Texture

Texture refers to the grain size in an igneous rock and relates directly to how the rock cooled.  There are two main types of textures:

Phaneritic Texture: A coarse-grained texture where crystals are visible with the naked eye.  These rocks are intrusive meaning they cooled very slowly beneath the earth’s surface allowing for large crystals to develop. The image of gabbro above has a phaneritic texture.

Aphanitic Texture: A fine-grained texture where crystals are not visible with the naked eye.  These rocks are extrusive meaning they cooled very rapidly above the earth’s surface where large crystals did not have time to develop.  The image of basalt above has a phaneritic texture.

Note: gabbro and basalt have the same mineral composition, but different textures.

Other textures include glassy (ie obsidian) produced from extremely rapid cooling, or frothy/cellular (ie. pumice) produced when gas bubble/vesicles are contained within the rocks.

Colour Index

Colour Index refers to the relative amount of ferromagnesian (FeMg) minerals present within the rock.  Ferromagnesian minerals contain high amount of iron (Fe) and Magnesium (Mg).  Minerals such as Olivine, Pyroxene and Hornblende are ferromagnesian minerals.

The colour index for igneous rocks ranges from Felsic (low FeMg minerals) to Ultramafic (~100% FeMg minerals).

Felsic: produced from magmas that are rich in K, Na, and Si and poor in Fe, Mg, and Ca.  These rocks are usually lighter in colour.  Felsic rocks have <15% FeMg minerals and are mostly composed of the minerals quartz, K-Feldspar and plagioclase feldspar.  The image of rhyolite above is a felsic igneous rock.

Intermediate: contain ~15-40% FeMg minerals and have more plagioclase than k-feldspar and quartz.  The image of diorite above is an intermediate igneous rock.

basalt with scale

Mafic rocks: produced from magmas that are rich in Fe, Mg, and Ca are relatively poor in K, Na, and Si.  These magmas produce rocks that are usually dark in colour.  Mafic rocks have >50% FeMg minerals and no quartz or k-feldspar. The image of basalt above is a mafic igneous rock.

IMG_9594

Ultramafic: these igneous rocks are made up of ~100% FeMg minerals. The image of peridotite above is an ultramafic igneous rock.

Identification Chart