Silicate Mineral Stability and Mineral Equilibria in the Great Lakes Jeffery C. Sutherland' Geology Department, Syracuse University, Syracuse, N.Y. 13210
Equilibriuin concepts involving silicate minerals and water are applied to chemical data from the North Channel and Lakes Erie, Ontario, and Huron, for understanding of chemical self-regulation in the Great Lakes. Equilibria involving silicates and water are inferred from aqueous chemical data. Concentrations of dissolved silica attain minimum values of 10-4.8mole/liter in surface waters of the remote lakes through dissolution of kaolinite, A ~ z S ~ Z O ~-t O 5HzO H ) ~ +.A1z(OH)6 + 2H4SiO4 kaolinite gibbsite aq. silica In deeper waters, metastable equilibria, C a montmorillonite $ gibbsite and muscovite e gibbsite may impose upper limits upon concentrations of SiOn(aq.). Silica concentrations in the enclosed waters of sediments from North Channel reach metastable equilibrium with amorphous silica at values of SiOg(as.) = 10-*.8 mole,'liter; values of less than mole/liter are imposed in sediments from the other lakes through C a montmorillonite @ kaolinite. The major chemical character of the Great Lakes is inherited from the carbonatesilicate mineralogy of bedrock, soils, and glacial drift in their drainage.
T
he Great Lakes drain several hundred thousand square miles of diverse igneous, metamorphic, and sedimentary rocks, soils, and glacial drift. Rain and snow melt flow over, mix with, and react with silicate minerals. Reaction solutions, unaltered minerals, chemically altered minerals, and well crystallized mineral weathering products flow into the lakes. There, solutions reside from three to five years (Erie) to ca. 500 years (Superior). Sediments react with open water and with water enclosed within them. The purpose of this study is t o determine whether silicate minerals are in thermochemical equilibrium in the Great Lakes. Where a mineral-water equilibrium regime is established, silicate minerals would act to control solute species activities through solution-precipitation reactions. The greater the size (volume, mass) of the regime, the less tractable is the
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system to be changed chemically. Investigations of mineral controls upon the chemistry of the Great Lakes may be useful steps in maintaining them as valuable resources. Goldrich (1938) made a quantitative study of chemical weathering of igneous and metamorphic rocks, where these are mantled with residual soils. His efforts yield a Mineral Stability Series, well-known to geologists as the reverse order of Bowen's Reaction Series for the igneous crystallization of minerals. Much simplified mineralogically, the series is : quartz (SO?), most stable; muscovite, KA13Si3010(OH)2; K feldspar, KAlSi30s; Na feldspar, NaA1Si,08; and C a feldspar, CaAlrSiz08. Jackson, Tyler, et a/. (1948) made a study of mineral stability sequence of the -2pminerals in sediments and sedimentary rocks. Their stability series, abbreviated, includes the clay minerals: gibbsite [Alz(OH)6],most stable; kaolinite, A12SizOj
