EXPERIMENTAL LEACHING OF ROCK CATIONS IN TWO PERCENT HCL SOLUTIONS

We determined solution concentrations for ten elements leached from two sedimentary, four igneous, and two metamorphic rocks, which had been submerged in two percent HCl for a week. In addition, we determined silicon relative release ratios (SRRR), the release rate of each element relative to that of silicon calculated by SRRR=10^{|log((Si/a)_l/(Si/a)_s)|}, where "a" represents the mole concentration of an individual element and subscripts l and s indicate the solution and rock mole ratios of the two elements, respectively. The rock samples used were palm-sized specimens of argillaceous shale, siliceous sandstone, biotite granite, diorite, gabbro, biotite gneiss, and garnet schist. The ten elements, listed in decreasing order of their average leachate solution concentrations in ppm for all rocks, are aluminum (878), calcium (801), iron (782), silicon (530), magnesium (311), potassium (73.6), phosphorus (68.4), sodium (65.3), manganese (25.4), and titanium (8.6). These concentration sequences basically agree with the conventional thinking about element solubility in highly acidic solutions. However, SRRR reveal an entirely different order from the above sequence. Phosphorus has the highest average SRRR (150.5), followed by manganese (118.8), titanium (79.9), calcium (44.6), iron (37.5), magnesium (30), aluminum (9.6), potassium (9.3), and sodium (3.9). Phosphorus also has the highest average SRRR in the igneous and metamorphic rocks. This consistently high SRRR for phosphorus is unexpected as it generally is believed that phosphorous can easily be occluded as hydroxyl phosphate precipitates due to the available aluminum, calcium, and iron and, therefore, should be one of the least released elements in solution. Possible explanations are that phosphorus is not easily occluded in a highly acidic solution or that fresh hydroxyl phosphate precipitates are still soluble in solutions that are only about two weeks old when analyzed. No statistically significant correlations exist between an element's leachate concentration and the concentration of that element in the original rocks. However, significant positive correlations exist for the concentrations in solution between calcium and magnesium, magnesium and iron, and aluminum and silicon. A possible explanation for this is that these pairs of elements behave similarly in highly acidic solutions.