Polymorphs - two minerals with the same chemical composition but totally different forms and properties. However, only a little over 2 dozens are common and are called rock-forming minerals. Ferromagnesian minerals dark in color an heavier A. Ferromagnesian Silicates 1. Olivine - high T silicate mineral, black to olive green, glassy luster and conchoidal fracture. Since Si - O bonds are stronger than bonds joining the silicate structures, cleavage is therfore parallel to the silicate chains Ex.
Minerals is long and fibrous.
Garnet - similar to olivine in structure tetrahedron linked by metallic ions. Usually brown to deep red. Muscovite - common member of mica family, light in color and pearly luster flat sheets with shiny surface. It is also very resistant to weathering. B. Evaporites - from evaporation of an enclosed area of the ocean or large lake.