Term: mica (mining)
A simple lightish mineral, having a shining silvery surface, capable of being split into very thin scales. The brilliant scales in granite and gneiss are mica.
View pictures relating to mining at Wisconsin Historical Images.
[Source: "Glossary of Miners' & Mineral Terms" in St. John, John R. A True Description of the Lake Superior Country ... (New York, W. H. Graham, 1846).]