Cafeosite, Ca4Fe2+3Fe3+2□O6S4, a new meteoritic oxysulfide, a redox indicator of metamorphic alteration of carbonaceous asteroids

Abstract

A natural iron-bearing oxysulfide, named сafeosite after its chemical composition, is a unique example of a mineral that simultaneously contains iron in three oxidation states: Fe³⁺, Fe²⁺, and intermediate between Fe²⁺ and Fe⁰ involved in metallic-type FeFe bonding. Cafeosite was discovered in metamorphosed carbonaceous chondrite Dhofar 225, which is classified as CM-anomalous but likely related to the CY (Yamato-type) group. The mineral occurs as tiny anhedral grains that coalesce into irregular aggregates up to 20 μm, commonly encrusted by micrometer-thick troilite or pyrrhotite rims. The grains are randomly disseminated within a chondrite matrix composed of thermally altered phyllosilicates. Associated accessory minerals are troilite, pyrrhotite, Fe-rich, Al-bearing olivine, unknown Al-bearing Fe sulfide, Al-rich chromite, kamacite, awaruite, pentlandite, escolaite, and perovskite. In reflected light, cafeosite is gray, with no internal reflections. Anisotropy is moderate, bireflectance in gray hues. Infrared microspectroscopy did not reveal any bands attributable to (OH)⁻, H₂O or CO₃²⁻ vibrations. Owing to the small grain size, the crystal structure of the mineral has been studied using synthetic analog, which was found to be isostructural with natural cafeosite based on electron backscatter diffraction (EBSD) data. Cafeosite is orthorhombic, space group Cmce (#64), a 17.4856(9), b 11.1516(5), c 11.1543(5) Å, V 2175.0(2) ų, Z = 8, D_x = 4.11 g cm⁻³. The crystal structure has been solved and refined to R₁ = 0.039 for 1105 unique reflections. Chemical composition of both natural and synthetic cafeosite corresponds to the formula Ca₄Fe²⁺₃Fe³⁺₂(□_1−xFe_x)O₆S₄ where (□_1−xFe_x) denotes structural vacancy partially occupied by semimetallic-type Fe (x = 0.2–0.3). The ideal endmember formula of the mineral is Ca₄Fe²⁺₃Fe³⁺₂□O₆S₄. Cafeosite was likely formed from previously altered precursor material of Dhofar 225, which, like common CM chondrites, consisted of phyllosilicates, Ca-bearing carbonates, tochilinite-like sulfides–hydroxides and pyrrhotite. During thermal metamorphism at temperatures between 750 and 900°C, sulfides–hydroxides were partly sintered with calcined carbonates and iron oxides, resulting in cafeosite formation. Due to varying and redox-dependent contents of Fe³⁺ and Fe²⁺, as well as the presence of metallic-type Fe in the structure, cafeosite could be regarded as a single-phase redox indicator alternative to the known triple-phase buffers, for example, iron–magnetite–pyrrhotite (IM-Po), iron–wüstite–pyrrhotite (IW-Po) and magnetite–wüstite–pyrrhotite (MW-Po) systems. Discovery of cafeosite provides insight into a previously obscured aspect of CY-chondrite formation: the redox conditions of thermal metamorphism on carbonaceous asteroids.