Showing papers in "American Mineralogist in 1954"

Study of orthopyroxenes from volcanic rocks

Abstract: Orthopyroxenes from volcanic rocks covering a composition range from bronzite to eulite were studied and the result is compared with the properties of orthopyroxenes from plutonic and metamorphic rocks. Orthopyroxenes crystallized at high temperatures contain from 0.104 to 0.057 ca in atomic proportion, whereas those crystallized at low temperatures contain less than 0.033 Ca. Pleochroism appears to depend on Ti content. The volcanic orthopyroxenes having compositions about Mg : Fe+2f Fe+rf Mn: 1 : 1 show optic angles (2V about X) 7' Iarger than those of the plutonic and metamorphic rocks. This departure gradually diminishes as the composition becomes either more magnesian or more ferriferous. unit cell dimensions a, b, and c all increase with Fe+z substituting for Mg. But the a and c dimensions increase more markedly with Ca substituting for Mg than with Fe+2. Aluminum substituting for Mg causes contraction of the b dimension.

On clouded plagioclase

Abstract: Various theories to account lor clouded plagioclase are reviewed. Observations indicate that the particles causing this efiect may consist of different minerals, and that many minerals, besides plagioclase, may show clouding. It is suggested that in clouded minerals there are minute surfaces of physical discontinuity which provide adequate passages for difiusion of material into and out of the crystals. In intermediate plagioclases these surfaces may consist of internal phase boundaries in the unmixed feldspar. Slight clouding is probably due to exsolution of iron present in the feldspar lattice at the time of its formation, but more intense clouding is believed to be the result of the migration of iron and other elements into the crystal after its formation. The geological significance of clouded plagioclase is discussed and it is shown that such feldspars cannot be used as sole criteria for thermal metamorphism.

Brannerite from California

Abstract: Brannerite is found in quartz veins about seven miles south of Coleville, Mono County, California. It is very similar in composition to brannerite from the type locality in Idaho and from elsewhere. Initially metamict brannerites from Idaho, California, and Morocco yield similar r-ray diffraction patterns after heating. It has proved impossible to index these patterns and so the crystallographic character of heated brannerite remains in doubt,