Macroscopic palygorskite from Lisbom Volcanic Complex
Suárez, Mercedes; García Romero, Emilia
European Journal of Mineralogy Volume 18 Number 1 (2006), p. 119 - 126
published: Mar 6, 2006
ArtNo. ESP147051801013, Price: 29.00 €
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(Al1.91Fe0.04Mg2.01)(OH)2(OH2)4Ca0.01Na0.074(H2O) very close to the ideal formula of a pure Mg-Al palygorskite, with almost no octahedral Fe, and no Al in tetrahedral sites. The cell parameters are a0 or a0sin β = 12.64 Å, b0 = 17.84 Å and c0 = 5.3 Å.