An Exploratory Study of Acetate Decomposition and Dissolution of Quartz and Pb-rich Potassium Feldspar at 150 °C, 50 MPa (500 Bars)

D. A. C. Manning, E. I. C. Rae and J. S. Small
Department of Geology, The University, Manchester M13 9PL, United Kingdom

Abstract: Experiments to explore the dissolution behaviour of Pb-rich orthoclase (1% PbO) and quartz have been carried out in the presence of pH buffered and unbuffered potassium acetate and lithium acetate solutions at 150 °C and 50 MPa (500 bars). In pH-unbuffered potassium acetate solutions Pb and Na solubilities (and pH) increase with increasing fluid acetate content, reflecting increased bulk dissolution of the feldspar; silica solubility decreases despite an increase in measured pH from 7.5 to 8.9. Similarly, in experiments at pH 6 using a potassium acetate pH buffer, quartz solubility decreases with increasing acetate content. The use of lithium acetate pH buffers (pH 6 at 25 °C) in experiments with orthoclase plus quartz results in the precipitation of the lithium chlorite cookeite, complicating interpretation of the fluid chemistry. It is also apparent that in the presence of orthoclase plus quartz (but not albite alone) acetate decarboxylation takes place at much higher rates than expected for the experimental configuration used. The observed effects are unlikely to be due to the presence of acetate alone; the influence of species produced by acetate decay (especially carbonate) must also be considered. This study provides little support for models which call upon acetate to enhance the solubility of aluminosilicate minerals, and suggests that acetate decarboxylation in nature may limit its involvement in dissolution processes. It emphasises the potential of feldspars as sources of elements for mineralisation, such as Pb.

Keywords: acetate • decomposition • quartz • potassium feldspar • dissolution

Mineralogical Magazine; June 1991 v. 55; no. 379; p. 183-195; DOI: 10.1180/minmag.1991.055.379.06
© 1991, The Mineralogical Society
Mineralogical Society (