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Geoscience - the earth

Classification of igneous rocks

• Introduction
• Grainsize
• Silica content
• Silica saturation
• International classification
• TAS classification
• Rock types
• Reference
• Terms

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Introduction

Igneous rocks are classified by using grainsize, silica content, and/or silica saturation. Other methods of classification include the International Union of Geological Sciences (IUGS) scheme and the Total Alkalies vs Silica scheme.

Grainsize

Based on the predominant grainsize that reflects the depth at which molten rocks form within the Earth. Three types of rock can be identified:

• Volcanic rocks solidify close to the Earth's surface. Because they cool quickly they have a finer-grained matrix (called groundmass). They may contain some larger crystals that formed earlier further down (called phenocrysts).
  
• Plutonic rocks form deeper within the Earth and the slower cooling allows them to crystallise as coarse-grained rocks.
  
• Hypabyssal or subvolcanic rocks form at intermediate depths (generally as dykes and sills) and so tend to be medium-grained.

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Silica content

Silica content (SiO₂) which also controls the minerals that crystallise is used to further classify igneous rocks as follows:

• Acid: usually above 63% silica (mostly feldspar minerals and quartz), e.g. granite.
• Basic: 45 to 55% silica (mostly dark minerals plus plagioclase feldspar and/or feldspathoid minerals), e.g. basalt.
• Ultrabasic: usually less than 45% silica (mostly dark minerals such as olivine and pyroxene), e.g. peridotite.

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Silica saturation

This is similar to modal composition, but allows distinction between feldspathoid-bearing and feldspathoid-free rocks.

• Rocks that are undersaturated in silica will crystallise silica-poor minerals like feldspathoids or olivine.
• Those that are just saturated in silica will lack feldspathoids or quartz.
• Those that are oversaturated in silica will crystallise quartz among the minerals.

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International classification

The International Union of Geological Sciences (IUGS) classification scheme is the standard scheme for igneous rocks and uses the minerals present. It is in the form of triangular diagrams where each apex of the triangle represents 100% of a mineral. It splits into feldspathoid-bearing (i.e. nepheline, sodalite) and feldspathoid-free rocks. Then, it takes the relative percentages of quartz (Q), plagioclase (P) and alkali feldspars (A). The value of Q, P, and A determines a given field for each rocktype. Similar schemes also classify ultramafic rocks (but using olivine, orthopyroxene and clinopyroxene) and gabbroic rocks.

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TAS classification

The TAS (Total Alkalies vs Silica) classification scheme uses chemistry to classify volcanic rocks. The analysis is recalculated to 100% after removing H₂O and CO₂. It is a simple X-Y graph with the X-axis being SiO₂ Wt% and the Y-axis being Na₂O+K₂O Wt%.

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Rock types

• Common plutonic rocks
• Common subvolcanic rocks
• Common volcanic rocks
• Other volcanic rock types
• Other types

Common plutonic rocks

Acid

• Granite: the most common igneous plutonic rock. Contains essential quartz, plagioclase and alkali feldspar, usually with hornblende and/or biotite and/or muscovite.
• Granodiorite: a plutonic rock with essential quartz and plagioclase, with lesser amounts of alkali feldspar and small amounts of hornblende and biotite.
• Tonalite: a plutonic rock with essential quartz and sodic plagioclase, usually with lesser biotite and amphibole.
• Aplite: fine-grained, composed mostly of quartz and feldspars.
• Pegmatite: a very coarse-grained, usually plutonic granitic rock.

Basic

• Diorite: a plutonic rock composed of sodic plagioclase, commonly hornblende, and/or biotite or augite.
• Gabbro: a coarse-grained plutonic rock with essential calcic plagioclase, pyroxene and opaque minerals, and/or hornblende or olivine. The plutonic equivalent of basalt.
• Monzonite: a plutonic rock with equal amounts of plagioclase and alkali feldspar, along with lesser amphibole and/or pyroxene.
• Syenite: a plutonic rock composed mainly of alkali feldspar, with subordinate sodic plagioclase, biotite, pyroxene, and amphibole.

Ultrabasic

• Anorthosite: a plutonic rock with mostly calcic plagioclase and small amounts of pyroxene.
• Dunite: a plutonic rock composed mostly of olivine.
• Clinopyroxenite: a plutonic rock composed mostly of clinopyroxene.
• Harzburgite: a plutonic rock composed of essential olivine and orthopyroxene.
• Hornblendite: a plutonic rock composed mostly of hornblende.
• Lherzolite: a plutonic rock composed largely of olivine, along with lesser clinopyroxene and orthopyroxene, and sometimes spinel.
• Orthopyroxenite: a plutonic rock composed mostly of orthopyroxene.
• Peridotite: a plutonic rock composed of olivine with lesser pyroxene and/or amphibole.
• Pyroxenite: a plutonic rock composed largely of pyroxene.

Common subvolcanic rocks

• Dolerite: a subvolcanic rock with essential plagioclase, pyroxene and opaque minerals. The subvolcanic equivalent of basalt.
• Porphyry: a general term for igneous rocks that contain phenocrysts in a finer-grained groundmass.

Common volcanic rocks

Acid

• Rhyolite: silicic volcanic rocks composed of quartz and alkali feldspar, with minor plagioclase and/or biotite. The volcanic equivalent of granite.
• Comendite: a porphyritic alkali-rich rhyolite with phenocrysts of quartz, alkali feldspar and sodic pyroxenes and/or amphiboles.
• Dacite: a common volcanic rock composed of quartz and sodic plagioclase, along with small amounts of biotite and/or hornblende, and/or pyroxene.

Basic

• Basalt: the most common volcanic rock on Earth. Composed of essential calcic plagioclase and pyroxene, sometimes with olivine, feldspathoids or interstitial quartz.
• Andesite: an intermediate, commonly porphyritic volcanic rock composed of plagioclase, pyroxene, hornblende and/or biotite.
• Latite: an intermediate volcanic rock with equal amounts of sodic plagioclase and alkali feldspar.
• Trachyte: a volcanic rock composed largely of alkali feldspar.

Ultrabasic

• Komatiite: these rocks are high in magnesium and crystallised at high temperatures. They commonly display a spinifex texture consisting of intergrown skeletal and bladed olivine and pyroxene crystals in a glassy groundmass. Most are Archean in age.

Other volcanic rock types

• Agglomerate: a rock of coarse pyroclastic material consisting dominantly of rounded fragments.
• Amygdaloidal: a volcanic rock containing mineral-filled cavities (or vesicles).
• Bomb: a volcanic projectile with an average diameter greater than 64 mm and whose shape or surface indicates that it was partially or completely molten during its formation and transport through the air.
• Ignimbrite: a pyroclastic tuff consisting of crystal and rock fragments in a matrix of glass shards that are usually welded together.
• Obsidian: a volcanic glass that is commonly dark in colour and glassy, with a water content less than 1%.
• Perlite: volcanic glass that exhibits numerous concentric cracks. Some are high in water and expand when heated.
• Pumice: a pyroclastic glassy volcanic rock containing abundant empty cavities (or vesicles), usually of rhyolitic composition.
• Scoria: a pyroclastic volcanic rock containing abundant empty cavities (or vesicles), usually of basaltic composition.

Other types

• Carbonatite: volcanic or subvolcanic rocks composed mostly of primary carbonate minerals, such as calcite, dolomite or Na-carbonates.
• Kimberlite: a volcanic or subvolcanic rock composed largely of serpentinised olivine, with variable amounts of phlogopite mica, orthopyroxene, clinopyroxene, carbonate and chromite. Characteristic accessory minerals include pyrope garnet, rutile, monticellite and perovskite, and in some cases diamonds.
• Lamproite: a volcanic or subvolcanic rocks that are rich in potassium and magnesium, composed of unusual rare minerals such as K-Ti-richterite, priderite, wadeite, jeppeite, Fe-orthoclase and leucite. The hostrock for diamonds in the AK1 pipe of the Argyle diamond mine, Western Australia.
• Lamprophyre: a name for a group of subvolcanic rocks that are strongly porphyritic in mafic minerals such as biotite, amphiboles and pyroxenes in a groundmass of feldspar. They commonly occur as dykes.

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Reference

Le Maitre, R.W., 1989. A classification of igneous rocks and glossary of terms. Blackwell Scientific Publications, 193p.

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Terms

• Acid - usually above 63% silica (mostly feldspar minerals and quartz), e.g. granite.
• Basic - 45 to 55% silica (mostly mafic minerals plus plagioclase feldspar and/or feldspathoid minerals), e.g. basalt.
• Ultrabasic - usually less than 45% silica (mostly mafic minerals such as olivine and pyroxene), e.g. peridotite.
• Felsic - mostly feldspar minerals and quartz, e.g. granite.
• Mafic - mostly pyroxenes and olivine plus plagioclase feldspar and/or feldspathoid minerals, eg. basalt.

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