Abstract: Apatite (Ca5(PO4)3(F,Cl,OH)), merrillite (Ca18Na2Mg2(PO4)14), and whitlockite (Ca9(Mg,Fe(2+))(PO4)6[PO3(OH)]) are the primary phosphate minerals found in most planetary materials including rocks from Earth, Moon, Mars, and asteroids. For many years, the terms merrillite and whitlockite have been used interchangeably in the meteorite literature. Much of the confusion regarding the relationship between terrestrial and extraterrestrial ‘whitlockite’ is based on the presence or absence of hydroge…