Redefinition of angastonite, CaMgAl₂(PO₄)₂(OH)₄ ⋅ 7H₂O, as an amorphous mineral

Abstract

A reinvestigation of the type angastonite specimen from the Penrice marble quarry, South Australia, shows that the published powder X-ray diffraction pattern for the mineral corresponds to a mixture of crystalline phases plus an amorphous phase. The published formula for the mineral, CaMgAl2(PO4)2(OH)4⋅7H2O, has been found to correspond to the amorphous phase. A proposal for the redefinition of angastonite as an amorphous phase has been approved by the IMA CNMNC. Amorphous angastonite is isotropic with an index of refraction in the range 1.33 to 1.40 and a measured density of 1.57(2) g cm−3. It is formed from incongruent leaching of minyulite in circulating Mg- and Ca-bearing solutions. The amorphous phase undergoes local recrystallisation to form penriceite and two other new crandallite-derivative layer phases. A cluster model for the local structure in the amorphous mineral is proposed based on crandallite-type segments. A brief review of amorphous minerals is given.