Composition of the rocks

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© AWMB /Armin Faber

Minerals are the building blocks of rocks. Most rocks are formed of several minerals such as granite with feldspar, quartz and mica. Few rocks consist largely of only one mineral, such as limestone with calcite or quartzite with quartz.

Minerals have an ordered internal structure and a specific chemical composition, and they themselves in turn consist of one or more elements or compounds.

The main rock-forming minerals are

• silicates (their main components are silicon, aluminium and oxygen) such as     feldspars (plagioclase and alkali feldspar), amphibole and pyroxene, quartz, mica, clay minerals, garnet and

• carbonates such as calcite (calcium carbonate) and dolomite (calcium magnesium carbonate).

Other groups form the phosphates, oxides, sulphides and sulphates. 

Common minerals in the three major rock groups are:

• Igneous rocks: quartz, feldspar, mica, pyroxene, amphibole, olivine
• Sedimentary rocks: quartz, clay minerals, feldspar, calcite, dolomite
• Metamorphic rocks: quartz, feldspar, mica, garnet, staurolite, kyanite.

The chemical and mineralogical composition of a rock determines the natural supply of nutrients and thus impacts on the choice of rootstock and varieties. The amount and type of clay minerals, having different properties with respect to water uptake and cation exchange capacity, has a strong influence on soil structure and workability, the behaviour of the soil water, on the binding and release of nutrients such as potassium and magnesium, and also on the susceptibility to erosion.