ABSTRACT Natural zeolites filling amygdales and veins in tertiary basaltic host rocks on the Isle of Skye (NW Scotland) have been studied in some detail by transmitted light microscopy, scanning electron microscopy and X-ray diffraction techniques . The zeolites and associated minerals so identified in order of their relative time of formation from early to late were nontronite, amorphous silica phases, carbonate phases, chabazite, phillipsite, wairakite, thomsonite, analcime, natrolite and stilbite-type minerals . Zeolite formation in the Skye basalts began with low Si/Al ratio Na zeolites and a gradual increase in Ca content and Si/Al ratio, ending up as Ca zeolites . They were probably formed as a consequence of late-stage hydrothermal activity, although, locally, contact metamorphism may control the process of zeolite formation RESUMEN Las zeolitas naturales como relleno en amigdalas y venas que ocurren en rocas basalticas Terciarias de la Isla de Skye (NW Escocia) han sido estudiadas en detalle por tecnicas de microscopia de luz transmitida, microscopia electronica de barrido y difraccion de rayos X. Las zeolitas y los minerales asociados identificados, en orden de su tiempo relativo de formacion, son nontronita, fases de silice amorfa, carbonatos, chabazita, filipsita, wairakita, thomsonita, analcima, natrolita y minerales tipo estilbita. La formacion de zeolitas en los basaltos de Skye comenzo con una zeolita rica en Na de baja relacion Si/Al y con el aumento gradual en el contenido de Ca y Si/Al, termino con una zeolita rica en Ca Estas probablemente se formaron como consecuencia de una actividad hidrotermal tardia, aunque localmente un metamorfismo de contacto pudo controlar el proceso de formacion de las zeolitas
Fine-grained and mainly reddish color, compact and slightly breccious and vesicular pyrometamorphic rocks (natural clinker) are associated to the spontaneous combustion of coal seams of the Cerrejon Formation exploited by Carbones del Cerrejon Limited in La Guajira Peninsula (Caribbean Region of Colombia). These rocks constitute remaining inorganic materials derived from claystones, mudstones and sandstones originally associated with the coal and are essentially a complex mixture of various amorphous and crystalline inorganic constituents. In this paper, a petrographic characterization of natural clinker, aswell as the application of the X-ray diffraction (Rietveld method) by mean of quantitative analysis of its mineral phases were carried out. The RIQAS program was used for the refinement of X ray powder diffraction profiles, analyzing the importance of using the correct isostructural models for each of the existing phases, which were obtained from the Inorganic Crystal Structure Database (ICSD). The results obtained in this investigation show that the Rietveld method can be used as a powerful tool in the quantitative analysis of phases in polycrystalline samples, which has been a traditional problem in geology.
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