Nordstrom and Archer reported in 2002 that thermodynamic data are critical as input to models that attempt to interpret the geochemistry of environmentally important elements such as arsenic. Unfortunately, the thermodynamic data for mineral phases of arsenic and their solubilities have been highly discrepant and inadequately evaluated. This paper presents the results of a simultaneous weighted least-squares multiple regression on more than 75 thermochemical measurements of elemental arsenic, arsenic oxides, arsenic sulfides, their aqueous hydrolysis, and a few related reactions.
The best-fitted thermodynamic database is related to mineral stability relationships for native arsenic, claudetite, arsenolite, orpiment, and realgar with Eh-pH diagrams and with known occurrences and mineral transformations in the environment to test the compatibility of thermodynamic measurements and calculations with observations in nature. The results provide a much more consistent framework for geochemical modeling and the interpretation of geochemical processes involving arsenic in the environment,
The authors concluded that sufficient physiochemical measurements exist to evaluate the thermodynamics properties for the simple arsenic minerals: native arsenic, arsenolite, claudetite, α-realgar, β-realgar, crystalline orpiment, and amorphous orpiment. The evaluation was done by examining the available literature, screening and selecting the data, and performing a simultaneous weighted multiple least-squares regression on substances and reactions of arsenic, its oxides, its sulfides, and their aqueous hydrolysis products. These results have been combined with known occurrences and mineral transformations observed in nature to provide a coherent framework for mineral reactions in the As-S-O-H2O system. The results show the following:
1. Claudetite is the most stable phase of arsenic trioxide under standard state conditions.
2. Claudetite and arsenolite are products of weathering of several arsenic sulfide minerals, of native arsenic, and of scorodite.
3. Claudetite and arsenolite are stable in equilibrium with waters of high pH.
4. Orpiment is more stable than realgar at standard state conditions and is stable in equilibrium with waters of low pH.
5. Orpiment and realgar can form under a wide variety of conditions that include hydrothermal mineralization, hot-spring environments, mine-fire sublimates, and fumarolic encrustations.
6. Realgar appears to be more stable than orpiment in high-temperature environments (100-200°C).
7. Native arsenic is stable only under strongly reducing conditions arid although it appears as a stable phase under standard state conditions, field observations indicate it only forms under hydrothermal conditions.
8. New polymorphs of realgar, pararealgar, and alacrinite have been characterized and reported in the literature and appear to form under earth’s surface conditions. Pararealgar forms by visible light radiation of realgar. No thermodynamic data currently exist for these phases.
[Note: Nordstrom and Archer found faults with some of the work by Robie and Hemingway's 1995 US. Geological Survey Bull. 2131. See N&A pp. 2 and 3 herein.
For Robie and Hemingway (1995), see: http://www.i2mconsultants.com/downloads/corp/Robie-Hemingway1995C.pdf].
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