OUTSTANDING PROPERTIES FOR MARBLES AND GRANITES

The physical and mechanical properties of higher interest for comparative evaluation of marbles and granites, representing the two main groups of dimension stones and coverings, include resistance to the abrasive consuming, to shear by uniaxial compression, to rupture by flexion traction and water absorption.

The resistance to the abrasive wearing out is normally proportional to the hardness of the constituent minerals of the rocks, as measured by the scale of Mohs. The calcite and dolomite, main constituents of marbles, show hardness of 3 and 3,5-4, respectively. The hardness of the main components of granites is significantly superior, to mention the quartz (hardness 7), the feldspars (6) and iron-magnesian minerals (4 to 6).

It is thus observed, that as a function of the mineral hardness, the granites are more resistant to the abrasive wearing out and also to scratches. Among granites it is expected the higher the resistance, the higher the amount of quartz; among marbles, the higher the resistance the higher the dolomitic character.

The consuming of the stones may also be caused by scarification (disruption) of the mineral grains, what seems to be conditioned not by the composition of it but because of the texture of crystals. The metamorphic texture of marbles of thick mass is over all grainoblastic of the saccharoid type, less resistant to the scarification than those common ones in granites.

It is possible that the texture differences and therefore the type of crystals imbrication also justifies the higher average values of the siliceous rocks as far as the resistance to compression and flexion is concerned, again enhancing their physical and mechanical superiority to marbles. Due to the modern techniques of plates setting in floors and façades with metallic bulkheads, the differential represented by its flexion is particularly remarkable.

In relation to the water absorption, which translates the percentage of intercommunicating empty spaces, or either, the actual porosity of the materials, values observed for the siliceous and silicated rocks are generally higher than those of the carbonate rocks. The granites and quartzites, even when polished and burnished, are still more subjected to stain and infiltration of liquids than marbles.

The physical and mechanical differences observed between marbles and granites recommend the discrimination of use and of application environments, however not implying in the use of distinct techniques or procedures for setting in coverings.