The Phosphates Class
The Phosphate Class is made up of minerals with a basic chemical unit of tetrahedral (AO4) groups with a negative three (-3) charge. The A can be either Phosphorus, Arsenic, Vanadium or Antimony. The basic chemical unit can be combined with metal ions on a one to one ratio or usually in more complex combinations with other ions such as hydroxide groups (OH), uranyl groups (UO2), a halogen or even water molecules.
The typical phosphate is vitreous to dull, often strongly colored, above average in density, average in hardness (4-7), and low in index of refraction unless ions such as lead are present. All other properties are variable. Many interesting and beautiful mineral specimens come from this class and although a large number of minerals are known to belong to this class, only some of the members of the Apatite Group are considered common.