Classification & Nomenclature of Igneous Rocks

Granite

Phaneritic rock with 20-60% quartz, alkali feldspar + plagioclase (Q 20-60%)

Granodiorite

Plagioclase > alkali feldspar with 20-60% quartz

Quartz monzonite

Roughly equal plagioclase and alkali feldspar with 5-20% quartz

Monzonite

Roughly equal plagioclase and alkali feldspar with <5% quartz

Syenite

Alkali feldspar dominant, <5% quartz

Quartz syenite

Alkali feldspar dominant with 5-20% quartz

Alkali feldspar granite

90% alkali feldspar + quartz

Tonalite

Plagioclase dominant with 20-60% quartz (quartz diorite)

Diorite

Plagioclase (An<50) + hornblende/biotite; salt-and-pepper appearance

Gabbro

Plagioclase (An>50) + pyroxene ± olivine; dark-colored

Anorthosite

90% plagioclase (phaneritic)

Norite

Orthopyroxene gabbro

Rhyolite

Volcanic equivalent of granite; high silica

Dacite

Volcanic equivalent of granodiorite/tonalite

Andesite

Intermediate volcanic; plagioclase-rich, color index

Basalt

Mafic volcanic; plagioclase + pyroxene ± olivine, color index >35% or <52% SiO₂

Trachyte

Volcanic equivalent of syenite

Latite

Volcanic equivalent of monzonite

Phonolite

Volcanic foid-bearing rock (feldspathoid + alkali feldspar)

Tephrite

Basanite-like with feldspathoids

Basanite

Foid-bearing basalt

Nepheline syenite

Phaneritic rock with nepheline (foid) + alkali feldspar

Foidolite

Plutonic rock dominated by feldspathoids

Peridotite

Ultramafic rock dominated by olivine + pyroxene

Dunite

90% olivine

Lherzolite

Olivine + orthopyroxene + clinopyroxene

Harzburgite

Olivine + orthopyroxene

Wehrlite

Olivine + clinopyroxene

Pyroxenite

Ultramafic rock dominated by pyroxene

Dolerite (Diabase)

Medium-grained hypabyssal equivalent of basalt (shallow intrusive)

Picrite

Mg-rich volcanic with 12-18% MgO and <3% alkalis

Komatiite

Very Mg-rich volcanic (TiO₂ <1%)

Meimechite

Very Mg-rich volcanic (TiO₂ >1%)

Boninite

Andesite or basaltic andesite with >8% MgO and <0.5% TiO₂

Lamproite

Highly alkaline, volatile-rich mafic rock (volcanic/dike)

Lamprophyre

Highly alkaline, volatile-rich mafic dike rock

Carbonatite

Igneous rock dominated by carbonate minerals

Spilite

Sodic altered basalt

Keratophyre

Sodic altered intermediate volcanic rock

Quartzolite

90% quartz (phaneritic)

Tephrite

Basanite-like foid-bearing mafic volcanic

Phonotephrite

Intermediate between phonolite and tephrite

Trachyandesite

Intermediate between trachyte and andesite

Trachybasalt

Intermediate between trachyte and basalt

Pumice

Highly vesicular volcanic glass (often rhyolitic)

Obsidian

Dense volcanic glass (often rhyolitic)

Pegmatite

Very coarse-grained igneous rock (usually granitic)

Aplite

Fine-grained granitic dike rock

Rapakivi granite

Granite with orthoclase phenocrysts mantled by plagioclase

Graphic granite

Intergrown quartz and feldspar in cuneiform pattern

Phaneritic

Crystals > ~0.1 mm, visible to naked eye; slow cooling, intrusive (plutonic) rock

Aphanitic

Crystals < ~0.1 mm, too small to see without magnification; rapid cooling, extrusive (volcanic) rock

Fine-grained (phaneritic)

< 1 mm diameter

Medium-grained (phaneritic)

1–5 mm diameter

Coarse-grained (phaneritic)

5–50 mm diameter

Very coarse-grained (phaneritic)

50 mm diameter; often called pegmatitic when granitic

Pegmatitic

Very coarse-grained texture, usually with compositional implications (late-stage granitic magmas)

Porphyritic

Texture with two dominant grain sizes: large phenocrysts in a finer groundmass

Phenocryst

Large, early-formed crystals in a porphyritic rock

Groundmass

Finer-grained matrix surrounding phenocrysts

Fragmental (pyroclastic)

Rock composed of disaggregated igneous fragments (bombs, blocks, lapilli, ash) from volcanic explosions

Pyroclastic

Fragmental igneous rocks formed by volcanic explosions or collapse

Felsic

Light-colored silicate minerals (quartz, feldspars, feldspathoids) and the rocks dominated by them

Mafic

Dark-colored silicate minerals rich in Mg and Fe (olivine, pyroxene, amphibole, biotite) and the rocks dominated by them

Ultramafic

90% mafic minerals

Leucocratic

Light-colored rock (based on color)

Melanocratic

Dark-colored rock (based on color)

Color index (M')

Volume % of dark (mafic) minerals

Silicic (acidic)

66 wt% SiO₂

Intermediate

52–66 wt% SiO₂

Basic

45–52 wt% SiO₂

Ultrabasic

< 45 wt% SiO₂

Mode

Volume % of each mineral in a rock (determined by point counting or visual estimation)

Q (Q')

Normalized % quartz (or other SiO₂ polymorph)

A (A')

Normalized % alkali feldspar (including albite An<5)

P (P')

Normalized % plagioclase (An5–An100)

F (F')

Normalized % feldspathoids (foids)

M (M')

Total % mafic minerals + accessories

IUGS Classification

Standardized system using modal mineralogy (QAPF diagram) for most igneous rocks

QAPF Diagram

Triangular diagram using normalized Quartz, Alkali feldspar, Plagioclase, Feldspathoids to name plutonic and volcanic rocks

TAS Diagram

Total Alkali vs. Silica chemical classification diagram for volcanic rocks when mode cannot be determined

Pyroclast

Individual fragment in pyroclastic rock (crystal, glass, or lithic)

Bomb

64 mm, molten during eruption (rounded, stretched)

Block

64 mm, solid during eruption (angular)

Lapilli

2–64 mm fragments

Ash

< 2 mm fragments

Tuff

Pyroclastic rock dominated by ash-sized particles

Lithic tuff

Tuff dominated by rock fragments

Vitric tuff

Tuff dominated by glass/pumice fragments

Crystal tuff

Tuff dominated by crystal fragments

Tuffite

Rock with 25–75% pyroclasts mixed with epiclasts

Epiclast

Sedimentary clast (non-volcanic origin)

Epivolcaniclastic

Secondary volcaniclastic deposit (reworked, not primary eruption)

Hyaloclastite (hyalotuff)

Aquagene tuff formed by magma shattering in water

First step in igneous rock classification

Determine texture: phaneritic (plutonic), aphanitic (volcanic), or fragmental (pyroclastic)

Why ignore M in QAPF normalization

Because most common rocks are named by the felsic minerals (Q+A+P or F+A+P); M is only used when >90%

How to distinguish gabbro vs diorite

Plagioclase composition: An>50 (gabbro) vs An<50 (diorite); also color and mafic minerals

How to distinguish andesite vs basalt (when mode unclear)

Color index (>35% mafics = basalt) or SiO₂ content (>52% = andesite)

Why quartz and feldspathoids never coexist

They are incompatible minerals; one or the other is used in QAPF

Purpose of TAS diagram

Chemical classification (Na₂O+K₂O vs SiO₂) for aphanitic/glassy volcanic rocks where mode cannot be determined