Gypsum commonly forms in several environments, ranging from sedimentary evaporite deposits to hydrothermal vein deposits. It can occur in massive granular form (alabaster), translucent white crystals (selenite is one variety), and as desert “roses.” It has extensive practical uses, such as fertilizer (for sulfur) and as building material (plaster of Paris, dry wall). Giant crystals of gypsum occur in Mexico’s Cave of Crystals and Cave of Swords.
CaSO4 · 2H2O; Hardness 2; Monoclinic
Calcite forms in many different geologic environments. Usually colorless or white it can occur in many tints due to the inclusion of minor constituents during formation. It has possibly more habits of occurrence than any other mineral.
CaCO3 Hardness 3; trigonal
Pyrite is an iron sulfide that forms at high and low temperatures and occurs, usually in small quantities, in igneous, sedimentary and metamorphic rocks. worldwide. The mineral is named after “pyr” (“fire”), because striking it against another hard material produces sparks. It is also known as “fool’s gold”
FeS2; Hardness 6-6.5; isometric