Original paper

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

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

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. μo333) through dissolution equilibria at 333 K. They are forSr-wood (svanbergite) ao = 699.1 ± .1 [pm] co = 1663.5 ± .4 [pm] ΔGof,333 = -1269.3 ± .3 [kcal · mol-1]Sr–As-wood (kemmlitzite) ao = 705.0 ± .1 [pm] co = 1693.6 ± .2 [pm] ΔGof,333 = -1175.6 ± .2 [kcal · mol-1]Ba-wood (no name) ao = 700.1 ± .1 [pm] co = 1732.0 ± .3 [pm] ΔGof,333 = -1270.5 ± .4 [kcal · mol-1]Ba–As-wood (weilerite) ao = 705.7 ± .1 [pm] co = 1746.9 ± .2 [pm] ΔGof,333 = -1175.9 ± .3 [kcal · mol-1]Pb-wood (hinsdalite) ao = 701.2 ± .1 [pm] co = 1683.1 ± .2 [pm] ΔGof,333 = -1142.8 ± .4 [kcal · mol-1]Pb–As-wood (hidalgoite) ao = 706.1 ± .1 [pm] co = 1705.0 ± .3 [pm] ΔGof,333 = -1050.7 ± .4 [kcal · mol-1]Ca–As-wood (arsenowoodhouseite) ao = 702.9 ± .1 [pm] co = 1648.4 ± .2 [pm] ΔGof,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.

Keywords

woodhouseitesarsenowoodhouseitesstandard free energy of formationlattice constantsgarrel diagramsdissolution equilibriathermodynamic datalateritic weatheringtable-mountains