Publication Date:
2018
Description:
〈div data-abstract-type="normal"〉〈p〉Tsangpoite, ideally Ca〈span〉5〈/span〉(PO〈span〉4〈/span〉)〈span〉2〈/span〉(SiO〈span〉4〈/span〉), the hexagonal polymorph of silicocarnotite, and matyhite, ideally Ca〈span〉9〈/span〉(Ca〈span〉0.5〈/span〉□〈span〉0.5〈/span〉)Fe(PO〈span〉4〈/span〉)〈span〉7〈/span〉, 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, 〈span〉P〈/span〉6〈span〉3〈/span〉/〈span〉m〈/span〉 or 〈span〉P〈/span〉6〈span〉3〈/span〉, with 〈span〉a〈/span〉 = 9.489(4) Å, 〈span〉c〈/span〉 = 6.991(6) Å, 〈span〉V〈/span〉 = 545.1(6) Å〈span〉3〈/span〉 and 〈span〉Z〈/span〉 = 2 for 12 oxygen atoms per formula unit, and that of matyhite was shown to be trigonal, 〈span〉R〈/span〉3〈span〉c〈/span〉, with 〈span〉a〈/span〉 = 10.456 (7) Å, 〈span〉c〈/span〉 = 37.408(34) Å, 〈span〉V〈/span〉 = 3541.6 (4.8) Å〈span〉3〈/span〉 and 〈span〉Z〈/span〉 = 6 for 28 oxygen atoms per formula unit. On the basis of their constant association with the grain-boundary assemblage: Fe sulfide + ulvöspinel + Al–Ti-bearing hedenbergite + fayalite–kirschsteinite intergrowth, the formation of tsangpoite and matyhite, along with kuratite (the Fe-analogue of rhönite), 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.〈/p〉〈/div〉
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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