GO 324A Rocks and Minerals
Susan Ward Aber

www.emporia.edu/earthsci/amber/go324/igneous.htm

Emporia State University
Emporia, Kansas USA
Earth Science Department

Introduction to Igneous Rock

Introduction    Texture    Structure
Color    Composition    References and Links

Introductions

Igneous rock has played a major role in the crustal evolution of Earth and other planetary bodies, and it is the precursor of sedimentary and metamorphic rock (Raymond, 1995, p. 7). Igneous rock solidifies from molten material or emanating volatile constituents, hot aqueous or gaseous solutions. This molten material or magma may contain mineral crystals, rock fragments, or gasses, along with the melt. Magma usually originates at great depths and moves up toward the Earth's surface.

Igneous rock has a variety of origins, including volcanic or extrusive, hypabyssal, and plutonic or intrusive. If the magma cools and solidifies at depth, then the igneous rock is termed plutonic or intrusive. An example of intrusive igneous rock is granite, which is the primary constituent of continent crust. If the magma cools after traveling near to or onto the surface, then it is termed volcanic or extrusive. An example of extrusive rock is basalt, which is the primary constituent of oceanic plates. Hypabyssal igneous rock crystallizes at medium depths and these rock types occur at boundaries of the continents and oceanic crustal rocks. An example of hypabyssal rock is andesite. These intermediate rocks may form in geologic structures such as sills, dikes, and batholiths.

Ah, an opportunity for you! If you are enrolled in this course, visit Wikipedia on Large Igneous Province, en.wikipedia.org/wiki/Large_igneous_province. Write a summary paragraph of what is meant by large igneous province. In addition, choose one of the following links and write a one page summary of the topic and its igneous geology significance. Choose from: Columbia River Basalt Group (http://en.wikipedia.org/wiki/Columbia_River_Basalt_Group), Geology of Venus (http://en.wikipedia.org/wiki/Geology_of_Venus), OR Ethiopian Higlands (http://en.wikipedia.org/wiki/Ethiopian_Highlands). Send this paper by October 16, to saber@emporia.edu for two extra credit points on the second exam. Place GO 324 extra credit points for exam 2 in the subject line of the email!

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Igneous Rock Textures

Igneous is from the Latin ignis meaning fire, which is characteristic of this rock type mode of formation. Differing textures result depending on conditions at the time of formation and crystallization. If the melt solidifies at depth, then large mineral grains, termed phenocrysts, will result. If the melt solidifies on or near to the surface, then small phenocrysts will result. What would happen if the melt began to crystallize at depth, but was moved near to or onto the surface? This change in the environment of formation will result in a rock with two different textures that is referred to as a porphyry. Common igneous rock textures are defined below.

Glassy - a smooth, glassy-looking texture in which no mineral phenocrysts occur; the resulting rock is actually amorphous or without a crystalline form or structure.
Vesicular - vesicles or holes are obvious creating a sponge-like appearance to the volcanic rock. This term is also applied to igneous rock without a crystalline mineral make-up, or amorphous material such as pumice or scoria.
Aphanitic - a dense texture where mineral grains are not visible to the unaided eye.
Phaneritic - an even-granular texture with mineral phenocrysts visible to the unaided eye.
Aphanitic Porphyry - visible phenocrysts embedded in the rock with an invisible or aphanitic groundmass or background.
Phaneritic Porphyry - an uneven-granular texture where larger visible phenocrysts are embedded in a rock with smaller, but visible, phenocrysts making up the groundmass or background.
Pegmatitic - very coarse, uneven-granular texture with phenocrysts larger than 1 cm in size.
Fragmental or Tuff - pyroclastic fragments, solidified ash particles, crystal and glass debris, cemented together.

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Igneous Rock Structures

Structural features in igneous rock may aid in identification as they may reveal rock mode of formation. Vesicular is a structural term that refers to a rock with numerous vesicles or cavities. This structure is formed when the rock quench cools and the former gas filled cavities simply solidify. Igneous rock displaying this structure include scoria and pumice. Spherulitic and amygdaloidal are two structures related to vesicular. When the vesicles are rounded or spherical, they may be filled sometime after the rock's initial formation with needlelike crystals (usually quartz or feldspars) and this is termed spherulitic. If the vesicles are almond-shaped, elliptical with pointed ends, an amygdaloidal structure results; these cavities are filled with secondary mineralization, usually quartz or zeolite minerals. When the mineral grains are roughly spherical the structure is termed orbicular (usually in phaneritic, felsic rocks). Flow banding is an igneous rock structure produced when continued flowing lava solidifies, and alternating layers result (Chesterman, 1978, p. 599). Xenoliths are stranger rock that is when older surrounding rock is broken and included in a magma forming an unrelated inclusion in the rock (Chesterman, 1978, p. 599-600).

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Igneous Rock Color

Igneous rocks are named and classified based on texture and mineral make-up. Color provides a measure for estimating the general mineral content. The color index for igneous rock is simply a measure of the proportion of dark and light minerals components (Chesterman, 1978, p. 600). Some of the common light-colored minerals include quartz, micas, and potassium- and sodium-rich feldspars. Common dark-colored minerals include hornblende, augite, biotite, olivine, and calcium-rich feldspars. The light-colored minerals are rich in low-temperature silicates with appreciable amounts of potassium, sodium, and aluminum such as orthoclase and muscovite. Dark-colored minerals are rich in high-temperature silicates with appreciable amounts of magnesium, iron, and calcium such as olivine and anorthite.

The color index is divided into three categories: leucocratic, mesocratic, and melanocratic. The lightest color term, leucocratic, contains two-thirds light minerals, one-third dark. Reds or pinks are considered a light mineral color; for example, orthoclase in rhyolite. Mesocratic include rocks of an intermediate status, one-half dark, one-half light; for example, albite and hornblende in diorite. Melanocratic is the darkest color term with two-thirds dark, one-third light colored minerals; for example olivine in basalt and peridotite. For the most part, greens are considered a dark mineral color.

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Igneous Rock Composition

Igneous rock with a carbonate mineral assemblage, primarily calcite, is not common, but may occur in carbonatite. Most igneous rock has a silicate composition and mineralogy and the following compositional terms are applied.

Felsic - a composition high in potassium and sodium rich silicates with a silica content of about 70%; these igneous rocks are mainly in continental crust.
Intermediate - variable between felsic and mafic.
Mafic - a composition high in calcium, magnesium, and iron rich silicates with a silica content of about 50%; these igneous rocks are mainly in oceanic crust.
Ultramafic - a composition very high in magnesium and iron with a silica content of less than 50%; these igneous rocks are found in the mantle.

Some of the common minerals found in igneous rock include feldspars, quartz, micas, hornblende, augite, and olivine. A more detailed mineral assemblage is given below.

Minerals associated with slow cooling molten magma include:
silicates: olivine, pyroxenes (augite), amphiboles (hornblende), micas, feldspars, quartz
phosphates: apatite
sulfides: pyrite, pyrrhotite
oxides: magnetite, chromite
native elements: diamond, platinum

Minerals associated with molten magma and vapors forming pegmatites include:
silicates: beryl, topaz, tourmalines, micas, spodumene (kunzite/hiddenite)
phosphates: apatite
oxides: corundum, cassiterite, columbite, uraninite
halides: cryolite
sulfides: molybdenite, arsenopyrite

Minerals associated with hydrothermal veins, forming in cracks and fissures as a result of precipitation from solutions include:
silicates: feldspars, quartz, epidote
sulfates: barite
carbonates: calcite, rhodochrosite, ankerite
oxides: hematite, ilmenite, rutile
halides: fluorite
sulfides: galena, sphalerite, chalcopyrite, pyrite

Minerals associated with igneous rock formed from vapors or sublimation of volcanic fumes include:
silicates: topaz, zeolites
oxides: hematite
sulfides: pyrite, cinnabar, realgar, stibnite
native elements: sulfur, arsenic, mercury.

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Recommended References and Notable Links

To the beginning!

Petrology Introduction
www.emporia.edu/earthsci/amber/go324/intro.htm 		Minerals
www.emporia.edu/earthsci/amber/go324/mineral.htm
Rocks
www.emporia.edu/earthsci/amber/go324/rock.htm 		Igneous
www.emporia.edu/earthsci/amber/go324/igneous.htm
Sedimentary Rock
www.emporia.edu/earthsci/amber/go324/sediment.htm 		Metamorphic Rock
www.emporia.edu/earthsci/amber/go324/metamor.htm
Course Field Trip
www.emporia.edu/earthsci/amber/go240/field_trip.htm 		Course Syllabus
www.emporia.edu/earthsci/amber/go324/syllabus.htm

This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. The curriculum is © by the author, 2001-2009. Last update 18 July 2009. For more information contact the course instructor, Dr. S. W. Aber, e-mail: saber@emporia.edu

To understand copyright, visit and lcweb.loc.gov/copyright/. All rights reserved. Susan Ward Aber.