The earth’s crust is made up of elements, some very common and others very rare. These elements are what make up minerals. The two most common elements are Oxygen (O) at 47% and Silicon (Si) at 28%.
Other common elements are Aluminium (Al) at 8%, Iron (Fe) at 5%, Calcium (Ca) at 3.5%, Sodium (Na) at 3%, Potassium (K) at 2.5% and Magnesium (Mg) at just 2%. All the other elements together make up only 1% of the earth’s crust by weight.
Elements make Minerals make Rocks
Minerals can be the pure element, like Gold, Silver or Copper, but most minerals are compounds of elements, like Quartz (SiO2), and Gypsum CaSO42H2O.
The most common mineral then is made up of the two most common elements, Oxygen and Silicon, SiO2. It’s a compound of the two elements and it’s name is Quartz (Silica). If other elements are added to quartz, for example in a molten state, then other compounds or minerals form. It’s a bit like a cooking reciepe, each mineral has it’s own ingredients. Most of the major rock forming minerals are variations of the silca compound with other elements added to the mix.
The silicate minerals then make up over 98% of the earth’s crustal rocks by weight. To look at all these would be beyond the scope of this page so we will look at the minerals involved in making up granite. For a rock to be classified or given the name granite it must have in it three essential minerals.
Quartz, Feldspar and Mica.
orthoclase

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These are all silicate minerals, that is to say they contain silicon and oxygen in their structure.
Quartz is pure silicate often called silica, it’s formula is SiO2. In granite there is free quartz, quartz which is not taken up in the making of the other silicate minerals.
Feldspar is the name given to minerals which are aluminosilicates, (AlSi3)O8. This arrangement of the elements leaves a surplus negative charge, which is balanced out by the addition of another element. Dependant on which element is present is the type of feldspar.

Examples:

KAlSi3O8 is the potassium feldspar orthoclase.

NaAlSi3O8 is the Sodium feldspar albite.

CaAl2Si2O8 is the calcium feldspar anorthite.

These can be illustrated as a triangular diagram where they form the end members of partly discontinuous solid solutions.

Important in granite are the potassium feldspars although most granites contain some plagioclase, normally only visable in
thin section.

 

After Cox, Price and Harte 1974.

 

Mica is the third essential mineral of granite, again an aluminosilicate mineral but with a complex chemical formula. There are two main members of the group, biotite and muscovite. In Dartmoor granite biotite is the essential mica and muscovite is a secondary mineral produced by alteration of the biotite. The chemical formula for biotite is (OH)4K2(Mg,Fe)6(Al2Si6)O20.

 





orthoclase feldspar biotite mica quartz