||Igneous Rock Classification|
James S. Aber
Emporia State University
Igneous rock texture
The names and terminology applied to igneous rocks can be quite confusing for geology students.
Various terms may refer to rock texture, mineral constituents, or chemical composition. Many
names with vague or poorly defined meanings have been applied over the years to the great variety
of rocks formed by cooling from magma or lava. Some of these names are quite strange sounding--trachyte, while others are in the common English vocabulary--granite.
From this often confusing situation, a commonly used classification for igneous rocks has emerged
based on two criteria--texture and chemical composition. Texture refers to the
size of crystals, presence of glass, and porosity of the rock. Texture is determined primarily
by how the magma or lava cooled. Following are common textural terms for igneous rocks.
- Phaneritic -- Large crystals that are clearly visible to the eye with or
without a 10-power hand lens. The entire rock is made up of large crystals, which are
generally ½ mm to cm in size; no fine matrix material is present. This
texture forms by slow cooling of magma deep underground in the plutonic
- Aphanitic -- Small crystals that cannot be seen by the eye with or
without a 10-power hand lens. The entire rock is made up of small crystals, which are
generally less than ½ mm in size. This texture results from rapid cooling in
volcanic or hypabyssal (shallow subsurface) environments.
- Porphyritic -- Texture in which an aphanitic matrix makes up part of the rock,
and large crystals are present within the matrix. The large crystals are called
phenocrysts. This texture implies two-stage cooling--an early stage of slow cooling
in which the phenocrysts grow, followed by a later stage of rapid cooling that forms the
- Pegmatitic -- Pegmatites are composed of quite large crystals--cm to tens of cm
in size. They typically occupy veins or layers within a larger plutonic body. The large
crystals form by slow cooling of magma.
- Glassy -- Non-crystalline (glassy) structure of the rock, in which no minerals
are present. Glass results from cooling that is so fast that minerals do not have a chance
to crystallize. This may happen when magma or lava comes into quick contact with much
cooler materials near the Earth's surface. Pure volcanic glass is known as obsidian.
- Vesicular -- This term refers to vesicles (holes, pores, or cavities)
within the igneous rock. Vesicles are the result of gas expansion (bubbles), which often
occurs during volcanic eruptions. Pumice and scoria are common types of vesicular rocks.
- Breccia -- A rock composed of broken, angular fragments of mixed composition.
Such texture forms in volcanoes, along fault zones, and in landslides.
Igneous rock composition
Classification based on chemistry takes into account the amount of silica (SiO2)
and the composition of feldspar minerals (K, Na, Ca). Igneous rock chemistry is determined
mainly by the source of the magma and any interactions between magma and the rocks through
which it migrates. Chemical composition usually is indicated by the minerals or color of an
igneous rock. Four main compositional categories result from this approach (based on
- Felsic -- Rich in feldspars and silica.
Silica content ranges from about 55% to > 70%. Potassium feldspar
makes up more than one-third of total feldspars; plagioclase (Na & Ca) feldspars are less
than two-thirds of total feldspars. Typical of continental crust.
- Intermediate -- Between felsic and mafic.
Silica content ranges from about 55% to 65%. Plagioclase feldspars make up more than two-thirds of total feldspars. Na-rich plagioclase predominates over Ca-rich
plagioclase. Found in association with subduction zones.
- Mafic -- Rich in magnesium and iron with less silica.
Silica content is 45% to 50%. Ca-rich plagioclase is the dominant
feldspar with little or no K- or Na-feldspars. Typical of oceanic crust.
- Ultramafic -- Still more magnesium and iron and even less silica.
Silica content is less than 45%, and little or no feldspar is
present. Derived from the mantle.
Igneous rock classification
Texture combined with chemical composition is the basis for modern classification of igneous
rocks. Although chemical composition cannot be "seen" in the field, it can be estimated based
on the main minerals of a rock specimen. The classification below considers the amount of
quartz and the types of feldspar (K, Na, Ca) varieties as primary indicators for chemical
Classification of common igneous rocks based on texture
Adapted from Travis 1955.
||Phaneritic (main minerals)
||Granite (>10% quartz, >2/3s K-feldspar)|
Syenite (<10% quartz, >2/3s K-feldspar)
Monzonite (1/3 to 2/3s K-feldspar)
||Granodiorite (>10% quartz, >10% K-spar, >2/3s Na-spar)|
Diorite (<10% quartz, <10% K-feldspar, >2/3s Na-feldspar)
||Gabbro (<10% quartz, >2/3s Ca-feldspar, olivine)
||Peridotite (pyroxene and olivine; no quartz or feldspar)
The color index (right column of table) indicates the percentage of dark-colored minerals in
the rock. Dark-colored minerals include olivine, pyroxene, biotite, hornblende, and iron
oxides, which are generally dark green to black. Quartz, feldspars, and muscovite are
light-colored. This includes pink and red potassium feldspars as well as gray Ca-rich
plagioclase. Color index is somewhat variable and works best with phaneritic rocks. The
color index is more difficult to apply to aphanitic rocks, as their hue and brightness
vary considerably. For example, rhyolite may be pale gray, green, or pink to dark red.
Andesite is usually moderate gray or green, and basalt is dark gray to black. However, the
colors for glassy rocks (obsidian, scoria, pumice) are meaningless as indicators for chemical
- * Lamprophyre is a special igneous rock of mafic composition. It is porphyritic and occurs in shallow (hypabyssal) intrusions. The phenocrysts consist of mafic minerals (biotite, hornblende and pyroxene); the same mafic minerals are found in the fine groundmass along with feldspars. Some dikes of the Spanish Peaks region are lamprophyres.
Return to Rocky Mountain geology.
- Travis, R.B. 1955. Classification of rocks. Colorado School Mines, Quarterly
50/1, 98 p.
GO 547/ES 747 © J.S. Aber (2000).