Original paper

Compounds of the crandallite-type: Synthesis, properties and thermodynamic data of pure crandallite and woodhouseite

Schwab, R.G.; Pimpl, T.; Schukow, H.; Stolle, A.; Breitinger, D.K.

Abstract

In spite of many efforts the synthesis of pure crandallite was not possible so far. Consequently also its properties especially the Standard Free Energy of Formation Δ Gof could not be determined directly; they had to be derived from mixed crystal data. The reason is a very small stability field in a very narrow range of rela- tively high pHs. To keep the run during synthesis within these restricted conditions Ca(H2PO4)2 and freshly precipitated Al(OH)3 have to be chosen as educts and held for at least 3 months at 200°C/15 bar. The lattice constants determined by X-ray powder diffraction are ao = 700.1 ± 0.1 [pm] and co = 1629.3 ± 0.2 [pm]. The Standard Free Energy of Formation ΔG of or chemical potential μo , respectively, was ascertained by dissolution equilibria at 333 K to –1341.2 ± 0.2 [kcal mol –1]1). Both lattice constants and Δ Gof values are fairly close to the respective data extrapolated from mixed crystals. Woodhouseite could be synthesized by the conventional method; the lattice constants are ao = 700.5 ± 0.1 [pm] and co = 1625.3 ± 0.2 [pm]. Its Standard Free Energy of Formation at 333 K is Δ Gof = –1267.3 ± 0.2 [kcal mol–1]. The Garrel diagrams based on the thermodynamic data for 333 K show, that the woodhouseite component becomes dominant even at extremely low concentrations of sulphate in the crandallite – woodhouseite mixed crystals. Natural mixed crystals forming either under low hydrothermal conditions or in the lateritic weathering zone of tropical climates are therefore always enriched in that component.