The terms Acid and Basic do not refer to the pH of the rock: it is based on the percentage of silica in the rock.

The rocks in the Granite 'family' contain more than 10 percent Quartz. In the two Intermediate families Orthoclase is dominant in the Syenite family and Plagioclase is dominant in the Diorite family. In the Basic rocks Plagioclase and 'Augite' (Pyroxene Group minerals) are dominant, and dark minerals dominate the Ultrabasic rocks.

Generally the Acid rocks are lighter in colour than the more basic ones.

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.