Alteration of dioctahedral micas in soils

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Virginia Polytechnic Institute

Artificial and natural weathering studies were conducted on four dioctahedral micas found in soil parent material of the Virginia Piedmont. Emphasis was given to the relationship between loss of K (also Na in certain cases), increase in CEC, and expansion characteristics.

Boiling treatments with several different solutions were used to promote artificial weathering. In terms of the three parameters of primary interest, the results were of three general types, depending upon the nature of the boiling medium. Boiling solutions of HCl and HOAc removed K and Na, but produced only slight expansion and a negligible increase in CEC. The acid solutions, being at low pH, liberated from the mineral large amounts of Al, which likely underwent hydrolysis later to form hydroxy-Al polymeric groups. These groups can occupy exchange sites, but they remain nonexchangeable. Boiling aalt solutions of NaCl and MgCl₂ removed K and Na in large amounts, produced considerable, but only a slight increase in CEC resulted. The explanation for the small increase in CEC was believed to be due also to blocking of exchange sites by nonexchangeable hydroxy-Al groups, because salts tend to accelerate the hydrolysis reaction. A boiling Na-citrate solution removed K effectively, produced marked expansion, and caused a large increase in CEC. Citrate complexes Al, which probably accounted for the high CEC. When a sample was treated with hot HCl prior to Na-citrate boiling, the loss of K and increase in CEC was stoichiometric. The equivalence was attributed to the removal of amorphous material by HCl and complexing of Al by Na-citrate.

Vermiculite and kaolinite were the natural weathering products of the dioctahedral micas studies. There was a striking similarity between the artificially-produced vermiculite and that occurring under natural conditions. The product formed under both circumstances expanded to approximate 14 A. when glycerol-solvated and contracted to 10 A. upon K-saturation and heating. Although the artificial weathering media were not representative of natural weathering conditions, the basic mechanism of mica alteration apparently was similar.

The soil micas altered easily in the laboratory compared to an Ontario muscovite. Thus, it was concluded that soil micas are quite different from the specimen-type micas. They may possess a lower charge or lack the structural control of specimen muscovites.

The difference in ease of weathers between dioctahedral and trioctahedral micas likely is attributable to the difference in chemical composition, especially in the octahedral layer. Dioctahedral micas appear to be more strongly bonded than trioctahedral micas as a result of the higher-charged Al⁺⁺⁺ ion in the octahedral layer. This stronger bonding would tend to restrict initial alteration. Subsequent alteration also would be hindered because, as weathering proceeds, greater hydrolysis of Al in dioctahedral micas would result in more fixation of positively-charged hydroxy-Al polymers in the interlayer position. This fixed Al inhibits expansion of dioctahedral micas. In trioctahedral micas, which contain Fe and/or Mg, this type of fixation does not occur during acidic weathering.

A significant finding was the occurrence of paragonite, the Na analogue of muscovite, in intimate association with muscovite. A relatively constant Na/K molar ratio of approximately one was maintained in both the parent rocks and soils. This indicated that equal molar quantities of paragonite and muscovite were present and that weathering of the two micas was similar. Paragonite is believed to be more widespread occurrence than previous work has indicated. Its influence on soil genesis, morphology, and classification may be significant.