Silicate Stones – Granites

In the area of ornamental and covering stones, the appellation “granite” designs a wide group of silicate stones, covering monzonites, granodiorites, charnockites, sienites, diorites, diabases/basalts and the granites.

The mineral composition of the "granites" is then defined by much variable associations of quartz, feldspars, micas (biotite and muscovite), amphiboles (specially hornblende), pyroxenes (aegirite, augite and hypersthene) and olivine. Some of the components can be absent in certain mineral associations; several other mineral accessories in much more reduced proportions have been observed . Quartz, feldspars, micas and amphiboles are the leading minerals in the granite and granitoid rocks.
Quartz is macroscopically recognized as the colourless or smoky (fumé) mineral, usually translucent, very common in the granites, also found in bluish color within specific rocks.
The feldspars (microclines, orthoclases and plagioclases), are the main markers of the silicate stones chromatic standards, conferring the reddish, rosy and cream-grayish colors on the homogeneous granites (isotropic) and oriented/active (anisotropic).

The mafic minerals (iron-magnesian silicates) confer the black color invariably impregnated in the matrix of the silicated rocks, especially amphibole (hornblende) and mica (biotite), commonly called "coal".

In granites with more leucite (light), and the like, thus with a smaller quantity of iron-magnesium minerals, the quartz and the feldspars compose usually between 85% and 95% of the rock.

The texture of the silicated rocks is determined by measurement of grain and crystal habits, the structure being defined by the distribution of these crystals. So, composition, texture and structure represent parameters of very large interest for characterization of granites and their distinction from the marbles.