Summary: Archerite, (K, NH₄)H₂PO₄, a new mineral, has been found in Petrogale Cave, 36 km east of Madura Motel (31° 54′S, 127°00′E) Western Australia. Archerite and biphosphammite together with aphthitalite, halite, syngenite, stercorite, oxammite, weddellite, whitlockite, guanine, newberyite, calcite, (NH₄)₂Ca(HPO₄)₂·H₂O, and an unknown occur as stalactites and wall and floor crusts.

Archerite is tetragonal with second order prisms and pyramids in crystals up to 2 mm in length with D 2·23, ω 1·513, ε 1·470, all close to artificial KH₂PO₄. The mineral is soft, has a white streak, and is soluble in cold H₂O. Cell dimensions of natural archerite were not determined and the composition was calculated from measurements of selected powder lines correlated with charts prepared from end-member data and by chemistry which gave 81% KH₂PO₄ and K₂O 10·8, NH₂ 3·46 respectively, the latter corresponding to (K_0·74(NH₄)_0·26)H₂PO₄.

The name is for Dr. M. Archer, Curator of Mammals, Queensland Museum. Type material is stored in the collections of the Government Chemical Laboratories, Perth, Western Australia.