Mafic

Mafic is an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron, and is thus a portmanteau of magnesium and ferric.[1] 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.

Chemically, mafic 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.

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. Most mafic-lava volcanoes are shield volcanoes, like those in Hawaii.

Rock texture Name of mafic rock
Pegmatitic Gabbro pegmatite
Coarse grained (phaneritic) Gabbro
Coarse grained and porphyritic Porphyritic gabbro
Medium grained Diabase or Dolerite, Microgabbro
Fine grained (aphanitic) Basalt
Fine grained and porphyritic Porphyritic basalt
Pyroclastic Basalt tuff or breccia
Vesicular Vesicular basalt
Amygdaloidal Amygdaloidal basalt
Many small vesicles Scoria
Glassy texture Tachylyte, sideromelane, palagonite
BasaltUSGOV
Basalt

See also

References

  1. ^ Schlumberger: Oilfield Glossary
Andesite

For the extinct cephalopod genus, see Andesites.

Andesite ( or ) is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and rhyolite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic. Along with basalts they are a major component of the Martian crust. The name andesite is derived from the Andes mountain range.

Anorthosite

Anorthosite ( ) is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Anorthosites are of enormous geologic interest, because it is still not fully understood how they form. Most models involve separating plagioclase crystals based on their density. Plagioclase crystals are usually less dense than magma; so, as plagioclase crystallizes in a magma chamber, the plagioclase crystals float to the top, concentrating there.Anorthosite on Earth can be divided into five types:

Archean-age anorthosites

Proterozoic anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth

Layers within Layered Intrusions (e.g., Bushveld and Stillwater intrusions)

Mid-ocean ridge and transform fault anorthosites

Anorthosite xenoliths in other rocks (often granites, kimberlites, or basalts)Of these, the first two are the most common. These two types have different modes of occurrence, appear to be restricted to different periods in Earth's history, and are thought to have had different origins.Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.

Basanite

Basanite ( ) is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture.

The mineral assembly is usually abundant feldspathoids (nepheline or leucite), plagioclase, and augite, together with olivine and lesser iron-titanium oxides such as ilmenite and magnetite-ulvospinel; minor alkali feldspar may be present, as illustrated by the position of the field for basanite in the QAPF diagram. Clinopyroxene (augite) and olivine are common as phenocrysts and in the matrix. The augite contains significantly greater titanium, aluminium and sodium than that in typical tholeiitic basalt. Quartz is absent, as are orthopyroxene and pigeonite. Chemically, basanites are mafic. They are low in silica (42 to 45% SiO2) and high in alkalis (3 to 5.5% Na2O and K2O) compared to basalt, which typically contains more SiO2, as evident on the diagram used for TAS classification. Nephelinite is yet richer in Na2O plus K2O compared to SiO2.

Basanites occur both on continents and on ocean islands. Together with basalts, they are produced by hotspot volcanism, for example in the Hawaiian Islands, the Comoros Islands and the Canary Islands.

Diabase

Diabase ( ) or dolerite or microgabbro is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic glass). Diabase is the preferred name in North America, yet dolerite is the preferred name in most of the rest of the world, where sometimes the name diabase is applied to altered dolerites and basalts. Many petrologists prefer the name microgabbro to avoid this confusion.

Diorite

Diorite ( ) is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite. Diorite is usually grey to dark-grey in colour, but it can also be black or bluish-grey, and frequently has a greenish cast. It is distinguished from gabbro on the basis of the composition of the plagioclase species; the plagioclase in diorite is richer in sodium and poorer in calcium. Diorite may contain small amounts of quartz, microcline, and olivine. Zircon, apatite, titanite, magnetite, ilmenite, and sulfides occur as accessory minerals. Minor amounts of muscovite may also be present. Varieties deficient in hornblende and other dark minerals are called leucodiorite. When olivine and more iron-rich augite are present, the rock grades into ferrodiorite, which is transitional to gabbro. The presence of significant quartz makes the rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassium feldspar) is present at greater than 10 percent, the rock type grades into monzodiorite or granodiorite. A dioritic rock containing feldspathoid mineral/s and no quartz is termed foid-bearing diorite or foid diorite according to content.

Diorite has a phaneritic, often speckled, texture of coarse grain size and is occasionally porphyritic.

Orbicular diorite shows alternating concentric growth bands of plagioclase and amphibole surrounding a nucleus, within a diorite porphyry matrix.

Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from the partial melting of a mafic rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building, such as in the Andes Mountains, as large batholiths. The extrusive volcanic equivalent rock type is andesite.

Felsic

In geology, felsic refers to 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.

They are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).

Gabbro

Gabbro ( ) is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich 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".

Geology of Ascension Island

The geology of Ascension Island is the geologically young, exposed part of a large volcano, 80 kilometers west of the Mid-Atlantic Ridge. The island formed within the last six to seven million years and is primarily mafic rock with some felsic rock.

Grayson Glades Natural Area Preserve

Grayson Glades Natural Area Preserve is a 31-acre (13 ha) Natural Area Preserve located in Grayson County, Virginia. Its centerpiece is an extremely rare wetland type known as a "mafic fen", which are situated upon soils rich in magnesium and fed by springs. The site is at the headwaters of a small stream system supporting additional mafic fens.Among the rare species found on the property are tuberous grass-pink (Calopogon tuberosus), ten-angled pipewort (Eriocaulon decangulare var. decangulare), large-leaved grass-of-parnassus (Parnassia grandifolia), queen-of-the-prairie (Filipendula rubra), sticky false-asphodel (Triantha glutinosa) and Canada burnet (Sanguisorba canadensis).The preserve is owned and maintained by the Virginia Department of Conservation and Recreation. It does not include improvements for public access, and visitors must make arrangements with a state-employed land steward prior to visiting.

Hawaiite

Hawaiite is an olivine basalt with a composition between alkali basalt and mugearite. It was first used as a name for some lavas found on the island of Hawaii.

In gemology, hawaiite is a colloquial term for Hawaii-originated peridot, which is a gem-quality olivine mineral. It occurs during the later stages of volcanic eruptions which happens to be when the alkaline metals are most present.

Labradorite

Labradorite ((Ca, Na)(Al, Si)4O8), a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573 and twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which are nearly at right angles and are more obvious, being of good to perfect quality. (The third direction is poor.) It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.

Layered intrusion

A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around 100 km2 (39 sq mi) to over 50,000 km2 (19,000 sq mi) and several hundred metres to over one kilometre (3,300 ft) in thickness. While most layered intrusions are Archean to Proterozoic in age (for example, the Paleoproterozoic Bushveld complex), they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland or the Rum layered intrusion in Scotland. Although most are ultramafic to mafic in composition, the Ilimaussaq intrusive complex of Greenland is an alkalic intrusion.

Layered intrusions are typically found in ancient cratons and are rare but worldwide in distribution. The intrusive complexes exhibit evidence of fractional crystallization and crystal segregation by settling or floating of minerals from a melt.

Ideally, the stratigraphic sequence of an ultramafic-mafic intrusive complex consists of ultramafic peridotites and pyroxenites with associated chromitite layers toward the base with more mafic norites, gabbros and anorthosites in the upper layers. Some include diorite, and granophyre near the top of the bodies. Orebodies of platinum group elements, chromite, magnetite, and ilmenite are often associated with these rare intrusions.

List of rock types

The following is a list of rock types recognized by geologists. There is no agreed number of specific types of rocks. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe rock types. Additionally, different classification systems exist for each major type of rock. There are three major types of rock: igneous, sedimentary, and metamorphic. They are all identified by their texture, streak, and location, among other factors.

Nephelinite

Nephelinite is a fine-grained or aphanitic igneous rock made up almost entirely of nepheline and clinopyroxene (variety augite). If olivine is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemble basalt in hand specimen. However, basalt consists mostly of clinopyroxene (augite) and calcic plagioclase.

Basalt, alkali basalt, basanite, tephritic nephelinite, and nephelinite differ partly in the relative proportions of plagioclase and nepheline. Alkali basalt may contain minor nepheline and does contain nepheline in its CIPW normative mineralogy. A critical ratio in the classification of these rocks is the ratio nepheline/(nepheline plus plagioclase). Basanite has a value of this ratio between 0.1 and 0.6 and also contains more than 10% olivine. Tephritic nephelinite has a value between 0.6 and 0.9. Nephelinite has a value greater than 0.9. Le Maitre (2002) defines and discusses these and other criteria in the classification of igneous rocks.

Nephelinite is an example of a silica-undersaturated igneous rock. The degree of silica saturation can be evaluated with normative mineralogy calculated from chemical analyses, or with actual mineralogy for completely crystallized igneous rocks with equilibrated assemblages. Silica-oversaturated rocks contain quartz (or another silica polymorph). Silica-undersaturated mafic igneous rocks contain magnesian olivine but not magnesian orthopyroxene, and/or a feldspathoid. Silica-saturated igneous rocks fall in between these two classes.

Silica-undersaturated, mafic igneous rocks are much less abundant than silica-saturated and oversaturated basalts. Genesis of the less common mafic rocks such as nephelinite is usually ascribed to more than one of the following three causes:

relatively high pressure of melting;

relatively low degree of fractional melting in a mantle source;

relatively high dissolved carbon dioxide in the melt.Nephelinites and similar rocks typically contain relatively high concentrations of elements such as the light rare earths, as consistent with a low degree of melting of mantle peridotite at depths sufficient to stabilize garnet. Nephelinites are also associated with carbonatite in some occurrences, consistent with source rocks relatively rich in carbon dioxide.

Nephelinite is found on ocean islands such as Oahu, although the rock type is very rare in the Hawaiian islands. It is found in a variety of continental settings. An example is the Hamada nephelinite lava flow in southwest Japan which occurred in the late Miocene age. Nephelinite is also associated with the highly alkalic volcanism of the Ol Doinyo Lengai volcanic field in Tanzania. Nyiragongo, another African volcano known for its semipermanent lava lake activity, erupts lava made of melilite nephelinite. The unusual chemical makeup of this igneous rock may be a factor in the unusual fluidity of its lavas.

Olivine nephelinite flows also occur in the Wells Gray-Clearwater volcanic field in east-central British Columbia and at Volcano Mountain in central Yukon Territory. Melilite olivine nephelinite intrusives of Cretaceous age are found in the area around Uvalde, Texas.

Norite

Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite, orthopyroxene, and olivine. The name norite is derived from Norge, the Norwegian name for Norway.Norite also known as orthopyroxene gabbro. Norite may be essentially indistinguishable from gabbro without thin section study under the petrographic microscope. The principal difference between norite and gabbro is the type of pyroxene of which it is composed. Norite is predominantly composed of orthopyroxenes, largely high magnesian enstatite or an iron bearing intermediate hypersthene. The principal pyroxenes in gabbro are clinopyroxenes, generally medially iron-rich augites.Norite occurs with gabbro and other mafic to ultramafic rocks in layered intrusions which are often associated with platinum orebodies such as in the Bushveld Igneous Complex in South Africa, the Skaergaard igneous complex of Greenland, and the Stillwater igneous complex in Montana. Norite is also the basal igneous rock of the Sudbury Basin complex in Ontario, which is the site of a comet impact and the world's second-largest nickel mining region.

Norite is a common rock type of the Apollo samples. On a smaller scale, norite can be found in small localized intrusions such as the Gombak norite in Bukit Batok, Singapore. It is also plentiful in the Egersund intrusion area of southwestern Norway, with titanium deposits to the east.

Phonotephrite

Phonotephrite is a strongly alkaline volcanic rock with a composition between phonolite and tephrite. This unusual igneous rock contains 7 to 12% alkali content and 45 to 53% silica content (see TAS diagram). It can be described as a mafic phonolite or a potassic tephrite. Phonotephrite lava flows and volcanic cones have been identified in Antarctica, North America and Africa.

Tephriphonolite

Tephriphonolite is a mafic to intermediate extrusive igneous rock in composition between phonotephrite and phonolite. It contains 9 to 14% alkali content and 48 to 57% silica content (see TAS diagram).Tephriphonolite has been found, for example, at Colli Albani volcano, Italy and in the Asuncion Rift, Paraguay.

Ultramafic rock

Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.

Volcanogenic massive sulfide ore deposit

Volcanogenic massive sulfide ore deposits, also known as VMS ore deposits, are a type of metal sulfide ore deposit, mainly copper-zinc which are associated with and created by volcanic-associated hydrothermal events in submarine environments.These deposits are also sometimes called volcanic-hosted massive sulfide (VHMS) deposits. The density generally is 4500 kg/m3. They are predominantly stratiform accumulations of sulfide minerals that precipitate from hydrothermal fluids on or below the seafloor in a wide range of ancient and modern geological settings. In modern oceans they are synonymous with sulfurous plumes called black smokers.

They occur within environments dominated by volcanic or volcanic derived (e.g., volcano-sedimentary) rocks, and the deposits are coeval and coincident with the formation of said volcanic rocks. As a class, they represent a significant source of the world's copper, zinc, lead, gold and silver ores, with cobalt, tin, barium, sulfur, selenium, manganese, cadmium, indium, bismuth, tellurium, gallium and germanium as co- or by-products.

Volcanogenic massive sulfide deposits are forming today on the seafloor around undersea volcanoes along many mid ocean ridges, and within back-arc basins and forearc rifts. Mineral exploration companies are exploring for seafloor massive sulfide deposits; however, most exploration is concentrated in the search for land-based equivalents of these deposits.

The close association with volcanic rocks and eruptive centers sets VMS deposits apart from similar ore deposit types which share similar source, transport and trap processes. Volcanogenic massive sulfide deposits are distinctive in that ore deposits are formed in close temporal association with submarine volcanism and are formed by hydrothermal circulation and exhalation of sulfides which are independent of sedimentary processes, which sets VMS deposits apart from sedimentary exhalative (SEDEX) deposits.

There is a subclass of VMS deposits, the volcanic- and sediment-hosted massive sulfide (VSHMS) deposits, that do share characteristics that are hybrid between the VMS and SEDEX deposits. Notable examples of this class include the deposits of the Bathurst Camp, New Brunswick, Canada (e.g., Brunswick #12); the deposits of the Iberian Pyrite Belt, Portugal and Spain, and the Wolverine deposit, Yukon, Canada.

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