In the Eye of the Beholder

Abstract

MINERALOGY Tiger's eye is the popular name for gem-quality quartz-rich material composed of gold to brown fibrous bands that exhibit chatoyancy (lustrous color changes with an undulating narrow band of white light, like a cat's eye). Blue tiger's eye (also called hawk's eye) gets its blue color and chatoyancy from crocidolite, an asbestiform amphibole. In tiger's eye, the brown colors are derived from the hydrothermal alteration of the crocidolite to goethite. Both gems were thought to be produced by pseudomorphism, in which quartz replaces most of the crocidolite while retaining its fibrous structure (like petrified wood or mineralized fossils). After 125 years of this textbook explanation, Heaney and Fisher have determined that tiger's-eye is formed instead by a repetitive crack-seal mechanism. Host rock that contains crocidolite is fractured by stress and heat; silica-saturated fluids enter the crack; and, while crocidolite grows out from the host rock, columnar quartz grows around and between the crocidolite. Thus, tiger's eye records the structural and thermal path of the host rock and can be used to reconstruct tectonic events.— LR Geology 31 , 323 (2003).