Mafic

Last updated

Basalt BasaltUSGOV.jpg
Basalt

A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar. Mafic materials can also be described as ferromagnesian.

Contents

History

The term mafic is a portmanteau of "magnesium" and "ferric" and was coined by Charles Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry Stephens Washington in 1912. Cross' group had previously divided the major rock-forming minerals found in igneous rocks into salic minerals, such as quartz, feldspars, or feldspathoids, and femic minerals, such as olivine and pyroxene. However, micas and aluminium-rich amphiboles were excluded, while some calcium minerals containing little iron or magnesium, such as wollastonite or apatite, were included in the femic minerals. [1] Cross and his coinvestigators later clarified that micas and aluminium amphiboles belonged to a separate category of alferric minerals. They then introduced the term mafic for ferromagnesian minerals of all types, in preference to the term femag coined by A. Johannsen in 1911, whose sound they disliked. [2] [3]

Minerals

The term mafic is still widely used for dark-colored ferromagnesian minerals. [3] Modern classification schemes, such as the International Union of Geological Sciences (IUGS) classification of igneous rocks, include some light-colored ferromagnesian minerals, such as melilite, in the mafic mineral fraction. [4] Accessory minerals, such as zircon or apatite, may also be included in the mafic mineral fraction for purposes of precise classification. [5]

Rocks

When applied to rocks, the term mafic is used primarily as a field term to describe dark-colored igneous rocks. [5] The term is not used as a rock classification in the IUGS classification scheme. [4] Mafic rocks are sometimes more precisely defined as igneous rocks with a high proportion of pyroxene and olivine, so that their color index (the volume fraction of dark mafic minerals) is between 50 and 90. [6] Most mafic volcanic rocks are more precisely classified as basalts. [7]

Chemically, mafic rocks are sometimes defined as rocks with a silica content between 45 and 55 wt%, corresponding to the silica content of basalt in the TAS classification. [8] Such rocks are enriched in iron, magnesium and calcium and typically dark in color. In contrast, the felsic rocks are typically light in color and enriched in aluminium and silicon along with potassium and sodium. The mafic rocks also typically have a higher density than felsic rocks. The term roughly corresponds to the older basic rock class. [9]

Mafic lava, before cooling, has a low viscosity, in comparison with felsic lava, due to the lower silica content in mafic magma. Water and other volatiles can more easily and gradually escape from mafic lava. As a result, eruptions of volcanoes made of mafic lavas are less explosively violent than felsic-lava eruptions. [10]

See also

Related Research Articles

In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava, and have low temperatures to keep the felsic minerals molten.

<span class="mw-page-title-main">Gabbro</span> Coarse-grained mafic intrusive rock

Gabbro is a phaneritic, mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term gabbro may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite.

<span class="mw-page-title-main">Amphibole</span> Group of inosilicate minerals

Amphibole is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain SiO
4
tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is Amp. Amphiboles can be green, black, colorless, white, yellow, blue, or brown. The International Mineralogical Association currently classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups.

<span class="mw-page-title-main">Andesite</span> Type of volcanic rock

Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

<span class="mw-page-title-main">Amphibolite</span> Metamorphic rock type

Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.

<span class="mw-page-title-main">Volcanic rock</span> Rock formed from lava erupted from a volcano

Volcanic rocks are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "pyroclastics," and these are also technically sedimentary rocks.

<span class="mw-page-title-main">Nepheline syenite</span> Holocrystalline plutonic rock

Nepheline syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive equivalent.

<span class="mw-page-title-main">Lamprophyre</span> Ultrapotassic igneous rocks

Lamprophyres are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide, and high nickel and chromium.

<span class="mw-page-title-main">Essexite</span> Igneous rock type

Essexite, also called nepheline monzogabbro, is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, Massachusetts, in the United States.

<span class="mw-page-title-main">Komatiite</span> Magnesium-rich igneous rock

Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% magnesium oxide (MgO). It is classified as a 'picritic rock'. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.

<span class="mw-page-title-main">Cumulate rock</span> Igneous rocks formed by the accumulation of crystals from a magma either by settling or floating.

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Cumulates can be deposited on top of other older cumulates of different composition and colour, typically giving the cumulate rock a layered or banded appearance.

The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma into a more evolved, silica rich end member. Rock types of the tholeiitic magma series include tholeiitic basalt, ferro-basalt, tholeiitic basaltic andesite, tholeiitic andesite, dacite and rhyolite. The variety of basalt in the series was originally called tholeiite but the International Union of Geological Sciences recommends that tholeiitic basalt be used in preference to that term.

Normative mineralogy is a calculation of the composition of a rock sample that estimates the idealised mineralogy of a rock based on a quantitative chemical analysis according to the principles of geochemistry.

<span class="mw-page-title-main">Silicic</span> Adjective to describe magma or igneous rock rich in silica

Silicic is an adjective to describe magma or igneous rock rich in silica. The amount of silica that constitutes a silicic rock is usually defined as at least 63 percent. Granite and rhyolite are the most common silicic rocks.

The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it fractionally crystallizes to become a felsic magma, which is low in magnesium and iron and produces rhyolite or granite. Calc-alkaline rocks are rich in alkaline earths and alkali metals and make up a major part of the crust of the continents.

Talc carbonates are a suite of rock and mineral compositions found in metamorphosed ultramafic rocks.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

<span class="mw-page-title-main">Igneous rock</span> Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.

<span class="mw-page-title-main">Color index (geology)</span> Ratio of dark colored to light colored minerals in a rock

Color index, as a geological term, is a measure of the ratio between generally dark mafic minerals and generally light felsic minerals in an igneous rock. The color index of an igneous rock is the volume percentage of mafic minerals in the rock, excluding minerals generally regarded as "colorless" such as apatite, muscovite, primary carbonates and similar minerals. Rocks can be sorted into classes by several systems based on their color index, including into leucocratic and melanocratic rocks, or into (mineralogically) felsic and mafic rocks.

References

  1. Cross, Whitman; Iddings, Joseph P.; Pirsson, Louis V.; Washington, Henry S. (September 1902). "A Quantitative Chemico-Mineralogical Classification and Nomenclature of Igneous Rocks". The Journal of Geology. 10 (6): 555–690. Bibcode:1902JG.....10..555C. doi:10.1086/621030. S2CID   140683261.
  2. Cross, Whitman; Iddings, J. P.; Pirsson, L. V.; Washington, H. S. (September 1912). "Modifications of the "Quantitative System of Classification of Igneous Rocks"". The Journal of Geology. 20 (6): 550–561. Bibcode:1912JG.....20..550C. doi:10.1086/621996. S2CID   129515388.
  3. 1 2 Jackson, Julia A., ed. (1997). "mafic". Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN   0922152349.
  4. 1 2 Le Bas, M. J.; Streckeisen, A. L. (1991). "The IUGS systematics of igneous rocks". Journal of the Geological Society. 148 (5): 825–833. Bibcode:1991JGSoc.148..825L. CiteSeerX   10.1.1.692.4446 . doi:10.1144/gsjgs.148.5.0825. S2CID   28548230.
  5. 1 2 Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 137. ISBN   9780521880060.
  6. Allaby, Michael (2013). "mafic". A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN   9780199653065.
  7. Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. p. 23. ISBN   978-3-540-43650-8.
  8. Nesse, William D. (2000). Introduction to mineralogy . New York: Oxford University Press. ISBN   9780195106916.
  9. Islam, M. R.; Hossain, M. E.; Islam, A. O. (2018). Hydrocarbons in Basement Formations. John Wiley & Sons. p. 80. ISBN   978-1-119-29453-5.
  10. "Volcanoes". Columbia University. Retrieved 18 June 2019.