Mica

The black mineral in this rock is mica. It looks a little like silver but it is a sheet silicate (phyllosilicate) mineral. Mica glitters in the sun and is very attractive and often confused with more valuable minerals. The formula is

 X₂Y_4–6Z₈O₂₀(OH,F)₄

in which X is K, Na, or Ca or less commonly Ba, Rb, or Cs;

Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.;

Z is chiefly Si or Al, but also may include Fe³⁺ or Ti.

Usually, there is an abundance of this type of mineral near where gold or other mineralizations are found.