Mica can be formed in igneous rock (rock that forms directly from molten lava), rocks like granite, but is also found in metamorphic rock. When sediments are subjected to extreme heat and pressure ...

These metamorphic rocks were divided by him into two groups—the Seve group, composed mainly of quartzites and schists, and the Köli group, consisting largely of mica-schists and clay-slates.

Biotite is black mica. Another rock found here is the gray-brown Julian schist, a metamorphic with twisted, streaked bands of darker and lighter material that originated as seafloor sediment ...

Minerals like feldspar and mica, making up plutonic rocks like granite and metamorphic rocks like gneiss and schist, tend to react with water and oxygen, decaying to clay minerals.

In this case, it’s a piece of schist, a type of metamorphic rock made of easily identified grains or flakes. Its presence often marks places where continents collided millions of years ago. “Sometimes ...