Macroscopic palygorskite from Lisbom Volcanic Complex

Abstract

The palygorskite of the Volcanic Complex near of Lisbom (Portugal) is particular both in the size of the fibres and in chemical composition. It appears as veins of very pure mineral. From hand specimen and optical observations it can be described as a macroscopic palygorskite. The crystals are large with exfoliation traces of several hundred microns to few millimetres in length. In thin section, this palygorskite is colourless, translucent, negative biaxial, with positive elongation and parallel extinction. The optically homogeneous fibres and laths are shown by the selected area electron diffraction to be composed of aggregates of much thinner fibres rotated differently around the c crystal axis which represents their common elongation direction. The chemical formula obtained by the X-ray EDS is Si8.02O20 (Al1.91 Fe0.04 Mg 2.01) (OH)2 (OH2)4 Ca 0.01 Na 0.07 4(H2O) very close to the ideal formula of a pureMg-Al palygorskite,with almost no octahedral Fe, and noAl in tetrahedral sites. The cell parameters are a0 or a0 sin q = 12.64 A° , b0 = 17.84 A° and c0 = 5.3 A° .