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

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

Source: Neues Jahrbuch für Mineralogie - Abhandlungen, Volume 181, Number 3, June 2005 , pp. 207-218(12)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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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 Delta

G°f,333 (resp.

^o333) through dissolution equilibria at 333 K. They are for

Sr-wood (svanbergite) a_o = 699.1 ± .1 [pm] c_o = 1663.5 ± .4 [pm] Delta

G^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] Delta

G^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] Delta

G^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] Delta

G^o_f,333 = -1175.9 ± .3 [kcal · mol^-1]

Pb-wood (hinsdalite) a_o = 701.2 ± .1 [pm] c_o = 1683.1 ± .2 [pm] Delta

G^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] Delta

G^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] Delta

G^o_f,333 = -1175.6 ± .3 [kcal · mol^-1]

The dehydration temperatures are:

Sr-wood (svanbergite) 480, 630 °C Pb-wood (hinsdalite) 575, 610 °C

Sr–As-wood (kemmlitzite) 630, 645 °C Pb–As-wood (hidalgoite) 615 °C

Ba-wood (no name) 525, 610 °C Ca–As-wood (arsenowoodhouseite) 590, 650 °C

Ba–As-wood (weilerite) 580, 600 °C

The 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: WOODHOUSEITES; ARSENOWOODHOUSEITES; STANDARD FREE ENERGY OF FORMATION; LATTICE CONSTANTS; GARREL DIAGRAMS; DISSOLUTION EQUILIBRIA; THERMODYNAMIC DATA; LATERITIC WEATHERING; TABLE-MOUNTAINS

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0077-7757/2005/0018

Publication date: 2005-06-01

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