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Economic Geology; December 1986; v. 81; no. 8; p. 1989-2007; DOI: 10.2113/gsecongeo.81.8.1989
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Phase relations of some cupric hydroxy minerals

Terri L. Woods, and Robert M. Garrels

Univ. South Fla. at St. Petersburg, Dep. Mar. Sci., St. Petersburg, FL, United States

Electrochemical laboratory studies of the basic copper salts, malachite (Cu 2 (OH) 2 CO 3 ), brochantite (Cu 4 (OH) 6 SO 4 ), and paratacamite (Cu 4 (OH) 6 Cl 2 ), have been conducted at ambient temperatures and pressures using copper metal-mineral electrodes. The scanning electron microscope showed well-crystallized patinas consisting of crystals usually greater than 2 X 0.5 X 15 mu m for brochantite, 5 X 5 X 5 mu m for paratacamite, and 0.2 X 0.2 X 1 mu m for malachite. Therefore surface energy corrections to calculations of free energy of formation values were not required. The few successfully patinated, copper metal electrodes were titrated with various sulfate, carbonate, and chloride solutions while monitoring the potential between the copper-cupric salt electrode and an Ag-AgCl-KCl reference electrode in an aqueous solution. The observed relationship between electromotive force (emf) and the activity of the titrant permits determination of free energy of formation values for the copper minerals studied. These titrations lead to values of -905.0 and -1341.8 kJ/mole for the free energies of formation (298 degrees C and 1 bar) of malachite and paratacamite, respectively. The recorded changes in emf appear to be a linear function of the log of the activity of the cupric ion in solution, but various junction potentials prevent use of the observed emf values for direct calculation of mineral stabilities. No solid solution between the minerals was observed and the phase changes occurred by partial or complete dissolution of the original phase on the surface of the electrode and precipitation of the new phase. Original textures, at the 100X magnification level, were preserved by the guest minerals.

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