Datolite is a calcium boron hydroxide nesosilicate, CaBSiO4(OH)... It is common in the copper deposits of the Lake Superior region of Michigan. It occurs as a secondary mineral in mafic igneous rocks often filling vesicles along with zeolites in basalt. Unlike most localities throughout the world, the occurrence of datolite in the Lake Superior region is usually fine grained in texture and possesses colored banding. Much of the coloration is due to the inclusion of copper or associated minerals in progressive stages of hydrothermal precipitation.This specimen comes from the Tamarack Mineral Company in Calumet, Michigan, on the Keweenaw Peninsula where I went on an interesting rockhounding field trip several years
ago.
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