The concentration of iron in chloride solutions equilibrated with synthetic granitic compositions; the sulfur-free system

The concentrations of iron in 1 N chloride solutions have been determined at temperatures from 400 degrees to 700 degrees C and a pressure of 1 kb for fluids in equilibrium with synthetic magnetite- or biotite-bearing granitic (quartz monzonite) compositions. K/H and Na/H ratios in solution were controlled by reactions similar to those observed in natural alteration assemblages involving alkali feldspar and plagioclase with aluminosilicate, muscovite, or montmorillonite. Oxygen fugacity was controlled by internal or external methods using hematite-magnetite, Ni-NiO, and quartz-fayalite-magnetite assemblages. Starting compositions of fluids were solutions of HCl, 2HCl:FeCl (sub 2) , HCl:NaCl, and NaCl:KCl:FeCl (sub 2) . Equal masses (i.e., low liquid/solid ratios) of solid and solution were used to expedite equilibration. Iron was added as synthetic magnetite, synthetic biotite, or in solution. Run times ranged from four days to several weeks.At 400 degrees C, the dominant cations in solutions are Na, K, and Ca, with Fe concentrations of 0.01 to 0.03 molal. At 500 degrees C, Na, K, and Fe are dominant with Fe concentrations of 0.10 to 0.17 molal. At 600 degrees C, the Fe concentration reaches a maximum of 0.20 to 0.25 molal and then appears to drop to between 0.10 to 0.15 molal at 700 degrees C. The effect of f (sub O (sub 2) ) on concentration is consistent with changes in the iron-bearing assemblage from magnetite to biotite, and along with charge balance calculations, suggests that ferrous chloride is the dominant species at low temperatures. These data indicate that in natural magmatic systems, the concentration of iron in chloride solutions coexisting with magnetite or biotite is extremely high. This high solubility may explain the large quantities of iron deposited in skarns and related deposits around some mineralized granitic stocks.