What is a granite in geology ?
Granite : definition
Granites are mineralogically, chemically and texturally varied, which has led to them being subdivided into many types. The most commonly used classification takes into account chemical criteria :
- alkaline granites with very abundant quartz, anorthoclase, sometimes albite and sodium minerals (reebeckite type amphibole, aegyrine type pyroxene).
- calc-alkaline granites, the most common, with basic quartz, feldspar and muscovite paragenesis, usually enriched with fairly abundant ferromagnesian minerals (biotite, amphibole,garnet), cordierite, andalusite, topaz, etc...
- monzonitic granites (or monzogranites), also very common, most often porphyroid, characterized by equal amounts of potassium feldspar and plagioclase feldspars, and biotite. They herald the transition to intermediate rocks, less rich in silica, of the diorite type.
On the basis of grain size, types are often distinguished from aplitic, fine-grained granites to porphyroid granites (frequently monzogranites), characterized by automorphic K-feldspar crystals of several centimeters (sometimes 15 to 20 cm in some French Hercynian granites). The volcanic equivalent of granite is rhyolite, a rock that is much less present on the planet.
.
.