Coesite prepared from hydrated silica gel between 50 and 80 kb and 700-900 C. in a Belt-type apparatus crystallized as hexagonal platelets flattened parallel to (010) and as lath-shaped crystals flattened parallel to (100) and elongated parallel to the c axis. Previous x-ray studies showed that coesite is pseudohexagonal and monoclinic; the biaxiality and orientation of its optical indicatrix also reveal its monoclinic character. The optical orientation is X = b and Z∧c=4°-6° with ²=120°. The optical constants are ± —1.5940, ²= 1.5955, ³=1.5990 (Na light; all ±0.0005); (+)2V=66.5° calculated from the indices of refraction, (+)2V=64° measured with the universal stage; weak horizontal dispersion with r< v. Coesite is colorless, white when powdered, and has a subconchoidal fracture and a vitreous luster.

Simple contact twins of the normal type with (021) as composition plane are common. They range from anhedral grains to tabular heart-shaped crystals to tabular kite-shaped crystals with increasing morphological development and appear to be flattened normal to the a axis. Twinned crystals have a re-entrant angle of 90°; some have a second re-entrant angle of 62°.

The measured density of coesite is 2.915± .005. Using unit-cell dimensions of a = c = 7.14 Å, and b = 12.37 Å, based on an indexed powder pattern, Z= 16, and the calculated density is 2.92. Refractive index-density relationships suggest that silicon is tetrahedrally coordinated in coesite in agreement with x-ray and infrared absorption results of other workers.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.