New minerals tsangpoite Ca-5(PO4)(2)(SiO4) and matyhite Ca-9(Ca-0.5 square(0.5))Fe(PO4)(7) from the D'Orbigny angrite

摘要

Tsangpoite, ideally Ca-5(PO4)(2)(SiO4), the hexagonal polymorph of silicocarnotite, and matyhite, ideally Ca-9(Ca-0.5 square(0.5))Fe(PO4)(7), the Fe-analogue of Ca-merrillite, were identified from the D'Orbigny angrite meteorite by electron probe microanalysis, electron microscopy and micro-Raman spectroscopy. On the basis of electron diffraction, the symmetry of tsangpoite was shown to be hexagonal, P6(3)/m or P6(3), with a = 9.489(4) angstrom, c = 6.991(6) angstrom, V = 545.1(6) angstrom(3) and Z = 2 for 12 oxygen atoms per formula unit, and that of matyhite was shown to be trigonal, R3c, with a = 10.456 (7) angstrom, c = 37.408(34) angstrom, V = 3541.6 (4.8) angstrom(3) and Z = 6 for 28 oxygen atoms per formula unit. On the basis of their constant association with the grain-boundary assemblage: Fe sulfide + ulvospinel + Al-Ti-bearing hedenbergite + fayalite-kirschsteinite intergrowth, the formation of tsangpoite and matyhite, along with kuratite (the Fe-analogue of rhonite), can be readily rationalised as crystallisation from residue magmas at the final stage of the D'Orbigny meteorite formation. Alternatively, the close petrographic relations between tsangpoite/matyhite and the resorbed Fe sulfide rimmed by fayalite + kirschsteinite symplectite, such as the nucleation of tsangpoite in association with magnetite +/- other phases within Fe sulfide and the common outward growth of needle-like tsangpoite or plate-like matyhite from the fayalite-kirschsteinite symplectic rim of Fe sulfide into hedenbergite, infer that these new minerals and the grain-boundary assemblage might represent metasomatic products resulting from reactions between an intruding metasomatic agent and the porous olivine-plagioclase plate + fayalite-kirschsteinite overgrowth + augite + Fe sulfide aggregates. Still further thermochemical and kinetics evidence is required to clarify the exact formation mechanisms/conditions of the euhedral tsangpoite, matyhite and kuratite at the grain boundary of the D'Orbigny angrite.