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Roman Wall of Tarragona. Spain. Portal de Sant Antoni.

Causes de la patologia

Limestone of the Miocene age from Tarragona quarries. This partially dolomitized calcisiltite rock presents an  idiotypic zoned fabric, with anomalous intracrystalline distribution, deficient in magnesium by geological processes. These crystals undergo a differential dissolution of their nuclei, which produces abundant intracrystalline moldic porosity and subsequent disaggregation favoring the formation of sand. 
This predisposition to alteration of diagenetic origin, is speeded by existing gypsum and other sulphates eflorescences, external to the rock.

Imatge de visu

Autor: J.L.Prada
Descripció: Altered ashlars from the Portal. The disaggregation processes and the formation of sand have
been produced by differential dissolution the anomalous dolomite crystalline.

Imatge detalli / macro

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Autor: Africa Pitarch
Augments: x100. MOP -thin section- crossed polar (XPL).
Descripció: These processes are very difficult to observe with stereomicroscope, because the dolomite crystals are smaller than a millimeter (micrometer scale)

Imatge Microscòpia

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Autor: Figure 1.Africa Pitarch. Figure 2. J.L.Prada
Augments: Figure1. x120. Figure 2. x1570
Descripció: Figure 1. MOP -thin section- crossed polar (XPL). Dolomitic limestone with distinctly zoned  crystal. The nuclei of euhedral rhomb-shaped crystal is unstained because it is not dedolomitized. The external zone is red stained because it is dedolomitized. These anomalous minerals undergo a differential dissolution and produce the subsequent disaggregation of material.
Figure 2. SEM-BSEI electron image. Differential dissolution of the nuclei produces abundant intracrystalline moldic porosity.

Patologies Associades

Flaking,

Sanding.

Soluble salts.


Altres Anàlisis

Figure1. X Ray image. Mg - EPMB CamecaX50-1500X
Figure 2. X Ray image. Ca - EPMB CamecaX50-1570X

Observacions

Figure 1. Anomalous intracrystalline magnesium distribution. The anomalous dolomite due to a deficiency in magnesium is frequently found in the nuclei of the dolomite crystal.
Figure 2. Differential dissolution of the nuclei of dolomite crystal.
 
These mechanisms may be detemined using the new techniques of electron microscopy -Electron Probe Micro Analyzer (EPMA)- that allow crystallochemycal studies. The figures show the anomalous intracrystalline distribution of calcium and magnesium. In conclusion, this mineral is non-stechiometric and deficient in magnesium. This composition is a favourable factor of stone decay process.

Bibliografia

PRADA, J.L .Caracterización de Formas y Procesos de Alteración observados en Piedra de Construcción de Edad Miocénica del Área Monumental Romana de Tarragona".1995. PhD.


Autor

Jose Luis Prada Pérez. prada43@wanadoo.es

Geologo


Institució o Empresa

ESCRBCC


Articles Publicats

PRADA, J.L.; VALENCIANO, A.; NAVARRO, A. 1995. Procesos de alteración de materiales pétreos en edificios de interés histórico. Acta GeolÓgica HispÁnica, vol 30, nº 13. Universidad de Barcelona. pp. 97-110.
 
PRADA, J.L.; VALENCIANO, A. 1995. Alteración de unas dolomitas zonadas del Miocéno de Tarragona. XVII Reunion Bienal de la Sociedad Española de Microscopía Electrónica. Oviedo. pp. 282-283

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