Eugene N. Cameron

Postcumulus and subsolidus equilibration of chomite and coexisting silicates in the Eastern Bushveld Complex

Analyses of chromite and coexisting olivine or bronzite in various textural associations in the Critical Zone of the Eastern Bushveld Complex are examined for evidence of the extent and nature of postcumulus or subsolidus equilibration of chromite and Fe-Mg silicates. Subsolidus equilibration of chromite inclusions in bronzite and olivine is indicated, and postcumulus or subsolidus equilibration of chromite with postcumulus bronzite is indicated in mottled anorthosite of the H unit. Compositional zoning of cumulus bronzite is inferred to be due to equilibration with cumulus chromite during the postcumulus stage. The main pattern of variation in MgFe ratio and Al2O3 content of chromite with modal per cent chromite is inferred, however, to have been produced during the cumulus stage. The data are consistent with control of chromite precipitation and composition by the oxygen fugacity of the magma, but the effects of changes in pressure or other variables of the magma system on chromite stability and composition are still largely unknown.

Opaque Minerals in Lunar Samples

Microscope study and electron microprobe analysis of lunar rocks and soil show that ilmenite, troilite, and native iron are accompanied by trace amounts of ulv�spinel, titanochromite (new mineral name), an unidentified Ti-Fe oxide, and a complex Zr-Y silicate. The assemblage requires a strongly reducing environment. Textures and modal proportions show that the rocks present are not a differentiation series. The restricted nature of the opaque mineral assemblage suggests a narrow range of composition for the materials from which the parent liquids of the rocks were generated. Textural variety mnust reflect differences in cooling rates, probably related to depths of formation.

Nonfuel mineral problems of the United States

Abstract Since World War II, United States dependence on imports for supplies of nonfuel minerals has significantly increased. Mineral supplies are now being drawn from many countries of the world. Rising dependence on imports has been accompanied by transfer of certain segments of American mineral industry to mineral-producing countries and by internationalization of American industry at various levels from mining, smelting, and refining to manufacturing. Treatment of deficiencies in United States production solely in terms of supply of deficient mineral raw materials is, therefore, no longer possible. A review of national policies affecting nonfuel mineral supply and the nature and strength of domestic mineral-based industry is overdue. A coherent, long-term national mineral policy aimed at reversing deterioration of mineral-producing and mineral-consuming industries of the United States is needed.

Postcumulus and subsolidus equilibration of chromite and coexisting silicates in the Eastern Bushveld Complex

Analyses of chromite and coexisting olivine or bronzite in various textural associations in the Critical Zone of the Eastern Bushveld Complex are examined for evidence of the extent and nature of postcumulus or subsolidus equilibration of chromite and Fe-Mg silicates. Sub-solidus equilibration of chromite inclusions in bronzite and olivine is indicated, and postcumulus or subsolidus equilibration of chromite with postcumulus bronzite is indicated in mottled anorthosite of the H unit. Compositional zoning of cumulus bronzite is inferred to be due to equilibration with cumulus chromite during the postcumulus stage. The main pattern of variation in Mg/Fe ratio and A12O3 content of chromite with modal per cent chromite is inferred, however, to have been produced during the cumulus stage. The data are consistent with control of chromite precipitation and composition by the oxygen fugacity of the magma, but the effects of changes in pressure or other variables of the magma system on chromite stability and composition are still largely unknown.

The influence of the immersion medium on the rotation properties of anisotropic minerals in reflected light

The rotation of the plane of polarization of incident light produced by reflection from a polished surface, cleavage, or crystal face of an anisotropic mineral changes as the immersion medium (oil, water, air) is varied. For a given mineral the changes produced are functions of the optical constants of the mineral and are therefore specific properties. This paper discusses the principles underlying changes in angles of rotation and related rotation properties with variation in the immersion medium and the possible applications of the changes to the study of minerals in reflected light. Preliminary observations suggest that the changes can be used as an aid in identifying ore minerals.