Layer Charge Decrease by Tetrahedral Cation Removal and Silicon Incorporation during Natural Weathering of Phlogopite to Saponite
1974
Sridhar, K. | Jackson, M. L.
A substantial layer charge decrease, from 210 to 140 meq/100 g, occurred during the natural weathering of phlogopite through vermiculite to saponite in Kansas. A comparison of the Fe²⁺ content and layer charge of the coarse fraction (72% phlogopite) and the fine fraction (71% saponite) indicated that iron oxidation cannot account for the layer charge decrease during the transformation. Control studies on iron oxidation during vermiculitization of biotites and lepidomelane did not correlate with layer charge decrease. Elemental analysis of the coarse and fine fractions revealed gains and losses of cations during the weathering of the mica flakes to fine particles. The Si⁴⁺ and Mg²⁺ increased by 2.66 and 0.84%, respectively, while Al³⁺ and total Fe³⁺ plus Fe²⁺ decreased by 0.34 and 1.21%, respectively, in the two fine fractions compared to the macroflakes. The elemental analyses also revealed the unique nature of this phlogopite which contained fewer moles of Al³⁺ than K⁺; the presence of some isomorphously substituted tetrahedral Fe³⁺ contributed 17% of the total negative layer charge. Loss of layer charge was found to be a result of the loss of isomorphically substituted tetrahedral Al³⁺ and Fe³⁺ by authigenic recrystallization with an increase in tetrahedral Si⁴⁺. In control studies, removal of isomorphously substituted tetrahedral Al³⁺ from Ontario phlogopitic vermiculite and Transvaal micaceous vermiculite by citrate chelation also resulted in an equivalent layer charge decrease.
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