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Layer Charge Decrease by Tetrahedral Cation Removal and Silicon Incorporation during Natural Weathering of Phlogopite to Saponite
Author(s) -
Sridhar K.,
Jackson M. L.
Publication year - 1974
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1974.03615995003800050041x
Subject(s) - phlogopite , vermiculite , saponite , mica , geology , weathering , mineralogy , chemistry , inorganic chemistry , materials science , clay minerals , geochemistry , metallurgy , mantle (geology) , paleontology
Abstract 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 Fe 2+ 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 Si 4+ and Mg 2+ increased by 2.66 and 0.84%, respectively, while Al 3+ and total Fe 3+ plus Fe 2+ 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 Al 3+ than K + ; the presence of some isomorphously substituted tetrahedral Fe 3+ 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 Al 3+ and Fe 3+ by authigenic recrystallization with an increase in tetrahedral Si 4+ . In control studies, removal of isomorphously substituted tetrahedral Al 3+ from Ontario phlogopitic vermiculite and Transvaal micaceous vermiculite by citrate chelation also resulted in an equivalent layer charge decrease.