In the natural stone sector it is known as weathered granite, “granito país” (in Galicia), those granites that have suffered some degree of weathering.
Weathering is the process of physical and chemical alteration experienced by rocks that are close to the surface or fissures, by the exposure and action of meteoric agents (temperature, rain, frost, etc.), and that in granites it is shown as a mineral alteration (oxidation of biotites, argillitation of feldspars …), increased porosity and micro-cracking, etc.
The weathered granite used as natural stone is selected from rocks with an incipient degree of weathering, although even so, they present certain changes with respect to healthy granite such as:
Changes in appearance
They present ochre, orange / yellowish or even brownish tones due to the oxidation of the biotite that ornamentally gives them a more rustic appearance
. Slight changes in the physical-chemical properties
Mineral alteration and increased porosity produce a decrease in resistance. They may have lower density, and greater porosity and water absorption than their corresponding healthy variety. However, these values are still very valid for use in construction.
Traditionally the weathered granites were commercialized in the local market, finding in Galicia a very common use in the rural zone, nevertheless, some varieties of weathered granite are becoming more important in the international market. In Galicia, more than 15 different varieties of weathered granites are extracted.
The weathered granites provide a warm appearance, combining exceptionally with other natural materials such as wood; their more and more habitual use in contemporary constructions allow to observe its perfect adaptation to these buildings and perfect combination with other materials.
Nuria Sánchez Delgado
Doctor in Geology
CENTRO TECNOLÓXICO DO GRANITO DE GALICIA