Arsenate subclass (and to a lesser extent the Vanadate Subclass)
of the Phosphate Class almost perfectly mirrors the true phosphates in many ways
including structure, diversity and properties. The only major difference is the
arsenate or vanadate ion replacing the phosphate ion.
The ions have the same charge and roughly the same size and therefore can usually
substitute for each other in the structure of minerals. In fact many solid solution
series exist between minerals that contain the phosphate ion as their principle anion
and those that contain the arsenate and/or vanadate ion as their principle anion.