COMPOSITE NATROLITE-MESOLITE CRYSTALS FROM THE COLUMBIA-RIVER-BASALT-GROUP, CLARKSTON, WASHINGTON

Chemical composition, optical properties, and X-ray-diffraction photog raphs confirm the occurrence of ''single'' crystals of natrolite cappe d by mesolite in cavities in the Columbia River Basalt Group near Clar kston, Washington. The indices of refraction for the natrolite tip are alpha 1.4781(3), beta 1.4814(3), and gamma 1.4895(3), with a measured 2V(z) of 62.9-degrees, and the index of refraction for the near-isotr opic mesolite tip is 1.5055(3). Chemical analysis by electron micropro be shows the natrolite and mesolite portions to be near their ideal co mpositions. Single-crystal X-ray diffraction shows a tripling of b for the mesolite portion of the composite crystal. Calcium and sodium do not readily exchange in natrolite and mesolite; thus, the formation of Ca-free natrolite at the beginning of growth and Ca-rich mesolite at the end of growth indicates a change in the environment during crystal growth.