A hexagonal mineral, Ca₅(SiO₄,PO₄,SO₄)₃(F,OH,Cl) ; apatite group; it is chlorellestadite if (Cl>OH,F), fluorellestadite if (F>OH,Cl), or hydroxylellestadite if (OH>F,Cl). Hydroxylellestadite occurs as veinlets in blue calcite associated with wilkeite, idocrase, and similar contact metamorphic minerals at Crestmore, Riverside County, CA.