Classifying Igneous Rocks

Texture - What does it look like? Refers mainly to mineral size

• Aphanitic - Rapid rate of cooling of lava or magma produces microscopic crystals. Typically occurs in extrusive / volcanic rocks
• Phaneritic - Slow cooling produces large crystals. Crystals are large enough that they can be identified without a microscope. Typically occurs in intrusive / plutonic rocks
• Porphyritic - Two distinct crystal sizes. Minerals form at different temperatures & rates. Large crystals (phenocrysts) are embedded in a matrix of smaller crystals.
• Glassy - Very rapid cooling of molten rock results in no minerals forming at all. Resulting rock is called obsidian.
• Pyroclastic - Various fragments ejected during a violent volcanic eruption. Textures often appear similar to sedimentary rocks
• Pegmatitic - Exceptionally coarse grained. Crystals can range from a few inches to several feet long. Form in late stages of crystallization of granitic magmas.

 

The shape of the crystals can vary, depending on mineral type and rate of cooling.

• Euhedral - grains completely bounded by crystal faces
• Subhedral - grains only partly bounded by crystal faces
• Anhedral - grains completely devoid of crystal boundaries

 

The grains (crystals) can also be arranged differently.

• Equigranular - all grains are roughly the same size.
• Slightly inequigranular - some grains are just a bit bigger than others
• Inequigranular - most grains are the same size, but a few are conciderably larger than others. Basically a porphyritic texture.