Resumen de Montmorillonitas cloritizadas de la formación Río Chiflón, La Torre, provincia de La Rioja, Argentina
X-rays, chemichal and genetical characters of a chloritized montmorillonite, found in the Río Chiflón Formation (La Torre, La Rioja,, Argentina) are described. The mineral is associated with minor amounts os illite, montmorillonite and chlorite. The principal non-clay associates are analcite and hematite. This chloritized montmorillonite occurs in red siltstones and sandstones of Triassic age. The sandstones and siltstones are organized in poorly developed fluviatil cyclothems.
The Li* and K* varieties of the clay have proved that the mineral is a smectite with a peculiar behavior under heating. Berween 200ºC and 600ºC the mineral collapses gradually from a basal spacing of 10 A to 13 A. This particularity has been interpreted as originated by interlayer contaminants of hydroxy-cation type.
The Mitchell and MacKenzie (1953) procedure for removing the free iron oxides and the Tamura (1958) method for extracting interlayer contaminants, have been worked on. Despite that the iron remotion has driven important quantities of Al2O3, SiO2 and cations, the X-rays properties have not been altered. The citrate treatment of Tamura on the interlayer was proved to be of the little effect in the removal of the hydroxy-cation interlayer. Instead of, it affected all the structure reducing its termal stability. The amorphous phase at the end of 9 hs. boiling with citrate, is between 20 to 30% of the 2 microns sample.
Considering the chemical composition of the extracts from Mitchell an MacKenzie, and Tamura methods, the interlayer hydroxy-cations may be Mg, Al, and Fe, in this order.
The minerlal presents a good and regular expansibility with Ethilene Glycol and Gycerol, wich are preserved in the Li* and K* varieties almost without change, up to a preheating of 400ºC. Over this temperature the expansion is more complex but still conspicuous. Jackson's explanation (1963) about intergradient clays between montmorillonite and chlorite has been applied. In this proposal, the expansion with organic liquids considered to be caused by the presence of hydroxy-cation islands, wich are attached to one or the other "mica" layer.
The enviroment of formation has been evaluated at the light of the well known occurrences of intergradient clays in soils, and the presence of analcite intimately associated with hamtite. It is supposed that the chloritization has taken place during a soil weathering process, in alkaline conditions with grat disponibility of sodium, and an alternting dry-humid seasons climate (oxidizing condictions). The stability of the interlayer developed under these circumstances, was increases during the long burying and diagenesis.
