Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites

Abstract

TEM, HRTEM, HVEM and SEM methods, coupled with energy dispersive X-ray analysis, have been used to study the microstructure and the phases comprising the matrix of carbonaceous chondrites Murchison, Cold Bokkeveld, Nawapali and Cochabamba. A wide variety of phyllosilicate morphologies occurs in each. Very small crystals and clasts of olivine, pyroxene and other unhydrated minerals are mixed intimately with the phyllosilicates. Intergrowths of carbonates and Sulfides within the phyllosilicates also occur, as well as a ubiquitous spongey material which is difficult to characterize, but contains elementary phyllosilicate units and embryo crystals. The identifiable large crystalline phyllosilicates are principally Fe-rich serpentine-group minerals and intermediate more Mg-rich chrysotilelike group members, with characteristic ~ 7.0–7.4 Å basal layer spacings. Complex interlayered and intergrown hydrous minerals also occur associated with the spongey material, and other poorly crystalline silicates and finely divided Fe-Ni sulphides. Fe/Si and Mg/Si ratios vary on a sub-micron scale, and the morphologies of the larger phyllosilicate crystals correlate broadly with these variations. Small crystals of sodium chloride and potassium chloride have been identified, occluded within a predominantly organic mass. The matrix minerals have a multistage history of formation in which the effects of aqueous alteration are dominant. Few, if any, of matrix minerals can be unmodified nebular condensates, although some clasts and inclusions have escaped alteration and predate the alteration process.