Compounds of the crandallite-type: Synthesis, properties and thermodynamic data of Ca – Sr – Ba – Pb-(arseno)-woodhouseites

Author： Schwab R. G. Pimpl T. Schukow H. Stolle A. Breitinger D. K.

Publisher： E. Schweizerbart'sche Verlagsbuchhandlung

ISSN： 0077-7757

Source： Neues Jahrbuch für Mineralogie - Abhandlungen: Journal of Mineralogy and Geochemistry, Vol.181, Iss.3, 2005-06, pp. : 207-218

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Abstract

The pure M²⁺ -(arseno)-woodhouseites with M²⁺ = Ca²⁺, Sr²⁺, Ba²⁺ and Pb²⁺ were synthesized at 200 °C/15 bar starting from the pure (hydr-)oxides and acids. The lattice constants were determined by x-ray powder diffraction and the Standard Free Energies of Formation ΔG°f,333 (resp. μ^o333) through dissolution equilibria at 333 K. They are forSr-wood (svanbergite) a_o = 699.1 ± .1 [pm] c_o = 1663.5 ± .4 [pm] ΔG^o_f,333 = −1269.3 ± .3 [kcal · mol⁻¹]Sr–As-wood (kemmlitzite) a_o = 705.0 ± .1 [pm] c^o = 1693.6 ± .2 [pm] ΔG^o_f,333 = −1175.6 ± .2 [kcal · mol⁻¹]Ba-wood (no name) a_o = 700.1 ± .1 [pm] c_o = 1732.0 ± .3 [pm] ΔG^o_f,333 = −1270.5 ± .4 [kcal · mol⁻¹]Ba–As-wood (weilerite) a_o = 705.7 ± .1 [pm] c_o = 1746.9 ± .2 [pm] ΔG^o_f,333 = −1175.9 ± .3 [kcal · mol⁻¹]Pb-wood (hinsdalite) a_o = 701.2 ± .1 [pm] c_o = 1683.1 ± .2 [pm] ΔG^o_f,333 = −1142.8 ± .4 [kcal · mol⁻¹]Pb–As-wood (hidalgoite) a_o = 706.1 ± .1 [pm] c_o = 1705.0 ± .3 [pm] ΔG^o_f,333 = −1050.7 ± .4 [kcal · mol⁻¹]Ca–As-wood (arsenowoodhouseite) a_o = 702.9 ± .1 [pm] c_o = 1648.4 ± .2 [pm] ΔG^o_f,333 = −1175.6 ± .3 [kcal · mol⁻¹]The dehydration temperatures are:Sr-wood (svanbergite) 480, 630 °C Pb-wood (hinsdalite) 575, 610 °CSr–As-wood (kemmlitzite) 630, 645 °C Pb–As-wood (hidalgoite) 615 °CBa-wood (no name) 525, 610 °C Ca–As-wood (arsenowoodhouseite) 590, 650 °CBa–As-wood (weilerite) 580, 600 °CThe corresponding 2 d and 3 d Garrel diagrams (model systems) show a generally higher thermodynamic stability of the arsenowoodhouseites. In the crand-wood mixed crystals Ca²⁺ and Sr²⁺ behave extremely antagonistic: Ca²⁺ favours the sulphate (wood), Sr²⁺ the phosphate (crand) component. In pure woodhouseites Ca²⁺ and Sr²⁺ exhibit an almost identical geochemical behaviour, however. The model systems can be used for the interpretation of natural element distribution patterns. In case of the lateritic weathering zone the behaviour of the ions and the formation of table-mountains connected with certain types of parent rocks and deposits (gold) can be explained perfectly.