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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

Eleven clinopyroxenes from the Sludyanka Crystalline Complex in Russia belonging to the ternary join NaV[Si.sub.2][O.sub.6]-NaCr[Si.sub.2][O.sub.6]-CaMg[Si.sub.2][O.sub.6] (natalyite-kosmochlor-diopside) were studied by means of X-ray single crystal diffractometry and electron probe microanalysis. The crystal chemical data show that the T site is almost completely occupied by Si, so that the Na ([V.sup.3+],[Cr.sup.3+]) [right arrow] Ca Mg substitution mechanism ensures charge balance. Changes in M1 site geometry are explained by the aggregate ionic radius, and are influenced by Mg occupancy and [V.sup.3+]/([V.sup.3+] + [Cr.sup.3+]) ratio. The M2 site geometry depends both on Na content and on the ([V.sup.3+],[Cr.sup.3+]) [right arrow] Mg substitution in M1 site. Changes in M2-O3cl bond length are mainly related to Na content, whereas the longest M2-O3c2 bond lengths are significantly affected by the [V.sup.3+]/([V.sup.3+] + [Cr.sup.3+])ratio of the M1 site. The T site geometry is affected by chemical and geometrical variations at the M1 and M2 sites, principally the M1 site occupancy.

A high-temperature in situ single-crystal X-ray diffraction study was performed from room T to 1150 [degrees] C on two crystals of Fe-free [P2.sub.1]/c clinopyroxenes of composition [Ca.sub.0.15][Mg.sub.1.85][Si.sub.2][O.sub.6] [cell parameters at room T: a = 9.651(2) [Angstrom], b = 8.846(2) [Angstrom], c = 5.202(1) [Angstrom], [beta] = 108.38(2) [degrees], V = 421.4 (2) [[Angstrom].sup.3] synthesized by isothermal annealing for 624 h at T = 1370 [degrees] C, P = 1 atm. A first order [P2.sub.1]]/c-C2/c phase transition was found slightly below 1000 [degrees] C [[T.sub.c] = 926(39) [degrees] C]. The transition was revealed by discontinuous changes in intensities and cell parameters. Prolonged heating at high temperature induced a non-reversible increase in the transition temperature up to more than 1150 [degrees] C, without apparent changes in the order of the phase transition. Coupling with strain due to incipient exsolution in a formerly almost defect-free sample is suggested to be responsible for increase in [T.sub.c]. TEM observations of a sample from the same starting material after further annealing for 72 h at T = 1050 [degrees] C, P = 1 atm are consistent with the proposed incipient exsolution model. Annealing was found to induce the formation of a mottled texture oriented parallel to (101). Results from structure refinement of data collected below the transition at T = 25,500, 650, 800, and 1000 [degrees] C showed only minor changes in the chain configurations, which are highly differentiated up to 1000 [degrees] C, confirming the strong first-order character of the transition.

Author's Abstract, COPYRIGHT 1999, Mineralogical Society of America Crystal structures and phase transitions of enstatite (MgSiO(sub 3)) were studied by in situ X-ray diffraction experiments using synchrotron radiation and a multi-anvil high-pressure apparatus at pressures to 12 GPa and temperatures to 1473 K. Low clinoenstatite with space group P2(sub 1)/c transforms to high-pressure C2/c clinoenstatite at high pressures and high temperatures, accompanied by a volume reduction of about 2.5%. The beta angle of this high pressure C2/c phase ranges from 101.4 degrees to 101.7 degrees, shows almost no variation with pressure and temperature, and is about 8 degrees smaller than that of the high-temperature C2/c phase previously reported. This confirms the suggestion (Hugh-Jones et al. 1994) that these two clinoenstatite phases differ at high pressures and high temperatures. The pressure-volume-temperature data for P2(sub 1)/c and high pressure C2/c clinoenstatite were fit to room-temperature third-order BirchMurnahan equations of state (EOS) using the parameters: volume of V(sub 0) = 415.4(5) Angstroms(super 3), isothermal bulk modulus of K(sub 0) = 108.5 (6.4) GPa, and its pressure derivative of K'(sub 0) = 4.5 (1.3) for the P2(sub 1)/c phase, and v(sub0) = 405.1(1.7)Angstroms(super 3), K(sub 0) = 106.4 (17.4) GPa, and K'(sub 0) = 5.4(2.7) for the C2/ c phase. These values are at ambient condiitons. For the C2/c phase, we determined the high-temperature EOS, expressed at P = 3/2 K(sub T) [V(sub T)/V)(super 7/3)-(V(sub T)/V)(super 5/3)]{1-3/4 (4-K'(sub T))[(V(sub T)/V)(super 2/3)-1]}, where K(sub T) = K(sub 0) + (mathematical expression not reproducible in ASCII) K(sub T)/(mathematical expression not reproducible in ASCII T)(sub p)(T-300), K'(sub T)= K'(sub 0), V(sub T) = V(sub 0) [exp(sum alpha (T)dT from 300 to T)], where thermal expansivity alpha(T) is a(sub 0) + a(sub 1) T. The parameters are V(sub 0) = 405.0 (2.6) Angstroms(super 3), K(sub 0) = 106.9 (25.9) GPa, K'(sub 0) -5.3 (3.9), a(sub 0) = 2.01 (44) x 10(super -5)K(super -1), a(sub 1) = 2.10 (1.1) x 10(super 8)K(super -2), and (mathematical expression not reproducible in ASCII) K(sub T) T)(sub p) = 0.021 (10) GPa/K. Although the K(sub 0) values are nearly the same with those of previous studies for both the P2(sub 1)/c and C2/c phases, the K'(sub 0) values are slightly smaller.

Author's Abstract, COPYRIGHT 1999, Mineralogical Society of America The kenetic of H extraction-incorporation in diopside single-crystals (Ca(sub 0.97)Na(sub0.02)Cr(sub 0.01)Mg(sub 0.97)Fe(sub0.036)Si(sub 1.99)O(sub 6) deduced by monitoring OH infrared absorption bands for samples heated from 973 to 1273 K at 0.1 atm and 1 atm of pH(sub 2), is independent of crystallographic orientation, P(sub 02), and pH(sub 2). The diffusion law is D = D(sub 0) exp[ -(126 +/- 24) kJ/mol/RT], with log D(sub 0) (in m(super 2)/s) = -6.7 +/- 1.1. Hydrogen self-diffusion obtained from H-D exchange in the same diopside samples over 873-1173 K,and along directions [001] and ]100]* at 1 atm total pressure is two orders of magnitude faster than H uptake and follows the diffusion law D(sub H) = D(sub 0) exp[ -(149 +/- 16) kJ/mol/RT], with log D(sub 0) (in m(super 2)/s =-3.4 +/- 0.89. Self-diffusion along [010] follows the diffusion law D(sub H) = D(sub 0) exp[ -(143 +/- 33) kJ/mol/RT], with log D(sub 0) (in m(super 2)/s) = -5.0 +/- 1.7 and is one order of magnitude faster than H uptake. The kinetics of extraction incorporation of H in this diopside follows the reaction Fe(super 3+) + O(super 2) + 1/2H(sub 2)(g) = Fe(super 2+) + OH- and are not rate limited by themobility of protons but more probably by the mobility of electron holes connected with the Fe oxidation-reduction process. The results suggest that the kinetics of H uptake inclinopyroxenes will increase with increasing Fe content until it is rate controlled by the kinetics of H self-diffusion. We predict a rate for H exchange in deipside appropriate to the upper mantle almost as fast as H exchange in olivine. The insensitivity of H solubility on temperature and P( sub 02) for samples recovered from low-temperature conditions (below 1273 K) and/or rapidly quenced samples let us suggest the use of OH concentration measurements in diopside as a potentialpH(sub 2) sensor.

The high-pressure behavior of the clinopyroxenes kanoite [Mn.sub.09][Mg.sub.1] [Si.sub.2][O.sub.6], MnSi[O.sub.3], and CrMg[Si.sub.2][O.sub.6] (all space group P[2.sub.1]/c) was studied by single-crystal X-ray diffraction in a diamond-anvil cell at room temperature. Phase transitions from P[2.sub.1]/c to C2/c clinopyroxene were found and reversed at 5.06 [+ or -]0.14 GPa for kanoite, 2.905 [+ or -] 0.005 GPa in MnSi[O.sub.3], and 3.60 [+ or -] 0.03 GPa in CrMg[Si.sub.2][O.sub.6]. The phase transitions are first-order in character and are accompanied by a volume decrease of approximately 2.3%. The structure of high-pressure C2/c kanoite was determined from single-crystal X-ray intensity data collected at 5.20 GPa, and is very similar to that of high-pressure (HP) C2/c-ferrosilite. Although the space group C2/c is the same as for the high-temperature (HT) kanoite form, the two phases have significantly different structures. The silicate chains are extremely kinked in HP kanoite while they are almost straight in HT kanoite. Compared with the transition pressure of clinoenstatite-clinoferrosilite, the new data suggest that the effective ionic radii of M1 and M2 cations do not exclusively control the transition pressure and that the HP C2/c clinopyroxenes with [Cr.sup.2+] and [Fe.sup.2-] gain additional stabilization energy from crystal field effects.

The high-pressure behavior of the clinopyroxenes kanoite [Mn.sub.09][Mg.sub.1] [Si.sub.2][O.sub.6], MnSi[O.sub.3], and CrMg[Si.sub.2][O.sub.6] (all space group P[2.sub.1]/c) was studied by single-crystal X-ray diffraction in a diamond-anvil cell at room temperature. Phase transitions from P[2.sub.1]/c to C2/c clinopyroxene were found and reversed at 5.06 [+ or -]0.14 GPa for kanoite, 2.905 [+ or -] 0.005 GPa in MnSi[O.sub.3], and 3.60 [+ or -] 0.03 GPa in CrMg[Si.sub.2][O.sub.6]. The phase transitions are first-order in character and are accompanied by a volume decrease of approximately 2.3%. The structure of high-pressure C2/c kanoite was determined from single-crystal X-ray intensity data collected at 5.20 GPa, and is very similar to that of high-pressure (HP) C2/c-ferrosilite. Although the space group C2/c is the same as for the high-temperature (HT) kanoite form, the two phases have significantly different structures. The silicate chains are extremely kinked in HP kanoite while they are almost straight in HT kanoite. Compared with the transition pressure of clinoenstatite-clinoferrosilite, the new data suggest that the effective ionic radii of M1 and M2 cations do not exclusively control the transition pressure and that the HP C2/c clinopyroxenes with [Cr.sup.2+] and [Fe.sup.2-] gain additional stabilization energy from crystal field effects.

A coherent set of partition coefficients for rare earth elements (REE) and high-field-strength elements (HFSE) between clinopyroxene, garnet, and basaltic melt in the pressure range 2.0-3.0 GPa have been derived from a series of doped, high pressure piston cylinder experiments. The data for clinopyroxene indicate that partitioning of these elements is not a strong function of pressure or temperature, except in the case of Ti which shows a mild temperature dependence. Melting in the spinel peridotite stability pressure range can produce the HFSE anomalies in some of the data.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) yields mass spectra of the upper several monolayers of an analytical surface. The applicability of TOF-SIMS to the characterization of C compounds on crack surfaces in rocks is demonstrated. A pyroxene grain recovered from the interior of a gabbro xenolith from the 1801 flow of Haulalai volcano, Hawaii, was chosen for this initial study because well-developed carbonaceous films are known to exist on many of the crack surfaces in these rocks. In addition to the anticipated major elements (Si, A1, Fe, Mg, Ca), several minor and trace elements (B, Li, Na, Ti, Mn, Co, Ni, Cu) were identified in the positive ion mass spectra. The unsputtered surface is covered with a hydrocarbon-rich layer, as indicated by the presence of numerous light [C.sub.x][H.sub.4] fragments as well as several masses of several hundred atomic mass units (amu). This layer is much better developed than the ubiquitous atmosphere-derived hydrocarbon layer observed on nominally clean, unsputtered surfaces and therefore must be indigenous. High concentrations of Ni and Cu are associated with the C-rich layer and may exist as organo-metallic compounds. Several C-N fragments, possibly indicative of biogenic compounds, were also identified in negative ion spectra. Imaging reveals the presence of localized islands enriched in oxides of Si, Al, Na, and Ca beneath the carbonaceous layer. This study demonstrates that TOF-SIMS can be used to provide unique and geochemically useful information on crack surfaces in rocks.

The intracrystalline distribution of Mg and [Fe.sup.2+] on the M1 and M2 octahedral sites in high-P C2/c (Mg,Fe)Si[O.sub.3] clinopyroxene has been determined on quenched samples using 57Fe Mossbauer spectroscopy. Although the recovered samples that were measured had the low-P ([P2.sub.1]/c) structure, the ordering state of the high-P polymorph is believed to have been preserved because the high-P C2/c [right arrow] low-P [P2.sub.1]/c transformation occurred at room temperature during the final stages of decompression of the experiments. Under such conditions significant cation diffusion is unlikely. Low temperature (81 K) Mossbauer spectra indicate that [Fe.sup.2+] is strongly ordered onto the M2 sites, with an average [K.sub.D] [Angstrom] 4.0(8) for the intersite exchange reaction: [Fe.sup.2+](M1) + [Mg.sup.2+](M2) = [Fe.sup.2+](M2) + [Mgs.up.2+](M1) (1) Because the samples were quenched from high temperature (1200-1275 [degrees]C) at 9.5 GPa, the temperature to which the measured ordering state corresponds remains problematic. However, a sample re-annealed at 1000 [degrees]C is more ordered than the original sample synthesized at 1275 [degrees]C, suggesting that high-temperature cation distributions in high-P clinopyroxene can be quenched in the multi-anvil experiments. Therefore, we conclude that the measured cation distributions correspond approximately to the conditions of the experiments. A small compositional dependence of cation ordering also exists, indicating subregular behavior across the binary join. A fit to the data yields [Mathematical Expression Omitted] = -11.9 (2.0) kJ/mol at 1275 [degrees]C for the exchange Reaction 1, which describes the ordering behavior on the octahedral sites in the clinopyroxenes. Comparison with data for orthopyroxene, which were obtained mostly at 1 atm, indicates that the state of ordering in orthopyroxene and high-P clinopyroxene is similar, especially if cation ordering in the high-P clinopyroxene has a small pressure dependence. The similarities are consistent with the observed structural similarities between the two polymorphs.

We present minor and trace element (REE, Sr, Y, and Zr) data for pyroxenes and mesostases in four porphyritic chondrules from the Semarkona ordinary chondrite. Apparent partition coefficients for clinoenstatite, orthoenstatite, pigeonite, and augite are compared with experimental and petrologic data from the literature, and the effects on apparent partition coefficients of the rapid cooling rates at which chondrules crystallized are evaluated. For most elements, the effects of cooling at rates of hundreds of degrees per hour cannot be distinguished from variations in equilibrium data resulting from differences in temperature or composition. However, for LREE apparent partition coefficients are significantly higher than comparable equilibrium data, and the ratio of HREE/LREE partition coefficients is lower, particularly for Ca-poor pyroxene. We attribute this flattening of REE patterns to the effect of rapid cooling. Apparent partition coefficients of all REE and Y in augite are higher than equilibrium data, particularly in one chondrule with a high [Al.sub.2][O.sub.3] content. We suggest that this may be attributed to an increase in the uptake of trivalent trace element cations in the pyroxene crystal structure as a result of charge-balanced substitutions with [Al.sup.3+] cations.

The effects of water content differences on the kinetics of diffusion-controlled growth of albite (Ab) and enstatite + diopside + spinel rims (Px) are studied using the reaction rim technique. Results show that based on the parabolic rates constants for Ab and Px reaction rims at 1000 degrees Centigrade and 700 MPa, water content has a small impact on grain boundary diffusion rates.

Experiments at 70-224 kbar, using a split-sphere anvil apparatus, reveal the melting relations on the enstatite-diopside join. For low pressures, melting in the enstatite-rich compositions is peritectic that turns eutectic at higher pressures. The diopside-rich clinopyroxene experiences azeotropic melting at 70-165 kbar and at temperatures lower than the eutectic. Relationships with the coexisting garnets and perovskites are studied with the help of the melting curve of diopside. Applications of the calculated temperature-pressure phase diagram from the study are discussed.

An X-ray structure-refinement procedure was developed to characterize orthopyroxene from the Serra de Mage meteorite. In the studied sample, the orthorhombic phase coexists with exsolution lamellae of C2/c augite, parallel to (100), with [Mathematical Expression Omitted], [Mathematical Expression Omitted]. Diffraction maxima of the monoclinic phase overlap those of the orthorhombic phase with h + l = 2n, yielding violations of the extinction conditions for space group Pbca and simulating the lower symmetry space group [P2.sub.1]ca. Structural parameters of the Pbca phase together with the [W.sub.Cpx] parameter, which expresses the fraction of the C2/c phase present, were refined first. [F.sub.c] values were calculated using the equation [Mathematical Expression Omitted], with [[[F.sub.c]].sub.Cpx] values taken from the structure refinement of an augite. Observed [F.sub.o] values were then corrected by subtracting the calculated contribution of the monoclinic phase with use of the equation [Mathematical Expression Omitted] and used for a further refinement of the orthorhombic phase. The final residual indices, [R.sub.obs], were 2.11 and 1.78% for two crystals with different augite contents. This refinement procedure confirms Pbca as the correct space group and provides more accurate structural parameters for the Serra de Mage orthopyroxene.

The CaGe[O.sub.3] and SrGe[O.sub.3] perovskites and CaGe[O.sub.3], SrGe[O.sub.3], and MnGe[O.sub.3] pyroxenoids have been studied at high-temperature by X-ray diffraction (XRD) and X-ray absorption fine-structure spectroscopy (XAFS). The diffraction studies show that the back-transformation of the perovskite begins at lower temperature for SrGe[O.sub.3] ([approximately]500 K) than CaGe[O.sub.3] ([approximately] 945 K). An intensity reduction of the Bragg lines and the presence of a diffuse band in the diffraction patterns show the occurrence of transient amorphous phases. This transient phase contains Ge in fourfold and sixfold coordination after the loss of CaGe[O.sub.3] and SrGe[O.sub.3] perovskites, respectively. The recrystallization of the stable pyroxenoids occurs at higher temperature in a second step of the transformation. For these compounds, the anharmonic character of the first Ge-O bond was extracted from the XAFS analysis. For the perovskites, we observed that the bulk thermal expansion and the Ge-O bond anharmonicity are closely connected. Both parameters are higher for cubic SrGe[O.sub.3] and simultaneously increase near 525 K when CaGe[O.sub.3] changes symmetry. For the pyroxenoids, our calculated tetrahedral thermal expansion is lower than for the bulk, with Ge-O bond anharmonicity very low. Close to the melting point, anomalous motion of the tetrahedral chains is suggested by a significant increase of the Ge-O bond length.

The crystal structure of the most K-rich natural pyroxene ever reported, a chromian diopside with 1.5 wt% [K.sub.2]O, has been refined (diffractometer data, filtered MoK[Alpha] radiation, by least-squares using XTAL to [R.sub.u] = 3.2%) to examine the effect of K on the average structure. The crystal with structural formula [Ca.sub.0.80][K.sub.0.073][Na.sub.0.023][Mg.sub.0.95][Fe.sub.0.06 ][Cr.sub.0.07][Al.sub.0.02][Si.sub.2][O.sub.6] was found as an inclusion in a Koffiefontein diamond. The refined structure is typical of clinopyroxene on the diopside-enstatite join: Mg in M2 leads to distortion that is modeled by site splitting, with M2[prime] (Mg + Fe) displaced 0.33 [Angstrom] from M2 (Ca + K + Na). Assignment of K to M2 is required to account for the electron density at that site. The average of eight M2-O distances (2.504 [Angstrom]) is slightly larger than for diopside (2.498 [Angstrom]). The effect of K on the average M2-O distance can be seen by calculating the average cation radius of atoms at M2 (+ M2[prime]) from the occupancy: 0.798-1.12 [Angstrom] ([8]Ca) + 0.073[center dot!1.51 (K) + 0.023[center dot]1.18 (Na) + 0.036[center dot]0.92 (Fe) + 0.070[center dot]0.89 (Mg) = 1.127 [similar to] [greater than or equal to]R ([8]Ca). The large size of K is mostly offset by Mg + Fe in M2[prime] in the diopside-like structure; this size-distortion balancing may facilitate K uptake in mantle clinopyroxene in K-rich environments. Large apparent thermal motion parameters of most atomic sites indicate sizable local distortions of the structure from substitution of K into M2.

A mathematical model predicts the outcome of trapped liquids in causing variations in incompatible trace-element distribution in cumulus minerals. Variations occur with different modes in mineral composition. Concentrations of incompatible elements present in low abundance in the rock exhibit extreme enrichment compared to that in the original cumulus mineral. The application of different models on Bushveld Complex shows that plagioclase have Rb contents of 1-5 ppm. The rare earth elements in clinopyroxenes may be twice as much contained in parent liquid.

Igneous cumulate rocks provide an important record of planetary magmatism, but there are pitfalls in their interpretation. The rocks are composed of cumulus minerals plus assemblages that crystallized from trapped melt. Cumulus minerals may react with the trapped melt and other cumulus phases during subsolidus reactions, thus losing a direct record of their igneous history. One of the best approaches for estimating the melt compositions parental to the cumulates is to analyze the cores of cumulus phases for elements with slow diffusion rates (e.g., REE) because these most reliably retain a record of the mineral-melt partitioning. Many of the cumulus orthopyroxene grains from the Stillwater Complex analyzed in this study have augite lamellae in their interiors but lamellae-free rims. Secondary ion mass spectrometer (SIMS) analysis shows that these orthopyroxene rims have lower Sr, Y, Zr, and Ce concentrations relative to the cores. These systematics were apparently caused by migration of augite exsolution lamellae, which preferentially incorporate these trace elements, out of the rims of the orthopyroxene grains. SIMS analyses for REE in orthopyroxene cores show lower LREE concentrations, (Ce/ Yb[).sub.n], and (Dy/Yb[).sub.n] than the isotope dilution data on mineral separates (Lambert and Simmons, 1987). We believe that these differences result from one or more contaminating phases (e.g., augite and plagioclase) in the orthopyroxene mineral separates. The SIMS data yield calculated parental melts similar to sills found below the Stillwater Complex, which have previously been suggested as possible parental melts (Helz, 1985). A major focus of this study was to determine whether the onset of plagioclase crystallization at the bronzitite zone-norite zone contact was the result of franctionation until plagioclase saturation of the magmas parental to the bronzitites, or if mixing of a second magma with higher Al activity was the cause of plagioclase cystallization. The data presented in this paper, coupled with existing data, support the magma-mixing model.

A study was conducted on the stability and mixing characteristics of synthetic jadeite-acmite pyroxene in the presence of quartz and albite. Results of experiments conducted at 600-900 degrees celsius generally support conclusions by other researchers that jadeite and acmite mixing is ideal. Acmite addition to jadeite was found to enhance the stability of pyroxene by under 300 MPa at a temperature of 900 degrees celsius.

Spectroscopic analysis of glass samples in pyroxene from boninite specimens from the Bonin Islands in Japan illustrates that water is a major constituent of boninite magmas. However, quenched glass from pillow lava rims in the region have poor water content as a consequence of degassing between the period of melt entrapment in the orthopyroxenes and the following sea floor eruption.

A new thermodynamic model, for studying Gibbs free energy of igneous pyroxenes, is based on general formulation and is dependent on the calibration of a quadrilateral subsystem. The model is made using experimental data of compositions of co-existing pyroxenes and silicate melt. The general formula of pyroxenes, under study, is given and the thermodynamics of multicomponent pyroxenes is discussed.

Application of modified Landau free energy expansion, having an additional linear term in the order parameter, Q, to normal and inverse spinels and to orthopyroxene solid solutions reveals the efficiency of modified expansion in describing the thermodynamics of nonconvergent cation ordering in various types of minerals. A comparative study of the Landau expansions with other models of nonconvergent ordering reveals the advantages of the modified expansion. The modified expansion accounts for contributions from both configurational and nonconfigurational entropies.

Estimates of Nd and Sr isotope compositions of abyssal peridotitic clinopyroxenes that occur in southwest Indian and American-Antarctic ridge feature zones that survived sea water changes reveal that abyssal peridotites are residues of mid-ocean-ridge basalt (MORB) melting. The peridotites originate from a depleted mantle source that resembles MORB sources. The peridotite clinopyroxene isotopic and MORB isotopic compositions help explain the composition of the sub-oceanic depleted mantle.

The use of a diffusion process equation and order-disorder properties of Fe and Mg in orthopyroxene helps derive an equation for determining the average Fe2+-Mg interdiffusion coefficient along the C and b directions of orthopyroxene, D(Fe-Mg)(c-b). Experimental data on the order-disorder process at different temperatures and compositions help determine D(Fe-Mg)(c-b). A comparative study suggests that increasing Fe/Mg ratio increases the tracer diffusion coefficient of Mg at X(Fe)=1.

ZrK edge x-ray absorption fine-structure spectroscopic studies of the environment of Zr in two inosilicates, Na- and Fe2+-rich clinopyroxene and an arfvedsonitic amphibole, from west Cameroon reveal the coordination of six O atoms with tetravalent Zr in the clinopyroxene. Spectral data suggest the localization of Zr in the M2 site in arfvedsonite. The local environment of Zr in these silicates is similar to that existing in Zr-bearing garnets and Zr-bearing silicate glasses. Short range structural environments explain the compatibility of Zr during crystallization of phonolites of the Rumpi Hills volcanic complex.

Single-crystal X-ray diffraction analysis of in situ Fe2+-Mg distribution in synthetic (Fe,Mg)2Si2O6 orthopyroxene of four different temperatures shows the anomalous nature of distribution over 1250 K. Iron-rich orthopyroxenes exhibit phase transition from orthopyroxene to clinopyroxene near 1225 K. Fe-Mg distribution over M1 and M2 sites is asymmetric between 1000-1200 K. The distribution data help derive thermodynamic parameters of orthopyroxene and activity-composition relations.

Thermodynamic analyses reveal that the exchange reaction of Fe2+ and Mg2+ between olivine and clinopyroxene depends on the temperature and is not affected by pressure changes. Ca concentrations of olivine systematically increase as the Fe content in the system increases. Fe-Mg partitioning between olivine and clinopyroxene at wide range of temperatures forms the basis of geothermometer calibration.

Studies of single crystal diffusion of MgSiO3 in a diamond-anvil cell reveal that there is a change in compression mechanism at a pressure of 4 GPa, giving rise to two different values for room pressure bulk modulus. Below 4 GPa, SiO4 tetrahydra remains incompressible, while at higher pressure, Si-O bond lengths are shortened. The degree of kinking of tetrahedral chains and the amount of tetrahedral tilt toward the (100) plane at bout 4 GPa show marked variations.

The chemical variations in clinopyroxenes are characterized by single crystal X-ray diffraction in association with electron microprobe analysis to delineate the intracrystalline constraints. The intracrystalline constraints are special features of certain magnetic environments. The experimental sample of clinopyroxene is crystallized from the peralkaline ultrapotassic melts. The variation characteristics reveals the presence of large amount of Si.

Thermodynamic properties of multicomponent pyroxenes in the composition space are investigated using a newly developed model. The thermodynamic properties are described by the final constituent CaMgSi2O6 and other exchange components. The model involves the molecular mixing type approximation which is capable of determining the molar configurational entropy variation.

The Sack and Ghiorso formulation for pyroxenes in a quadrilateral subspace is calibrated. The calibration is applicable to quadrilateral pyroxenes having miscibility gap characteristics. The calibration model is consistent with the Berman's thermodynamic database and solution theory of orthopyroxenes and olivines.

Combined electron microprobe and secondary ion mass spectrometry methods were used to study sector-zoned arginine crystals from a peralkaline nepheline syenite in Ilimaussaq, South Greenland, for major and trace elements. Sector enrichment regulation depends on the site properties and the capacity of a site to accommodate a cation.

OH incorporation in the pyroxene structure was examined through preparation of a set of pyroxene crystals of diopside composition, doped with different cations. OH was taken in relation to the gas mixing ratio by samples grown in the H2.CO2 gas mixing furnace. The samples grown in the gas mix took much less than the Fe3+-rich samples grown in air that were heated in H2.

Spectral analysis of the S-type asteroid population shows that absorption bands are less intense than those of the pure mafic silicate assemblages. This shows that an additional phase consisting of ferrous nickel metal phase is present. This is caused by the presence of fully differentiated, partially differentiated and undifferentiated objects inside the S-asteroid population.

Ophiolitic basalts composed of plagioclase-clinopyroxene associated with glass fragments do not provide information about the petrogenesis process and structure of magma in Ligurian Tethys. Clinopyroxenes in ophiolitic basalts are analyzed by ion microprobe method. Geochemical modeling of magma reveals that IL and EL basalts are not mantle source composition products but of fractional melting in asthenospheric MORB sources.

The saturation temperature of orthopyroxene and olivine melts are obtained from irregular silicate solution models with a precision of plus or minus 10K in crystallization and melt phases. Thermo chemical properties are used to calcute the mineral melt and energy produced in the two components. Eruption temperatures of the two species are less than the temperature of Atlantic mid-oceanic ridge basalts, indicating the occurrence of fractional crystallization processes.