The dominant building block of this mineral class is the PO4- tetrahedron. Phosphorus, arsenic, and vanadium may substitute for each other within the tetrahedron.
The typical mineral is vitreous to dull, often strongly-colored, above average average in density, has an average hardness, and a low index of refraction. The minerals of this class can have very complex chemical relationships and structures. It is a large mineral class, but many members are rare.