Ensayos acelerados realizados en el laboratorio, permiten estudiar el material cristalino depositado sobre la superficie de areniscas cuando están en contacto con disoluciones acuosas de diferente composición.

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ESPAÑA I'tícha de rccepcióii: 09-XII-1993 RESUMENSe analizan granulométricamente 11 muestras de arenisca de las Catedrales de Salamanca. El objetivo del estudio es establecer si existe relación directa entre el tamaño de las partículas de tales piedras y el deterioro superficial de alguno de los sillares de las fachadas. Esto se lleva a cabo con la ayuda de la espectrometría de difracción por rayos láser.Los resultados muestran que no hay conexión entre la finura textural de las piedras estudiadas y el grado de deterioro superficial que éstas presentan. SUMMARY 11 sandstone samples from the cathedrals of Salamanca were analysed granulometrically. The study was to establish whether there is a direct connection between the particle size of such stone and the surface deterioration of some of the ashlars in the facades. This was carried out with the help ofdifraction spectrometry by laser beams.The results show that there is no connection between the nature of the stone texture and their degree of surface deterioration.

Three weathering profiles of granitic rocks and two arkoses in the NW of Madrid Province have been studied.Rocks are granodiorites and adamellites with albites showing variable degrees of sericitization, and no-weathered potassic feldspars. Clay fraction in the weather~horizons from these rocks displays high content in beidellite, and minor amounts of illite and 7A-halloysite.On the other hand, beidel1ite, illite and well crystallized kaolinite are clay minerals in the arkosic rocks.Microfabric characteristics from SEM studies show abundant weathered albites with clean surfaces and formation of beidellite in all areas of the samples. Moreover, spatial relations between albite and beidellite are not observed.In pedochemical weathering of albite, sodium from this mineral should be exchanged with protons from soil solution with very quick kinetics. This initial alkalinization produces an increase in the silica solubility released from albite network. In these environments, a continued contact bc¡,tween weathering solution and albite causes a silica and alumina enrichment and beidellite or 7A-halloysite precipitation. Microclimatic variations in weathering environment may cause fluctuations in silica and alumina activities with resultant pij changes. So, when silica activities are high, beidellite neoformation is favoured; on the contrary, 7A-halloysite is precipitated.