Datolite

specimen number:
2133298

location:
Isle Royale National Park, Keweenaw Co, Michigan, USA

description:
A mass of datolite, this polished on one face. Good thumbnail for this classic material.

Isle Royale National Park in Michigan contains datolite nodules, which are calcium boron silicate masses that formed roughly 1.1 billion years ago within the Mesoproterozoic Portage Lake Volcanics. These nodules precipitated from low-temperature hydrothermal fluids that filled gas vesicles and fractures within basaltic lava flows, often incorporating micro-inclusions of native copper or native silver that yield a striking porcelain-like texture with red, pink, or white coloration when cut. Prehistoric Indigenous peoples quarried the island for copper rather than datolite, leaving the extraction of these nodules to historic copper mining operations. Formal commercial mining on Isle Royale occurred in three main periods between 1843 and 1899, notably at the Minong Mine (1874 to 1883) and the Island Mine (1873 to 1875), where miners encountered datolite as a byproduct in the copper-bearing basalt lodes. Today, because Isle Royale is a protected national park established in 1940, all traditional mining, quarrying, and mineral collecting are strictly prohibited, preserving any remaining embedded nodules in their natural geological context.

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