Granite, natural design

Today we would like to issue a reminder that granite is a natural rock that has been formed by complex geological processes, and this makes for an infinite range of textures and colours.

It is formed from the solidification (crystallisation) of melted silicate or magma (700-1200ºC) that rises from the mantle and whose cooling takes place in a deep area of the crust, a reason why this cooling is slow and there is time for large crystals to form.

The final appearance of the granite, mainly defined by its texture and colour, besides providing the granite with its aesthetic component, tells the story of its formation. That is why the pieces of this natural stone are unique. They are rocks whose position in the crust occurred millions of years ago.

The texture is defined by the grain size, the distribution among the components of the rock, etc., which is a result of its formation process. For example, the cooling speed of the magma from which it is formed reflects that the greater the size of the crystals forming the granite, the slower it will have taken to cool the magma from which it is formed.

granitos gallegos — Figure 1. Images of Galician granites (10x10cm): From left to right: “GRIS ALBA”, “GRIS MONDARIZ”, “ROSAVEL”, from smaller to larger grain size, that is, which one has suffered the quickest and slowest cooling during their formation.

In terms of the distribution of the components, the above figures, for example, represent granites with granular textures, but when these rocks have suffered deformation at a regional scale, they present banded textures that are defined by the accumulation of certain and generally oriented minerals. In this case, they are not “granite” in a strict sense but are metamorphic rocks, although commercially they are considered as such

Granito — Figure 2. Images of “granites” with banded textures (10x10cm): SALMON PINK, GUAYABA-GUAYANA RED, MULTICOLOUR RED.

The colour is determined by its chemical composition. Each mineral forming the rock may have a typical colouring, but in turn, small impurities in the composition of a mineral can change that colour. For example, alkaline feldspars are those that usually present most variation with white, pink, reddish, yellowish tones, etc. Therefore, Galician granites from the south of Pontevedra usually have pink feldspars.

Granite is a rock formed mainly by the following minerals: quartz, feldspars (alkaline feldspar and plagioclase) and mica. If we look at granite well, we will be able to simply distinguish the different minerals just by viewing it:

Granito Rosa — Figure 3. Image of the “Rosa Porriño” granite (10x10cm). In pink, alkaline feldspars; in milky white, plagioclase; grey quartz (translucent crystals) and in black, biotite (black mica).

So remember, your granite countertop or surfaces will be unique and tell a story that was shaped millions of years ago, you will not find another that is exactly the same

Nuria Sánchez Delgado
PhD in Geology
TECHNOLOGICAL CENTRE OF GRANITE IN GALICIA