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Structural morphology of gypsum (CaSO4·2H2O), brushite (CaHPO4·2H2O) and pharmacolite (CaHAsO4·2H2O)

1991; Elsevier BV; Volume: 108; Issue: 1-2 Linguagem: Inglês

10.1016/0022-0248(91)90376-g

1873-5002

W.M.M. Heijnen, P. Hartman,

Minerals Flotation and Separation Techniques

A uniform description of the crystal structures of gypsum (CaSO4·2H2O), brushite (CaHPO4·2H2O) and pharmacolite (CaHAsO4·2H2O) is presented. From these structures, a PBC analysis leads to the following F forms: {020}, {011}, {⦶1}11 {120} and {⦶1}22 (for brushite and pharmacolite the counterfaces (⦶h⦶k⦶l) are included). Attachment energies of the gypsum F faces have been calculated in an electrostatic point charge model for various charge distributions in the water molecule and in the sulphate ion. The theoretical habits are all platy or tabular {020} with {120}, {011} and {⦶1}11, the latter being smaller than {011}, in disagreement with observation. Arguments are given that {011} can grow in layers built12d011, and therefore faster, at a supersaturation for which {⦶1}11 cannot grow in half layers, thus removing the discrepancy between theory and experiment. The curious triangular or trapeziumlike habit of brushite has been related to its crystal structure. The polar habit is ascribed to different adsorption of water and cosolutes on opposite faces. The most important forms are {020} with {⦶1}20, 11{⦶1} and {111}, the latter being an S form. Its appearance is ascribed to a habit change.