Marine sedimentary attapulgite of Yucatán, México

Abstract

Attapulgite or palygorskite is located in the vicinity of Ticul, Yucatan Peninsula, Mexico. The area is bounded, in its western part, by the Sierra de Ticul, an Eocene biocalcarenite reef, in contact to the N and E with calcitic and dolomitic marine sediments of biogenic and chemical precipitation, aragonite formation, and calcite and dolomite neoformation. In these sediments, attapulgite occurs as greenish plastic pseudospherical masses, lenses, or horizontal layers concordant with the limestone, being of chemical precipitation due to the reduction of Si⁴⁺ and Al³⁺ solubility by Mg²⁺ in an alkaline medium (pH 8-10).

In the immediate limestone, there are abundant indications of microfauna, cavities with acicular calcite, and anhydrous, amorphous, silicic precipitation gels with an index of 1.48-1.53, in which the attapulgite subsequently crystallized, all within an environment of coastal lagoons with shallow, quiet waters and abundant biogenic activity.

Mineralogical studies confirm the monoclinic crystallization of attapulgite in a cell with dimensions b=17.95 Å and α sin β=12.86 Å, presenting its interplanar distances and intensities. Chemical analysis indicates 54.50% SiO₂, 11.48% Al₂O₃, 10.55% MgO, 0.86% CaO, 0.31% FeO, 1.88% Fe₂O₃, 0.37% TiO₂, 9.00% H₂O^-, and 10.95% $H₂O⁺, allowing the calculation of the formula:

(Si_7.60 Al_0.31)(Al_1.60 Mg_2.22 Fe²⁺_0.03 Ti_0.04) O₂₁ Ca_0.13

with heating losses at 80°, 200°, 400°, and 700°C of 9.00%, 2.39%, 6.60%, and 1.96% respectively. In infrared absorption, proton-oxygen stretching of structural and zeolitic water is observed at 3580 cm^-1 and 3515 cm^-1, and of constitutional OH at 1640 cm^-1 and 1440 cm^-1. Tetrahedral substitution of Si⁴⁺/Al³⁺ and the increase in the (Si, Al)—O distance are recorded between 1100 cm^-1 and 900 cm^-1, and the octahedral substitution of Al³⁺ by Mg²⁺ and Fe³⁺ is confirmed by intense absorptions at 515 cm^-1, 490 cm^-1, and 445 cm^-1. Electron microscopy reveals typical acicular crystallization in fibers of 0.5 to 1.0 microns in cross-section and up to 10 microns in length, with acicular termination.

Abstract from:

Gómez-Caballero, J. A. (2005). Historia e índice comentado del Boletín del Instituto de Geología de la UNAM. Boletín de la Sociedad Geológica Mexicana: Volumen Conmemorativo del Centenario Aspectos históricos de la Geología Mexicana, 57I(2), 149-185. http://dx.doi.org/10.18268/BSGM2005v57n2a3