Emilia García Romero

Mg-RICH SMECTITE “PRECURSOR” PHASE IN THE TAGUS BASIN, SPAIN

The pink clays from the Tagus basin, Spain, were characterized by X-ray difraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Chemical data were obtained by plasma emission spectroscopy and analytical electron microscopy (AEM), and specific surface and cation-exchange capacity were measured also. The data indicate that these pink clays are primarily stevensite. This Mg-rich smectite is characterized by poor crystallinity, a high degree of structural disorder, trioctahedral character (pure magnesian), a very low cation-exchange capacity, a very small crystal size (which generates an abnormally high specific surface area), and a deficiency of octahedral cations. On the basis of the very small crystal size, a large number of edge dislocations, the lack of periodicity (turbostratic) in the structure, and a cellular (spherical) texture observed by TEM, we consider this occurrence to be an early stage of crystallization. Unlike other precursor clay materials described in the literature, this clay is not an alteration of volcanic ash, but it was generated by precipitation from a Si- and Mg-saturated medium.The pink clays from the Tagus basin, Spain, were characterized by X-ray difraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Chemical data were obtained by plasma emission spectroscopy and analytical electron microscopy (AEM), and specific surface and cation-exchange capacity were measured also. The data indicate that these pink clays are primarily stevensite. This Mg-rich smectite is characterized by poor crystallinity, a high degree of structural disorder, trioctahedral character (pure magnesian), a very low cation-exchange capacity, a very small crystal size (which generates an abnormally high specific surface area), and a deficiency of octahedral cations. On the basis of the very small crystal size, a large number of edge dislocations, the lack of periodicity (turbostratic) in the structure, and a cellular (spherical) texture observed by TEM, we consider this occurrence to be an early stage of crystallization. Unlike other precursor clay materials described in the literature, this clay is not an alteration of volcanic ash, but it was generated by precipitation from a Si- and Mg-saturated medium.

The Maya Blue Pigment

Abstract Maya blue is an artificial pigment fabricated by the Maya in the early first millennium AD. The pigment was used in Prehispanic times from the southern Maya region to most Mesoamerican cultures. Maya blue is made from indigotin mixed with palygorskite. This light blue pigment is surprisingly resistant to degradation in adverse environmental conditions, as it is unaffected by the attack of acids, alkalis, oxidants, reducing agents and organic solvents. Scientists do not fully agree in the mechanism of this organoclay interaction. The fabrication and distribution of this pigment probably required a sophisticated technical and commercial infrastructure. This chapter reviews the present knowledge on Maya blue. The first part corresponds from its rediscovery in 1931 to the current studies, followed by a review of the properties of Maya blue, including structural and organoclay interaction mechanisms. Archaeological artefacts containing Maya blue helped in understanding the cultural and economic links of the Maya with other Mesoamerican cultures.

Macroscopic palygorskite from Lisbom Volcanic Complex

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° .