperature, suggesting that changes in IR indicate changes in molecular state of OH. Based on: (1) the decreases of integral absorbance of OH bands with increasing temperature, and (2) comparison with Paterson's relationship (Paterson 1982), we suggest that OH absorption coefficients are temperature dependent, so it is necessary to apply different absorption coefficients when determining OH content from Beer-Lambert law at different temperatures and sample temperatures should be reported in quantitative IR studies Keywords: IR, varying temperature, OH, clinopyroxene DOI: 10.2138/am.2010.3501

A new thermodynamic model for sodic pyroxenes involving jadeite-diopside-hedenbergite-acmite is presented. This model allows for ordering of Mg, Al, [Fe.sup.2+], and [Fe.sup.3+] on the M1 sites, with coupled Na and Ca ordering on the M2 sites. It is calibrated on the basis of experiments in three chemical subsystems together with available information on ordering in different pyroxenes and on the limited calorimetric data. Central to the determination of the parameters of the model is the use of relationships among the end-member Gibbs energies and the interaction energies in the various possible non-independent sets of end-members. An important aspect of this model, which uses the symmetric formalism, is that Fe-Mg (FM) and Al-[Fe.sup.3+] (AF') mixing is not assumed to be ideal. The model accounts successfully for the experiments at both 600[degrees]C and at higher temperatures involving ordered and disordered pyroxenes coexisting with albite and quartz in NCMAS, NCFAS, NF'AS systems as well as the available calorimetry. It is also able to predict the positions and slopes of tielines in coexisting jadeite + omphacite and omphacite + augite found in nature at lower temperatures. Although the model requires a large number of energy parameters, some of these are not critical to the behavior of the model, while for others the constraints from experiment, calorimetry, ordering state and solvi lead to very limited allowable combinations. The model places some restrictive constraints on the shape of the phase relations in the jadeite-augite-acmite system. Petrological applications of the model are illustrated via a phase diagram example for a MORB bulk composition eclogite. Keywords: Thermodynamics, omphacite, activity-model, eclogite, phase equilibria, metamorphic petrology, mixing properties

Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions, trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3-4):191-233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges.

The Jinbaoshan ultramafic intrusion is a sheet-like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins. PGE are enriched in several disseminated sulfide zones in the intrusion. Olivine from the intrusion has low Fo and depleted Ni contents compared to olivine from coeval Emeishan picrites. Whole rock major and trace element concentrations suggest that the Jinbaoshan wehrlites originally contained Keywords Wehrlite * Olivine * Sulfide & Platinum-group elements * Isotopes * Melt/rock reaction

Room-temperature [sup.57]Fe Mossbauer spectra of synthetic orthopyroxenes equilibrated between 900-1000 [degrees]C and spanning the compositional range from ferrosilite to nearly enstatite were acquired. Peak areas corresponding to [Fe.sup.2+] in the M1 and M2 sites of the structure were corrected for effects of thickness and differential recoil-free fractions (f) on the two sites using a constant value of C = 1. 1, the correction parameter. The distribution coefficient, [K.sub.D], for Fe and Mg in M1 and M2 was calculated for all samples, and results were compared to [K.sub.D] values from previous studies where C was assumed to be unity. Our results demonstrate that choice of C dramatically influences calculated [K.sub.D] values. Thus, the failure of previous studies to consider C corrections may well be responsible for differences between Mossbauer and XRD studies of site occupancies noted by previous workers. Further characterizations off for pyroxenes of variable compositions are needed. Keywords: Mossbauer, recoil-free fraction, orthopyroxene, enstatite, ferrosilite

A series of Ti-doped diopside samples synthesized by fluxgrowth methods under reducing conditions were investigated by microanalytical methods and optical absorption spectroscopy, to assess Ti incorporation mechanisms, valence states, and related electronic transitions. Chemical characterization show that Ti occurs both in the tri-and tetravalent states, with [Ti.sup.4+] preferentially ordered to the tetrahedral position whereas [Ti.sup.3+] is restricted to the M1 position. Charge-balance is maintained by incorporation of Na and minor B, stemming from the flux compound. Polarized optical absorption spectra reveal three major absorption features. Two relatively narrow bands centered at 18 500 and 15 700 [cm.sup.-1], mainly polarized in the crystallographic c-direction, display absorbance values that correlate with the calculated [Ti.sup.3+] (M1) concentration, and are assigned to spin-allowed d-d transitions in [Ti.sup.3+]. A broader band centered around 24 500 [cm.sup.-1] displays absorbance values that are well correlated with the product of [sup.VI][Ti.sup.3+] and [sup.IV][Ti.sup.4+]. Based on the spectral characteristics of this band, including a strong polarization in the crystallographic b-direction, we assign this band to an [sup.M1][Ti.sup.3+]-[Ti.sup.4+] intervalence charge transfer process. Keywords: Crystal synthesis, diopside, optical spectroscopy, pyroxene, major and minor elements, Ti in pyroxene, order-disorder

The behavior of multivalent elements such as Fe, Cr, V, and Eu in magmatic systems reflects the [fo.sub.2] of the environment. In particular, Eu behavior in pyroxene from basaltic systems has been demonstrated to be an effective measure of [fo.sub.2]. We selected two nearly isochemical lunar pigeonite basalts (15058, 15499), a lunar high-Ti basalt (75035), and Pasamonte (representing asteroid 4 Vesta, an unequilibrated eucrite), to explore other potential variables that may affect this indicator of [fo.sub.2]. All of these basalts crystallized at an [fo.sub.2] of approximately IW-1, yet they experienced different cooling and crystallization histories and their pyroxenes exhibit a wide range of compositional trajectories within the pyroxene quadrilateral. There are several variables that influence the Eu/Eu * recorded in pyroxene that may compromise the determination of [fo.sub.2]. Previous experimental studies show that pyroxene composition influences the ability of pyroxene to accommodate REE and fractionate [Eu.sup.2+] from [Eu.sup.3+]. We demonstrate that in addition to the influence of Ca in the M2 site, the Al content in the pyroxene and its influence on coupled substitutions will also influence the fractionation of [Eu.sup.2+] from [Eu.sup.3+]. For example, the coupled substitution [sup.T][Si.sup.4+] + [M.sup.2][R.sup.2+] [right arrow] [sup.T][Al.sup.3+] + [sup.M2][REE.sup.3+] may accommodate [REE.sup.3+] in preference to [Eu.sup.2+], which is too large. Different pyroxene growth surfaces will incorporate [Eu.sup.2+] and [Eu.sup.3+] differently due to differences in growth rate, Al content, and site configuration. In consort with the pyroxene composition, fractionation of [Eu.sup.2+] from [Eu.sup.3+] will be aided by the Al content of the basaltic melt, which increases the activities of network-forming components such as Ca[Al.sub.2][O.sub.4] and Fe[Al.sub.2][O.sub.4] in the melt during pyroxene crystallization. The Al content will result in changing the partitioning behavior of [Eu.sub.3+] while having very little effect on [Eu.sup.2+]. Melt composition, the appearance of plagioclase on the liquidus, and the kinetics of plagioclase crystallization are influenced by cooling rate. Data from the four basalts selected also suggest that [Eu.sup.2+]/[Eu.sup.3+] in the melt remains buffered even with extreme differences in cooling rate and plagioclase crystallization kinetics. Unexpectedly, many of these same variables affect the substitution of multivalent V. If the [fo.sub.2] determined from Eu behavior in pyroxene is not placed within a petrologic and crystal-chemical context, errors of 1 to 2 log units may result. The influence of these variables may be reduced by using multiple, co-crystallizing phases (i.e., plagioclase and pyroxene) and ratioing [D.sub.Eu] to adjacent REE ([D.sub.Sm.Gd]). Keywords: Eu valence, V valence, pyroxene, oxybarometer, Moon, 4 Vesta

Synthetic samples with different chemical compositions along the hedenbergite--aegirine (Ca[Fe.sup.2+][Si.sub.2][O.sub.6]--Na[Fe.sup.3+] [Si.sub.2][O.sub.6]) solid-solution series have been investigated by single-crystal X-ray diffraction and [sup.57]Fe Mossbauer spectroscopy. All compounds show C2/c symmetry, both at 298 K and at low temperature (100 K). The structural changes within the hedenbergite--aegirine series are dominated by the M1 site while the M2 site plays a minor role. Replacement of [Fe.sup.2+] by [Fe.sup.3+] increases the polyhedral distortion of the M1 site and causes an increased repulsion between neighboring M1 sites. The changes in M1-site geometry also induce distinct alterations within the kinking state of the tetrahedral chains, but the changes in tetrahedral bond lengths and angles are small. In addition to the single-crystal X-ray diffraction experiments, a large number of synthetic samples were investigated by [sup.57]Fe Mossbauer spectroscopy at 298 K and, for three selected compositions, between 80 and 700 K. Here, substantial line broadening of the [Fe.sup.2+] resonance absorption was observed as an aegirine component is substituted. Two different groups of local distortion environments were observed for [Fe.sup.2+] within the solid-solution series, which change relative proportions and numeric value of the quadrupole splitting as a function of chemistry and temperature. This line broadening cannot be ascribed to discrete next-nearest-neighbor (NNN) configurations of Ca and Na as has been done in the literature. Above ~250 K, additional resonance absorption appears in the spectra of samples with aegirine components between 20 and 75 mol%. This absorption gains intensity with increasing temperature, while the [sup.57]Fe hyperfine parameters approach values intermediate between [Fe.sup.2+] and [Fe.sup.3+]. This effect is ascribed to fast electron delocalization between [Fe.sup.2+] and [Fe.sup.3+] at elevated temperature. Keywords: Synthesis, crystal structure, Mossbauer spectroscopy, hedenbergite--aegirine series, clinopyroxenes

Rhodonite (ideally MnSi[O.sub.3]) is a triclinic pyroxenoid that typically contains significant concentrations of Ca, Fe, and Mg. A variety called fowlerite contains concentrations of Zn up to 10 wt% and has been found only at Franklin, New Jersey. Data from electron microprobe analyses, single-crystal X-ray structure refinements, and preliminary Mossbauer spectra suggest marked differences in the crystal chemistry of rhodonite and fowlerite. Cations in Zn-poor rhodonites and in fowlerites exhibit distinct substitutional trends. Substitution of Zn in fowlerite occurs entirely at M4, and Ca displays a stronger preference for M5 in fowlerite than in Zn-poor rhodonite. Further, the distinctive compositional variations in fowlerite lead to strongly mutually correlated lattice parameter variations distinct from those in Zn-poor rhodonite. Mean M-O distances in fowlerite, with the exception of , are strongly correlated with all unit-cell edges. For Zn-poor rhodonites, neither nor are significantly correlated with cell edges, and the mean bond lengths for M1, M3, and M5 are correlated strongly with only a and b. The most striking distinctions between fowlerite and typical rhodonite occur in the geometrical details of M4 and M5. Distortion in the M4 site of magnesian rhodonite, typical Mn-rich rhodonite, and fowlerite increases with both increasing electron population and mean electronegativity. The M4 site assumes increasing amounts of tetrahedral character with increasing electron density (i.e., increasing Zn concentration). As M4 becomes more Zn-rich, tetrahedral, and covalent in character, M5 becomes more Ca-rich, less distorted, and more ionic in character. This suggests that 'pure' fowlerite (nominally [Ca.sub.0.2][Zn.sub.0.2][Mn.sub.0.6][O.sub.3], or CaZn[Mn.sub.3][O.sub.15]) may contain an unequivocally four-coordinated M4 site occupied solely by Zn, and with Ca largely restricted to M5.

We report chemical and mineralogical data for one atmosphere melting experiments conducted on alkalic rocks from the Mt. Erebus volcanic region: DVDP2 basanite, two hawaiites (DVDP2 and a nepheline-bearing variety), and an anorthoclase phonolite. Temperatures between 1,224 and 1,049[degrees]C were investigated at [f.sub.O2]~QFM. DVDP2 basanite appears to be an intermediate pressure liquid or a cumulate, because only olivine coexists with melt from above 1,224-1,160[degrees]C. High-Ca pyroxene joins olivine in the crystallization sequence at 1,138[degrees]C. These minerals are joined by plagioclase at a temperature between 1,120 and 1,104[degrees]C. In contrast, DVDP2 hawaiite appears to be relatively evolved, because it is multiply saturated with olivine, plagioclase, and high-Ca pyroxene near its liquidus (between 1,120 and 1,104[degrees]C). Plagioclase crystallizes in the Ne-hawaiite by 1,160[degrees]C followed by olivine below 1,120[degrees]C. The liquidus of anorthoclase phonolite is between the lowest temperatures investigated, 1,089 and 1,049[degrees]C, and plagioclase is the liquidus mineral. Our results indicate that DVDP2 hawaiite can be derived from a DVDP2 basanitic parental magma by crystal fractionation at low pressures, that the nepheline hawaiite is an olivine cumulate, and that the liquids parental to the anorthoclase phonolite represent the end products of crystal fractionation. They also allow us to illustrate how the Ti-content of pyroxene may be used as a petrogenetic indicator of processes and events in the evolution of the Erebus volcanic system.

Microorganisms are believed to create microenvironments leading to reaction products not predictable from equilibrium thermodynamics and to unique biomineral morphologies. Unambiguous evidence for such environments is, however, rare in natural samples. We have used scanning transmission x-ray microscopy and spectromicroscopy at the sub-40-nm scale, coupled with transmission electron microscopy, to examine bioweathering products on a meteoritic Fe-Mg-orthopyroxene colonized by a filamentous microorganism. Our measurements reveal an amorphous Al-rich layer beneath the microorganism, calcium carbonates of unique morphology intimately associated with polysaccharides adjacent to the microorganism, and regions surrounding the microorganism with different iron oxidation states. Our results confirm the presence of different microenvironments at this microorganism-mineral interface and provide unique nanometer-scale views of microbially controlled pyroxene weathering products. aluminosilicate | biomineralization | geomicrobiology | STXM | Urey reaction

THE QUESTION OF WHERE SUBSTANtial K resides in the mantle is fundamental for understanding the source for K in basalts that originate from partial melting of the mantle, the distribution [...]

Electron-diffraction patterns of clinopyroxene with an intermediate composition between kosmochlor and diopside show the presence of satellite reflections in diffuse streaks running perpendicular to (111) planes of the C2/c structure. The position of these satellite reflections indicates the existence of an incommensurate (INC) phase. Based on the characteristics of the diffraction patterns, it is suggested that the INC phase consists of a regular arrangement of kosmochlor-rich and diopside-rich slabs with a periodicity 2.6 to 6.0 times the (111) spacing in the C2/c structure. This is a new ordering scheme in clinopyroxenes.

Precision Lattice-Parameter Determination of (Mg,Fe)[SiO.sub.3] Tetragonal Garnets BECAUSE (MG,FE)[SIO.sub.3] PYROXENES are abundant minerals in the upper mantle, many investigators have studied the stability relations and the chemical and physical properties [...]

About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/ (Mg+[Fe.sup.2+]) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt%), [Na.sub.2]O (2.4 wt%) and [H.sub.2]O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The [[delta].sup.18]O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ~7%0. This value is consistent with the previous estimates for the Bushveld magma and with the 6180 values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatilerich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na--and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.