Negative Charge of Dioctahedral Micas as Related to Weathering
Negative charge and related characteristics of dioctahedral micas have been investigated by conducting artificial weathering studies on a muscovite from Ontario, Canada, and saprolitic micas from Virginia. Emphasis is given to the relationship between loss of potassium, increase in net negative charge, or cation exchange capacity, and expansion characteristics.
Boiling solutions of acetic acid, sodium chloride and magnesium chloride removed potassium and sodium from a saprolitic mica but produced a negligible increase in cation exchange capacity. Expansion of the mica structure occurred following treatment with salt solutions but not with acid solutions. Boiling solutions of sodium citrate removed potassium effectively, produced marked expansion, and caused a large increase in cation exchange capacity. The small increase in cation exchange capacity resulting from acid and salt treatments is attributed to the hydrolysis of exchangeable aluminum to form hydroxy-aluminum polymeric groups, which can occupy exchange sites but remain non-exchangeable. The results of the sodium citrate tratement are attributed to the complexing of aluminum by citrate, thus releasing the hydroxy-aluminum polymeric groups from exchange positions and permitting an increase in cation exchange capacity.
Molten lithium nitrate treatment of specimen muscovite produced a greater increase in cation exchange capacity than in the case of muscovite separated from soil-forming mica phyllite. The observed difference in charge characteristics, concomitant with the greater expansion of the muscovite, suggest that soil micas and specimen-type micas differ in their alteration tendencies.
When interlayer hydroxy groups are eliminated and account is taken of residual sodium, potassium, and water, dioctahedral micas apparently lose no negative charge on expansion to vermiculite-like minerals.
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