Nierite (Si3N4), a new mineral from ordinary and enstatite chondrites

Abstract

Abstract— Nierite (Si3N4) is a new mineral that has been found in perchloric acid‐resistant residues of three ordinary chondrites (Adrar 003 [LL3.2], Inman [L3.4] and Tieschitz [H3.6]) and one enstatite chondrite (Indarch [EH4]). This mineral occurs as very small (∼2 × 0.4 μm) lath‐shaped grains, which have been characterised by transmission electron microscopy. The d‐spacings of nierite are, within errors, comparable to those of synthetic α‐Si3N4, which has trigonal symmetry (P31c, a = 0.7758 nm, c = 0.5623 nm, V = 0.2931 nm3 Z = 4). Energy‐dispersive x‐ray analyses confirm that nierite is a Si‐ and N‐rich mineral. A few nierite crystals in the Indarch, Inman and Tieschitz residues are intergrown with whiskers of another nitride. Only two crystals of this additional nitride were found that were of sufficient size to give electron diffraction patterns uncontaminated by nierite reflections. The d‐spacings of this second nitride are comparable to those of β‐Si3N4, the hexagonal polymorph of synthetic Si3N4.The majority of nierite crystals in Indarch are interpreted to have formed by exsolution of Si and N from kamacite, perryite and schreibersite during parent‐body metamorphism. Some grains have evidence for two discrete episodes of nierite crystallization. The origin of nierite in Adrar 003, Inman and Tieschitz is not known, but formation during exsolution is again possible. The petrographic relationships between nierite and β‐Si3N4 in Indarch, Inman and Tieschitz suggests that the β‐Si3N4 whiskers predated nierite and acted as a seed on which nierite crystals later nucleated. The nierite/β‐Si3N4 ratio in ordinary chondrites is controlled by their metamorphic grade and possibly also their oxidation state.