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.