Mario Tribaudino

The high-temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6): structural and thermodynamic behavior

Abstract A high-temperature in situ single-crystal X-ray diffraction study was performed from room T to 1150 °C on two crystals of Fe-free P21/c clinopyroxenes of composition Ca0.15Mg1.85Si2O6 [cell parameters at room T: a = 9.651(2) Å, b = 8.846(2) Å, c = 5.202(1) Å, b = 108.38(2)°, V = 421.4 (2) Å3] synthesized by isothermal annealing for 624 h at T = 1370 °C, P = 1 atm. A first order P21/c-C2/c phase transition was found slightly below 1000 °C [Tc = 926(39) °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 °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 Tc. TEM observations of a sample from the same starting material after further annealing for 72 h at T = 1050 °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 °C showed only minor changes in the chain configurations, which are highly differentiated up to 1000 °C, confirming the strong first-order character of the transition.

Primary trapped melt inclusions in olivine in the olivine-augite-orthopyroxene ureilite Hughes 009

We describe the first known occurrence of primary melt inclusions in a ureilite. The ureilite is Hughes 009, one of a small number of ureilites whose primary mineralogy is olivine-augite-orthopyroxene, rather than olivine-pigeonite. Hughes 009 has a coarse-grained, equilibrated texture typical of ureilites, and homogeneous primary mineral compositions: olivine — mg 87.3; augite — mg 89.2, Wo 37.0, Al2O3 = 1.6 wt.%; orthopyroxene — mg 88.3, Wo 4.9. It shows only limited secondary reduction effects and no petrographically recognizable carbon phases, which indicates that its original carbon content was lower than in most ureilites. The melt inclusions occur in olivine crystals. They are concentrated in the central regions of their hosts, showing elongate (mostly 20–60 μm in maximum dimension), negative olivine crystal shapes and parallel alignment. These and other features indicate that they were trapped during initial growth of their hosts from a liquid, and are likely to be representative samples of that liquid. They consist of glass and single, subhedral crystals of high-Ca pyroxene, with minor Cr-rich spinel and metal-phosphide-sulfide spherules. They are surrounded by halos of olivine with rounded outlines defined by tiny bits of metal and thin arcs of glass. Pyroxenes within each inclusion show zonation patterns indicating that they nucleated at the olivine/ liquid interface with compositions close to that of the primary augite, and then grew inward with dramatically increasing Al2O3 (to 10.8 wt.%), Wo (to ≈50), TiO2 and Cr2O3 contents. Glasses within each inclusion are relatively homogeneous. Glasses from all inclusions show well-defined trends of CaO, TiO2, Cr2O3, Na2O and SiO2 vs. Al2O3, (16–23 wt.%) that can be modelled as resulting principally from crystallization of various amounts of the pyroxene. The halos, which represent olivine that grew from the trapped melts, are zoned in Cr and Ca with concentrations decreasing inward, reflecting cocrystallization of pyroxene; they have homogeneous Fe/Mg identical to that of the primary olivine, indicating reequilibration with the host. We develop a petrologic model for the postentrapment history (crystallization, reaction and reequilibration) of the inclusions, based on which we reconstruct the composition of the primary trapped liquid (PTL). The PTL was saturated only with olivine. This result implies that Hughes 009 is a cumulate (consistent with the high Mn/Mg ratio of its olivine and a low abundance of graphite) and that the composition of the PTL is close to that of its parent magma. The low-pressure equilibrium crystallization sequence predicted by MAGPOX calculations for the PTL (olivine → augite → plagioclase → pigeonite) is not, however, consistent with the primary mineralogy of Hughes 009. If the conditions of these calculations are, indeed, appropriate, then complex processes such as magma mixing must have been involved in the petrogenesis of this ureilite. This conclusion is consistent with other evidence that the olivine-augite-orthopyroxene ureilites record a more complex magmatic evolution than is evident in the olivine-pigeonite ureilites. TEM investigations of microtextural features in all phases and XRD determination of Fe2+-Mg site distribution in orthopyroxene have elucidated the cooling and shock history of this ureilite. Hughes 009 experienced an extremely high cooling rate (7 ± 5°C/h at the closure T of 630°C) late in its evolution, and two distinguishable shock events—the first at peak pressures of 5 to 10 GPa, resulting in mechanical polysynthetic twinning in augite and orthopyroxene and mild undulatory extinction in olivine; and the second at lower pressures, resulting only in brecciation and redistribution of metal. Its late history is similar to that of most ureilites, and probably reflects impact excavation.

The Raman spectrum of diopside: a comparison between ab initio calculated and experimentally measured frequencies

The Raman spectrum of diopside has been calculated by using three purely Density Functional Theory (DFT) Hamiltonians (PBE, WCPBE, LDA), the Hartree-Fock Hamiltonian (HF) and three hybrid HF/DFT ones (B3LYP, WC1LYP, PBE0). A comparison has been done between the calculated frequencies with those measured by Raman spectroscopy on a natural sample, along with several different orientations and beam polarizations, or retrieved from literature; such a comparison demonstrated the excellent performances of the hybrid Hamiltonians in reproducing the vibrational spectrum of the mineral, in line with what it is generally observed in literature concerning other mineral phases. In particular, the mean average absolute discrepancies of the calculated frequencies with respect to the experimental data were: 3.2 (WC1LYP), 4.7 (B3LYP), 6.5 (PBE0), 18.0 (PBE), 9.7 (WCPBE), 7.3 (LDA), and 40.6 cm −1 (HF). The very good quality of the WC1LYP results allowed for a reliable assignment of all of the experimentally observed Raman signals, and the corresponding assignments to specific patterns of atomic vibrational motion (normal modes).

High pressure behavior, transformation and crystal structure of synthetic iron-free pigeonite

Abstract A single-crystal high pressure X-ray investigation was performed up to P = 6.5 GPa on a synthetic clinopyroxene of composition Ca0.15(1)Mg1.85(1)Si2.00(1)O6 [Di15En85, unit-cell parameters at room pressure: a = 9.6525(6)Å, b = 8.8461(2)Å, c = 5.2036(5)Å, β = 108.370(5)°, V = 421.68(4)Å3]. A first order P21/c-C2/c displacive phase transition was found at P = 5.1 GPa; the transition was revealed by the disappearance of the b reflections (h + k = odd) and by sharp changes in the unit-cell parameters. Reversals through the transformation show that, if present, hysteresis is smaller than 0.1 GPa. The volume variation has been described by a third-order Birch-Murnaghan equation of state with V0 = 421.68(8) Å3, KT0 = 102(2) GPa, and K′ = 8(1) for the low-symmetry phase (P21/c) and with V0 = 411.06(3) Å3 and KT0 = 108(2) GPa for the high-symmetry phase (C2/c), with K′ fixed to the value obtained for the low-symmetry form. The axial compressibility shows the following scheme: βb > βa ≅ βc > βasinβ for both phases. In comparison with pure clinoenstatite, Di15En85 shows a similar step in unit-cell parameters at the transition, the disappearence of hysteresis and a decrease of transition pressure and of bulk modulus. Full intensity data sets were collected at room pressure, 2.6 and 4.5 GPa for the P21/c phase and at P = 6.2 GPa for the C2/c phase. A slight increase of the intensity of h + k odd reflections and of the difference in the A and B chain kinking angles were observed. A comparison of the structural behavior of the P21/c phase at high temperature and high pressure shows opposite behavior for M2-O bond lengths and O3-O3-O3 kinking angle.

The crystal structure of pyroxenes along the jadeite–hedenbergite and jadeite–aegirine joins

Abstract The crystal-structures of seven synthetic pyroxenes along the jadeite.hedenbergite (Jd57Hd43, Jd26Hd74, Jd0Hd100) and jadeite.aegirine (Jd100Ae0, Jd76Ae24, Jd35Ae65, Jd0Ae100) joins were refined using data collected by means of single-crystal X-ray diffraction (space group C2/c, R4σ between 2.2 and 3.4%). The M2 and M1 polyhedral volumes and bond lengths increase with increasing aegirine and hedenbergite content, moreover the Ca for Na substitution along the jadeite-hedenbergite join changes the M2 coordination from 6 + 2 to 4 + 4, with remarkable tilting of the tetrahedral chains. The value of the displacement parameters follows the trend UeqM2 > UeqO2 > UeqO3 > UeqO1 > UeqM1 ≈ UeqT for all samples belonging to the jadeite-aegirine join and for pure hedenbergite; in contrast,, for pyroxenes with intermediate compositions between hedenbergite and jadeite the trend is UeqO1 > UeqO2 > UeqM2 > UeqO3 > UeqM1 ≈ UeqT, with O1 and O2 having anomalously large displacement parameters, probably due to different local structural configuration around the cations with different size and charge. Cation substitution at the M1 site of Na-pyroxenes gives rise to a different structural deformation with respect of the double substitution at both the M1 and M2 sites in (Na,Ca)(M3+,M2+)Si2O6 pyroxenes as the rigid tetrahedral chains try to accommodate both the increasing size of the M1 site and the different coordination requirement of the M2 site

Raman spectroscopy of (Ca,Mg)MgSi2O6 clinopyroxenes

Abstract The Raman spectra of eight clinopyroxenes synthesized along the join diopside-clinoenstatite (Di- Cen, CaMgSi2O6-Mg2Si2O6) were measured. The splitting of the 670 cm-1 mode of the Ag symmetry, observed in the composition Di52En48 to clinoenstatite, was interpreted as evidence of a C2/c-P21/c phase transition. The transition was also revealed by deviation from the linear dependence of the peak position vs. composition and by the appearance of several new peaks in the samples richer in clinoenstatite. Analysis of peak positions vs. structural changes suggests that for the M2 polyhedron, in which Ca substitution for Mg occurs, a different deformation mechanism acts in Ca richer and poorer P21/c pyroxenes, and that Ca richer P21/c pyroxenes deform with the same mechanism as C2/c pyroxenes. The frequency of the peak at 670 cm-1 was found to change linearly with the kinking angle of the tetrahedral chains for C2/c and of the B chain for P21/c, whereas the position of the peak ascribed to the A chain was little affected by the kinking angle. Peak broadening in C2/c Ca-rich homogeneous pyroxenes was interpreted to be a consequence of the positional disorder of the Ca and Mg in the M2 cavity: peak broadening increases with increasing Mg content for peaks assigned to M2-O vibrations, but it changes little for peaks assigned to chain bending, which suggests that cation substitution in the M2 cavity occurs with little interaction with the silicate chain. Furthermore peak broadening was observed in intermediate pyroxenes as a consequence of mottled textures, antiphase domains, and compositional inhomogeneity.

Tilts and tetrahedra: The origin of the anisotropy of feldspars

Abstract Following the ideas of Helen Megaw, we describe the changes in the conformation of the tetrahedral framework of feldspars in terms of just four distinct tilt systems of rigid tetrahedra. These systems are based on the four allowed tilts of a ring of four corner-linked tetrahedra with point symmetry 2. Of the four tilt systems, only two result in significant volume change of the unit cell. We show that all of the essential features of the structures, unit-cell parameters and volumes of the AlSi3 feldspars, and their expansion and compression induced by changes in pressure, temperature, and composition at crustal pressures, are generated by the simultaneous application of these two tilts to an un-tilted framework of regular tetrahedra. In combination these two tilts impose significant anisotropy upon the expansion of the unit cell of the feldspar with the majority of the expansion accommodated by the expansion of d(100). This demonstrates that the fundamental reason for the anisotropy of feldspars lies in the topology of the tetrahedral framework. A comparison of the actual tilts observed in alkali feldspars with the model shows that the tilts maximize the shortest O-O distances in the structure, and therefore O-O repulsions along with the volume requirement control the values of the tetrahedral tilts in alkali feldspars and thus the anisotropy of the structure. Therefore, the bonding requirements of the bridging O atoms and the directionality of the bonding to the extraframework cations only play a secondary role in modifying this basic pattern of anisotropy, which is intrinsic to the common topology of the framework of all feldspars.

Thermoelastic and thermodynamic properties of plagioclase feldspars from thermal expansion measurements

Abstract Powder X‑ray diffraction patterns between 90 and 935 K have been collected for nine plagioclase samples, with different compositions and degree of Al-Si order. The refined volumes have been modeled using the Wallace and Suzuki formulations based on the Mie-Grüneisen EOS. No significant difference has been found between the Suzuki and Wallace formulations, and between the Einstein and Debye approximations of lattice energy. A Wallace model with the first derivative of the bulk modulus constrained to the experimentally determined values leads to refined Grüneisen parameters between 0.49 and 0.41, without any definite trend between albite and anorthite; the Einstein temperature in intermediate plagioclase is θE - 650 K, but it is lower in albite [θE = 453(5) K]. A good fit with experimental heat capacity data for the An60Ab40 composition has been found using two Einstein-like oscillators with θE1 = 230(3) K and θE2 = 952(7) K, XθE1 = 0.391(5). The change with temperature in An60Ab40 of the Grüneisen parameter is small at T > 150 K, with a slight decrease with temperature. Similar results could be obtained by independent refinement of an Einstein model with two oscillators to the volume data for the same composition [θE1 = 205(30) K, θE2 = 873(52) K, and X = 0.36(4)]. The components of the thermal strain tensor with temperature have been calculated and confirm that the greatest deformation is along the a* axis, i.e., along the extension direction of the crankshaft chains of the feldspar structure. Anomalous behavior of the strain tensor components in the a-c plane has been observed in albite and An27Ab73, and is related to an increase in the c unit-cell parameter with decreasing temperature.

Cordierite-anorthoclase hornfels xenoliths in Stromboli lavas (Aeolian Islands, Sicily): an example of a fast cooled contact aureole

High-grade hornfels xenoliths in the 60 ka old calcalkaline basaltic andesite lava flows of Stromboli (Omo lavas; Paleostromboli II period) consist of fine- to medium-grained holocrystalline rocks. Feldspar (mainly anorthoclase) and cordierite represent ca . 85 vol% of the modal mineralogy. Aluminiferous spinel (hercynite), sillimanite, ± corundum, ± ilmenite, ± chlorapatite are also present. Major and trace element compositions, together with Sr, Nd and Pb isotopic data, indicate that the source rocks are metapelites compatible with upper continental crust-derived materials. According to common petrogenetic grids for pelitic rocks ( e.g. NaKFMASH), a pressure range of 2–3.5 kbar and a peak temperature close to 800° C can be inferred for the mineral assemblage. A honeycomb texture and wide, high-T compositional range of the ternary feldspars (Ab 35–70 Or 3–63 An 2–49 ) suggest incipient melting followed by quenching. Accordingly, the xenoliths may well represent contact metamorphosed rocks (contact aureole), compatible with metapelites belonging to the continental crust basement of Stromboli. TEM (Transmission Electron Microscopy) observations of alkali-feldspar showed the presence of a cross-hatched pattern oriented parallel to (010) and (001), and of albite twinning in the orthoclase-enriched and -depleted samples, respectively. Both textures are related to a monoclinic to triclinic phase transition in a highly disordered feldspar. A highly disordered Al-Si configuration is also corroborated by cell parameters. Incipient decomposition textures, but no exsolution lamellae, are present in the intermediate alkali-feldspars. These results support a fast cooling of the hornfels xenoliths, of the order of degrees per hour. The high-T feldspar textures were preserved because the xenoliths, stoped into the uprising basaltic andesite host magma, were rapidly transported to the surface shortly after they reached the contact aureole peak temperature.

Average structure and M2 site configurations in C2/c clinopyroxenes along the Di-En join

In order to clarify the structural configurations observed in Diss in the Ca-rich region of the Di-En join (in which TEM observations show neither exsolution microstructures nor evidence of spinodal decomposition) single crystals large enough for X-ray diffraction analyses, with composition (Ca0.66Mg0.34)MgSi2O6, have been equilibrated close to the solvus atT=1350° C for 317 h, and quenched at room temperature. The refinement in C2/c space group shows that in the M2 site Ca and Mg are fully ‘ordered’ in two split positions (M2occ: 0.66 Ca; M2′occ: 0.34 Mg). Since the average structure shows a relevant elongation of anisotropic thermal ellipsoids of the O2 and O3 oxygen atoms, the refinement has been carried out according to a split model for O2 and O3 atoms: Ca appears 8-coordinated (as in diopside) and Mg shows a sixfold coordination similar to that of high-pigeonite. This coordination for Mg is significantly different from the fourfold coordination (Zn-like in Zn-cpx) proposed previously and it is a more probable coordination for Mg from a crystalchemical point of view. The same results were obtained refining a Di80En20 cpx, equilibrated atT=1230° C, according to the same O-split model. The data support the coexistence of a Di-like configuration for Ca and of a highPig-like configuration for Mg away from the solvus also. AtT very near toTsolidus the different configurations, observed at room temperature in the quenched samples, should converge and Ca and Mg should retain a single disordered configuration in the M2 site.