Monipite, MoNiP, a new phosphide mineral in a Ca-Al-rich inclusion from the Allende meteorite

Abstract

Monipite (IMA 2007-033), MoNiP, is a new phosphide mineral that occurs as one 1 × 2 mm crystal in a Type B1 Ca-Al-rich inclusion (CAI) ACM-2 from the Allende CV3 carbonaceous chondrite. It has an empirical formula of (Mo_0.84Fe_0.06Co_0.04Rh_0.03)(Ni_0.89Ru_0.09)P, and a P6̅2m Fe₂P type structure with a = 5.861, c = 3.704 Å, V = 110.19 ų, and Z = 3. The calculated density using our measured composition is 8.27 g/cm³, making monipite the densest known mineral phosphide. Monipite probably either crystallized from an immiscible P-rich melt that had exsolved from an Fe-Ni-enriched alloy melt that formed during melting of the host CAI or it exsolved from a solidified alloy. Most of the original phosphide in the type occurrence was later altered to apatite and Mo-oxides, leaving only a small residual grain. Monipite occurs within an opaque assemblage included in melilite that contains kamiokite (Fe₂Mo₃O₈), tugarinovite (MoO₂), and a Nb-rich oxide [(Nb,V,Fe)O₂], none of which has previously been reported in meteorites, together with apatite, awaruite (Ni₂Fe), and vanadian magnetite.