new icn messageflickr-free-ic3d pan white
Granite 28 | by James St. John
Back to photostream

Granite 28

Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.


Granite is a common intrusive igneous rock. Garden-variety granites are composed of quartz, potassium feldspar (K-feldspar), sodic plagioclase feldspar, hornblende amphibole, and mica. Granites have a felsic chemistry. Felsic igneous rocks are generally light-colored, have >65% silica (“silica” = SiO2 chemistry) (felsic has also been defined as >70% silica), are rich in potassium (K) & sodium (Na), and are dominated by the minerals quartz and K-feldspar.


Most granites have a phaneritic texture (coarsely crystalline; all crystals between 1 mm and 1 cm in size), the result of relatively slow cooling of magma deep underground. Other granites may be porphyritic (a mix of large & small crystals) or pegmatitic (almost exclusively composed of very large crystals - almost all >1 cm in size).


Gray, glassy crystals = quartz

Pinkish crystals = potassium feldspar

Whitish gray crystals = plagioclase feldspar

Black crystals = hornblende amphibole


0 faves
Taken on November 21, 2019