Igneous Rocks

tutor: Mike Horne

Igneous rocks are generally classified by their whole-rock chemistry, based mainly on the percentage of silica in the rock. Rocks with more silica in them tend to be lighter coloured, less dense, form at lower temperatures, have a more viscous magma/lava. The rocks can be split into 'families' and within each family there are coarse-, medium- and fine-grained members.

Granite family -

Granite - coarse grained acid plutonic rock - essential minerals quartz, (>10%), orthoclase feldspar (pink/red/cream), plagioclase feldspar (white), biotite mica, muscovite mica, hornblende (<10%); accessory minerals include - tourmaline, zircon, pyrite and magnetite.

Microgranite - medium grained equivalent

Rhyolite - fine grained volcanic equivalent - pale coloured, may have flow banding.

Obsidian - very fine grained - volcanic glass

Pegmatite - very coarse grained equivalent.

Syenite family -

Syenite - coarse grained intermediate plutonic rock. White/grey/pink/red. Essential minerals - more orthoclase than plagioclase (but up to 90% total feldspars), biotite, hornblende; accessory minerals - quartz (< 10%), nephaline (never occurs with quartz), augite, olivine, zircon, pyrite, magnetite. Example - Larvikite.

Micro-syenite - medium grained equivalent, often porphyritic - example Rhomb Porphyry.

Trachite - fine grained volcanic equivalent.

Diorite family -

Diorite - coarse grained intermediate plutonic rock. Darker than granites and syenites. Black, green and white speckled. Essential minerals - more plagioclase than orthoclase feldspar, hornblende, biotite; accessory minerals - augite, olivine, quartz, zircon, pyrite, magnetite.

Andesite - fine grained volcanic equivalent, often porphyritic. 60-70% plagioclase plus augite.

Gabbro family -

Gabbro - coarse grained basic plutonic rock. Dark coloured. Essential minerals - plagioclase (up to 60%), augite (green and black), olivine (often rounded); accessory minerals - hornblende, biotite, serpentine. May have a banded texture.

Dolerite (known as Diabase in USA) - essential minerals plagioclase, augite, olivine; accessory minerals - hornblende, biotite, quartz, magnetite. Sometimes amygdaloidal. Example - Whin Sill in northern England.

Basalt - fine grained volcanic equivalent. May be vesicular, amygdaloidal or ropey. May form large lava flows from vents, including pillow lavas. May have polygonal jointing in thick lavas - example Giant's Causeway in Ireland.

Ultrabasic family -

Mostly dark, coarse grained rocks, often dominated by one mineral.

Peridotite - olivine and augite.

Serpentenite - altered peridotite.

Pyroxenite - augite

Picrite - olivine, augite, hornblende plus some plagioclase.

Anorthosite - pale rock containing >90% plagioclase feldspar.