What are arsenates in mineralogy ?

Arsenates : definition

Arsenates form a subclass belonging to the mineralogical class of "phosphates, arsenates, vanadates"; they are built from tetrahedral anionic groups (AsO4)3- and different cations.

They are relatively rare, frequently hydrated minerals, the most common of which seem to be scorodite and olivenite. Equivalent (PO4)3- and (VO4)3- tetrahedral units are the basis of phosphates and vanadates. P5+, As5+, V5+ are easily replaced in the anionic groups, there are many transition terms between phosphates, arsenates and vanadates : pyromorphite Pb5(PO4)3Cl, mimetite Pb5(AsO4)3Cl and vanadinite Pb5(VO4)3Cl, for example between them form extended (but not complete) series.
Adamite from Mapimi, Mexico
Mimetite from Pingtouling, China
Roselite from Bou Azzer, Morocco