The Estremoz marbles: an updated summary on the geological, mineralogical and rock physical characteristics

Abstract

Marbles have been used by human beings for the creation of buildings, monuments and art since antiquity. Impressive architecture and art all over the world are still being constructed from marble today. The Portuguese Estremoz marble is one of the important marble varieties used for these objects. The name Estremoz is used for many of the varieties, exploited and sold from this region. The present study gives an updated summary on the geological, mineralogical and rock physical characteristics with special attention to the potential physical weathering behaviour. Ten representative marble varieties extracted and sold in this region were chosen for analysis with regard to their mineralogical composition, microfabric and their thermal, thermohygric and bowing behaviour. Experimental bowing tests, carried out on selected samples, have been analysed and compared to the results measured at the State Theatre in Darmstadt. The mineralogical composition is similar for all samples, even though the colours range from white, to pinkish and dark grey. Monochromatic colours are seldom. Randomly distributed veins and spots in different colours determine the aesthetic properties. The main component is calcite (87–98%), while two samples show a lower amount of dolomite (3 and 11%). The microfabric of the studied marbles are quite different, with grain sizes ranging between 40 µm and 2.5 mm. Distribution of grain sizes are equi- and inequigranular to seriate. The grain boundary geometry ranges from polygonal to amoeboid. All samples show a preferred orientation of the grain shape, often most pronounced in the XZ-plane. Measurements of texture showed, that the maximum c-axis densities of the marbles are similar and with 2.09–3.64 mrd moderately to strongly pronounced. In some cases, a weakly to moderate girdle-like shape of the intensity distribution could be observed. The thermal dilatation coefficient α varies between 0.16 and 12.97 × 10−6 K−1 and shows a strong directional dependence. All samples showed an irreversible length change after repeated heating cycles under dry and wet conditions that ranges between 0.05 and 0.59 mm/m. Marble panels used for the bowing test showed a maximum bowing intensity of 0.5 mm/m. The State Theatre in Darmstadt, Germany, is one example of the exterior application of Estremoz marble. The façade is cladded with panels of 3 cm in thickness. They show weathering phenomena like fractures and concave and convex bowing. Bowing intensities have been measured up to more than 9 mm/m. Weathering phenomena are influenced by the rock physical properties which can be seen in the results of the thermal and thermohygric behaviour as well as for the bowing tests. By comparing these results, the residual permanent length change can be used as a measure of the vulnerability to thermal weathering.