The chemistry and genesis of opaque minerals in kimberlites

The chemistry and genesis of opaque minerals in kimberlites Article

Haggerty, SE. (1975). The chemistry and genesis of opaque minerals in kimberlites . 9(C), 295-307. 10.1016/0079-1946(75)90024-5

Detailed electron-microprobe analyses of zonal trends in the spinel mineral group in kimberlites show that the early formed spinels are either highly enriched in the series picrochromite-chromite, or alternatively in the series MgAl2O4FeAl2O4; in both compositional types, progressive enrichment trends induced by solid-liquid or solid-gas reaction result in an overall increase in Fe + Ti, and a specific increase in Fe+3. The range of primary spinel compositions previously reported in kimberlites has been widened towards higher ratios of Al Cr. Homogeneous and oscillatory zoned spinels, skeletal and subhedral, are present in garnet kelyphitic reaction mantles. The unique characterization of kimberlitic spinels is in high MgO, FeO(Fe2O3) and TiO2 contents; whereas kimberlitic ilmenites are fingerprinted by high MgO, intermediate Fe2O3 contents, and by the presence of Cr2O3. Accessory minerals identified are Fe, FeNi, pyrite, pyrrhotite, pentlandite and chalcopyrite. Compositional data on two new minerals are presented and the first occurrence of terrestrial armalcolite is reported. © 1975.