No abstract

Abstract. Cryolite, Na3A1F6[=2Na+(Nag.sAlg.~)F3] is a mixed fluoride perovskite, in which the corner-sharing octahedral framework is formed by alternating [NaF6] and [A1F6] octahedra and the cavities are occupied by Na § ions. At 295 K, it is monoclinic (~ phase), space group P21/n with a=5.4139(7), b=5.6012(5) and c = 7.7769 (8) A and fl = 90.183 (3), Z = 2. A high temperature single crystal X-ray diffraction study in the range 295-900 K indicates a fluctuation-induced first-order phase transition from monoclinic to orthorhombic symmetry at TO~885 K, in contrast to a previous report that it becomes cubic at ,-~ 823 K. The space group of the high temperature 1~ phase is Immm with a = 5.632 (4), b= 5.627 (3) and c= 7.958 (4)/~, Z=2 at 890 K. Above To, the coordination number of the Na § ion in the cavity increases from eight to twelve and the zigzag Nal-A1 octahedral chains parallel to c become straight with the Nal -F--A1 angle = 180 ~ The phase transition is driven by two coupled primary order parameters. The first corresponds to the rotation of the nearly rigid [A1F6] group and transforms according to the F4 + irreducible representation of Immm. Coupled to the [A1F6] rotation is a second primary order parameter corresponding to the displacement of the Na2 + ion in the cavity from its equilibrium position. This order parameter transforms according to the X~ irreducible representation of Immm. Following Immm ~ P21/n phase transition, four equivalent domains of P21/n are determined relative to Immm, which are in an antiphase and/or twin relationship. The abrupt shortening of the octahedral A1-F and Na--F bonds and a sudden change in orientations of the atomic thermal vibration ellipsoids above To indicate a crossover from displacive to an order-disorder mechanism near the transition temperature. The fl phase is interpreted as a dynamic average of four micro-twin and -antiphase domains of the ~ phase. This view is consistent with the entropy of phase transition, ASt .... (11.43 JK-1 mol-1) calculated from heat capacity measurements (Anovitz et al. 1987), which corresponds closely to R ln4 (11.53 JK -1 tool-l), where 4 is the number of domains formed during the phase transition. The dynamic nature of the/~ phase is independently confirmed from a considerable narrowing of the 27A1 nuclear magnetic resonance (NMR) line-shape above To .

Liu (1986Liu ( , 1987 reported the diffraction pattern of a hydrous magnesium silicate resulting from the breakdown of serpentine at 22 GPa and 1000 ЊC and named this material ''phase D.'' Since that time there have been several reports of the synthesis of phase D, but its confirmed composition and crystal structure have not yet been reported. We synthesized a new dense hydrous magnesium silicate at 20 GPa and 1200 ЊC and solved its crystal structure (R w ϭ 0.015 and R ϭ 0.014). The single crystal has composition Mg 1.11 Si 1.89 H 2.22 O 6 (ideal formula: MgSi 2 H 2 O 6 ), cell parameters a ϭ 4.7453(4), c ϭ 4.3450(5) Å , and V ϭ 84.74(2) Å 3 , and space group P31m. The crystal structure is relatively simple with all the Si occupying octahedral sites in a layer similar to that of brucite, but with one of every three octahedra vacant. The MgO 6 octahedra are located above and below each vacant octahedral site. All O-H bonding occurs between SiO 6 octahedral layers. This is the only high-pressure hydrous magnesium silicate structure reported to date that contains all octahedrally coordinated Si. The calculated density of phase D (d cal ϭ 3.50 g/cm 3 ) is substantially greater than any other high-pressure hydrous magnesium silicate phase.

Key indicatorsSingle-crystal X-ray study T = 273 K Mean (e-O) = 0.001 Å R factor = 0.019 wR factor = 0.052 Data-to-parameter ratio = 21.0 For details of how these key indicators were automatically derived from the article, see

This trend can be explained by polybaric graphite-CO-CO 2 equilibria in the Martian mantle. Shergottites would have formed at pressures between 1.2 and 3.0 GPa, and nakhlite parent liquids formed at pressures >3.0 GPa, consistent with geochemical and petrologic data for the shergottites and nahklites. Carbon buffering in the Martian mantle could be responsible for variation in fO 2 in Martian meteorites (rather than assimilation or crustal interaction), as well as C-H-O fluids that could be the source of ~30 ppb CH 4 detected by recent spacecraft missions. The conundrum of an oxidized current mantle and basalts, but reduced early mantle during core-mantle equilibrium exists for both the Earth and Mars. A polybaric buffering role for graphite can explain this discrepancy for Mars, and thus it may not be necessary to have an oxidation mechanism like the dissociation of MgFe-perovskite to account for the oxidized terrestrial mantle.

No abstract

No abstract

Abstract-Tuite, c-Ca 3 (PO 4 ) 2 , was first discovered as the high-pressure phase of whitlockite in shock veins of the Suizhou L6 meteorite. This study reports the finding of tuite in a shock vein of the same Suizhou chondrite as a product of decomposition of chlorapatite, where it coexists with coarse-grained ringwoodite, majorite, lingunite, fine-grained majorite-pyrope solid solution, and magnesiow€ ustite. Moreover, we also successfully synthesized tuite with a multianvil apparatus from chlorapatite at 15 GPa and 1573 K over 24 h. Both natural and synthetic tuite crystals were examined by means of optical microscopy, scanning electron microscope, electron microprobe analysis, X-ray diffraction, and Raman spectroscopy. Our results suggest that the Na 2 O, MgO, and Cl contents in natural tuite may serve as good indicators for distinguishing the precursor phosphate mineral, chlorapatite or whitlockite.