An index of crystallinity for quartz

Abstract

A crystallinity index (CI) for quartz is based on the degree of resolution of the d(212) X-ray reflection, and measurements are scaled to give a maximum CI of 10 for such quartz samples. Examples of poorly crystallized quartz (CI of <1.0-3.0) are chrysoprase from Australia, cherts of the Monterey Shale of California, and Caballos Novaculite of Texas. Well-crystallized quartz (CI 8.0-10.0) occurs in Franciscan cherts of California and in the Arkansas Novaculite at places where these rocks seem to be strongly metamorphosed. The crystallinity index seems to be largely a function of crystalline size (up to about 1 µ diameter) but may also be affected by lattice distortions induced by mechanical stress. That time alone will not improve crystallization is shown by a silicified wood of Callixylon with a CI 1.2 from the Devonian of Ontario, Canada.