The average abundance of elements in rocks indicates that magnesium is substantially less abundant than calcium in all rock types and therefore should be less available in solution in water.
Table __(from Hem) Showing the average composition in mg/L of igneous and some types of sedimentary rocks.
The geochemical behavior of magnesium is substantially different from that of calcium because magnesium ions are smaller. In igneous rocks, magnesium is typically a constituent of the dark colored, ferromagnesian minerals. In altered rocks, magnesium mineral species such as chloride, montmorillonite, and serpentine often occur.
Sedimentary forms of magnesium include carbonates such as magnesite and mixtures of magnesium with calcium carbonate. Dolomite has a definite crystal structure in which calcium and magnesium are present in equal amounts.
The ion pair, magnesium sulfate, has about the same stability as the species calcium sulfate, and magnesium complexes with carbonate or bicarbonate have approximately the same stability as the similar species of calcium. However, magnesium carbonate solubility relationships cannot be represented as simply as those of calcium carbonate because there are many different forms of magnesium carbonates and hydroxycarbonates and they may not dissolve reversibly.
Waters in which magnesium is the predominant cation are somewhat unusual. Most limestone contains a moderate amount of magnesium. Water from dolomite, which is at or below saturation, should contain nearly equal concentrations of calcium and magnesium in terms of miliequivalents per liter (meq/L). Water that is near or above saturation, however, may have lost some calcium by calcite precipitation so the water will attain a concentration of magnesium greater than that of calcium.
Ion exchange minerals in rocks and in soil may absorb magnesium a little more strongly than calcium, but this effect evidently is not very important as a control over magnesium concentrations.