Publication: Tendencias geoquímicas durante el proceso
de alteración de los granitos de la
Catedral de Toledo.
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Publication Date
1992
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Instituto Geológico y Minero de España (Madrid)
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Abstract
Este trabajo presenta un estudio geoquímico de elementos mayores, menores, traza y tierras raras, durante los procesos de alteración de los materiales utilizados en la construcción de la Catedral de Toledo. Para ello se establece un estudio comparativo entre los contenidos geoquímicos en las rocas de la Catedral con diferente grado de alteración y las muestras frescas procedentes de las canteras utilizadas para dicha obra. Así, se observa que los granitos estudiados presentan una perdida de los contenidos en Si02 durante la alteración, pérdida producida por los procesos de sulfatación de silicatos. Esta misma tendencia presente en Na20 Y el K20, mientras que el P205 posee una tendencia a la ganancia, la cual está relacionada con la actividad biológica de las palomas existentes en el exterior de la catedral. En cuanto a los elementos traza existen procesos de ganancia (Zr, Cr, Sr Y Ba), pérdida (Th) o estabilización (Zn, Ga) durante los procesos de alteración. Por su parte, las tierras raras evolucionan de forma separada; así, los elementos de número atómico inferior 64 disminuyen sus contenidos, mientras que los de número atómico mayor de 64 los aumentan al progresar la alteración. Por último, y desde un punto de vista general, atendiendo al índice de alteración potencial (IAP), se puede establecer que los monzogran1tos utilizados en la Catedral tienen un grado de alteración que puede ir desde muy alto hasta un valor alto-medio, mientras que el otro gran grupo de rocas silicatadas utilizado en la construcción, las granodioritas, presentan una alteración de tipo medio.
A geochemical study of major, minor, trace and rare earth elements during weathering processes in the rocks used in the building. of Toledo Cathedral is developed using the comparation between geochemical contents in the rocks of the Cathedral with different degrees of weathering and the geochemical contents of samples from the quarries used in the building_ of the Cathedral. The results show a decrease in Si02 contents during weathering of the granites. This decrease is closely related to sulfation. processes. Na20 and K20 exhibit a similar trend to that shown by Si02. However, P2O5 exhibits an increase related to biological activity of pingeons living in the Cathedral. In relation to trace elements enrichment processes (Zr, Cr, Sr , Ba), depletion processes (Th) and processes with a slight variation (Zn, Ga) are observed in this study. The evolution of rare earth elements during weathering processes can be explained with two trends: elements whith atom1c number < 64 exhibit a decrease in the rare earth contents while elements with atomic number > 64 exhibit an increase in these contents. Finally, from a general point of view and attending the potential weathering index (PWI), the results obtained in this work show that monzogranites used in the Toledo Cathedral present PWI values ranging from very high to medium-high and the other type of silicate rocks used in the building (granodiorites) presents a PWI value of medium type.
A geochemical study of major, minor, trace and rare earth elements during weathering processes in the rocks used in the building. of Toledo Cathedral is developed using the comparation between geochemical contents in the rocks of the Cathedral with different degrees of weathering and the geochemical contents of samples from the quarries used in the building_ of the Cathedral. The results show a decrease in Si02 contents during weathering of the granites. This decrease is closely related to sulfation. processes. Na20 and K20 exhibit a similar trend to that shown by Si02. However, P2O5 exhibits an increase related to biological activity of pingeons living in the Cathedral. In relation to trace elements enrichment processes (Zr, Cr, Sr , Ba), depletion processes (Th) and processes with a slight variation (Zn, Ga) are observed in this study. The evolution of rare earth elements during weathering processes can be explained with two trends: elements whith atom1c number < 64 exhibit a decrease in the rare earth contents while elements with atomic number > 64 exhibit an increase in these contents. Finally, from a general point of view and attending the potential weathering index (PWI), the results obtained in this work show that monzogranites used in the Toledo Cathedral present PWI values ranging from very high to medium-high and the other type of silicate rocks used in the building (granodiorites) presents a PWI value of medium type.