Nuwaite (Ni6GeS2) and butianite (Ni6SnS2), two new minerals from the Allende meteorite: Alteration products in the early solar system

1Chi Ma, 1John R. Beckett
American Mineralogist 103, 1918-1924 Link to Article [https://doi.org/10.2138/am-2018-6599]
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
Copyright: The Mineralogical Society of America

Nuwaite (Ni6GeS2, IMA 2013-018) and butianite (Ni6SnS2, IMA 2016-028) are two new chalcogenide minerals, occurring as micrometer-sized crystals with grossular, Na-bearing melilite, heazlewoodite, and Ge-bearing Ni-Fe alloys in veins and as mono-mineralic crack-filling material in igneous diopside in the Type B1 Ca-Al-rich inclusion (CAI) ACM-2 from the Allende CV3 carbonaceous chondrite. The chemical composition of type nuwaite is (wt%) Ni 65.3, S 10.3, Ge 8.2, Te 7.9, Sn 5.1, and Fe 1.7, with a sum of 98.5 and an empirical formula of (Ni5.95Fe0.16)(Ge0.60Sn0.23)(S1.72Te0.33). The simplified formula is Ni6(Ge,Sn)(S,Te)2, leading to an end-member of Ni6GeS2. The chemical composition of type butianite is (wt%) Ni 62.1, Sn 8.9, Te 10.3, S 8.9, Ge 5.3, Fe 1.3, sum 99.1, giving rise to an empirical formula of (Ni5.93Fe0.13)(Sn0.52Ge0.41)(S1.56Te0.45). Butianite’s simplified formula is Ni6(Sn,Ge) (S,Te)2 and the end-member formula is Ni6SnS2. Both nuwaite and butianite have an I4/mmm inter-growth structure with a = 3.65 Å, c = 18.14 Å, V = 241.7 Å3, and Z = 2. Their calculated densities are 7.24 and 7.62 g/cm3, respectively. Nuwaite and butianite are the first known meteoritic minerals with high Ge and Sn concentrations.
Nuwaite and butianite are very late-stage, vapor-deposited, alteration products, filling in pores within preexisting grossular-rich alteration veins and cracks in igneous Al,Ti-diopside. These phases and associated heazlewoodite and Ge-bearing alloys are observed only within the Ca-,Al-rich inclusion (CAI) and not outside it or at the inclusion-matrix interface. As only sections in one half of ACM-2 contain nuwaite/butianite, they were probably derived through a relatively low fO2-fS2 sulfidation process, in which a highly localized, low-temperature Ge-, Sn-bearing fluid interacted with a portion of the host CAI. It is likely that the fluid became relatively more Sn- and Te-enriched with time and that crack fillings post-date vein fillings, possibly due to a late remobilization of vein sulfides.

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