This variety of plagioclase, called albite , is rich in sodium. Potassium feldspar Potassium feldspar is another member of the feldspar mineral family. Feldspar crystals are stubby prisms, often pink to white. Mica Micas are another group of silicate minerals composed of varying amounts of potassium, magnesium, iron as well as aluminum, silicon and water. All micas form flat, book-like crystals that peal apart into individual sheets on cleavage planes.
Crystals cleave into smooth flakes.
Mica is so soft that you can scratch it with a fingernail. The amphiboles are a family of silicate minerals that form prism or needle-like crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts along with silicon, oxygen, and water. Olivine Olivine is another silicate mineral containing iron and magnesium. It is a green, glassy mineral that forms at high temperature.