Arcillas con superespaciados en la Formación Los Rastros (provincia de San Juan, Argentina)

• Gerardo E. Bossi ^[1]
  1. [1] Instituto y Fundación Miguel Lillo
• Localización: Acta geológica lilloana, ISSN-e 0567-7513, Vol. 10, Nº 12 (Texto completo no disponible), 1970, págs. 243-278
• Idioma: español
• Títulos paralelos:
  • Clays with superspacing in the Los Rastros Formation (province of San Juan, Argentina)
• Texto completo no disponible (Saber más ...)
• Resumen

  • A quasiregular mixed-layer clay mineral from Los Rastros Formation of the río Ischigualasto aerea, in the province of San Juan, Argentin, is described. The mineral was found particularly in tuffaceous yellowish to grayish sediments, and is characterized by a basal superspacing of 30 to 32 A, depending on the interchangeable cation and the liquid associated.

    The X-ray analysis of the Li*, K*, NH4* y Mg* homoinic varities indicated an illite-beidellite mixed-layering with some montmorillonite replacing beidellite. Ther expandable portions seem to have a high charge considering their vermiculite-like behavior under K* saturation. Furthermore, the Mg**clay has an abnormal heating behavior that could be related to intergradational properties of the expansilbe layer with chloritic interlayers.

    The chemical analysis of the <2 µ fraction from two samples, quite unrelated geologically have shown that the mineral is rich in aluminium and very poor in magnesium. Both analysis are almost identiclal indicating that the chemical composition should be critical in conexion with the occurrence of a quesiregular mixed-layering instead of an irregular variety.

    The fourier transformation proposed by MacEwan (1956) indicates clearly that the interstratification is not regular buy close (quesiregular), with pAB=.75 and pBB=0, and an illite-beidellite ratio of 57-43.

    Additional complications arise from the fact that montmorillonite is present as a third phase making under certain conditions a tricomponent interstratification BAB'. (Ca**-beidellite/illite/Na*-montmorillonite) within the main AB sequence (illite-bedellite). Some evidences picked up from the X-ray diffracton diagrams indicated that the BAB' stacking could be present up to a maximun of 30% Under these conditions the montmorillonite associated to bedellite is less than 10%.

    The mineral is considered to be formed in the early (and also during the late) diagenesis from a beidellitic clay with some kind of "2M" stacking. This beidellitic clay have been formed under fluvial bituminous flood plain environment Los Rastros Formation from tuffaceous sediments and tuffs. According to Millot (1964) these fluvial coal formations are developed in alternating wet and dry hot temperate climates, where illite is stable and the illtization of smectites a nomal trend