![Sulfidic norite (platinum-palladium ore) Stillwater Mine MT](https://upload.wikimedia.org/wikipedia/commons/e/e5/Sulfidic_norite_%28platinum-palladium_ore%29_Stillwater_Mine_MT.jpg)
Cumulate rocks are
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
s formed by the accumulation of
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s from a
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
either by settling or floating. Cumulate rocks are named according to their
texture
Texture may refer to:
Science and technology
* Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object
* Texture (roads), road surface characteristics with waves shorter than road roughness
* Texture (c ...
; 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.
Formation
Cumulate rocks are the typical product of precipitation of solid crystals from a
fractionating magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
chamber. These accumulations typically occur on the floor of the magma chamber, although they are possible on the roofs if
anorthite
Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon.
Mineralo ...
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
is able to float free of a denser mafic melt.
Cumulates are typically found in
ultramafic intrusions, in the base of large ultramafic
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
tubes in
komatiite
Komatiite () is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite wa ...
and
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
rich
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
flows and also in some
granitic
A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quart ...
intrusions.
Terminology
Cumulates are named according to their dominant mineralogy and the percentage of crystals to their
groundmass
The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.
The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, ca ...
(Hall, 1996).
* ''Adcumulates'' are rocks containing ~100–93% accumulated magmatic crystals in a fine-grained groundmass.
* ''Mesocumulates'' are rocks with between 93 and 85% accumulated minerals in a groundmass.
* ''Orthocumulates'' are rocks containing between 85 and 75% accumulated minerals in groundmass.
Cumulate rocks are typically named according to the cumulate minerals in order of abundance, and then cumulate type (adcumulate, mesocumulate, orthocumulate), and then accessory or minor phases. For example:
* A layer with 50%
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
, 40%
pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
, 5%
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
and 5% groundmass (in essence a
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 che ...
) would be termed a ''plagioclase-pyroxene Adcumulate'' with accessory olivine.
* A rock consisting of 80% olivine, 5%
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
and 15% groundmass is an ''olivine mesocumulate'', (in essence a
peridotite
Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
).
Cumulate terminology is appropriate for use when describing cumulate rocks. In intrusions which have a uniform composition and minimal textural and mineralogical layering or visible crystal accumulations it is inappropriate to describe them according to this convention.
Geochemistry
![Layered Gabbro in Oman](https://upload.wikimedia.org/wikipedia/commons/a/a9/Layered_Gabbro_in_Oman.jpg)
Cumulate rocks, because they are
fractionates of a parental magma, should not be used to infer the composition of a magma from which they are formed. The chemistry of the cumulate itself can inform on the residual melt composition, but several factors need to be considered.
Chemistry
The chemistry of a cumulate can inform upon the temperature, pressure and chemistry of the melt from which it was formed, but the number of minerals which co-precipitate need to be known, as does the chemistry or mineral species of the precipitated minerals. This is best illustrated by an example;
As an example, a magma of
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
composition that is precipitating cumulates of
anorthite
Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon.
Mineralo ...
plagioclase plus
enstatite
Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO3) – ferrosilite (FeSiO3). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and m ...
pyroxene is changing composition by the removal of the elements which make up the precipitated minerals. In this example, the precipitation of anorthite (a
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
aluminium
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
) removes calcium from the melt, which becomes more depleted in calcium. Enstatite being precipitated from the melt will remove magnesium, so the melt becomes depleted in these elements. This tends to enrich the concentration of other elements - typically sodium, potassium, titanium and iron.
The rock that is made of the accumulated minerals will not have the same composition as the magma. In the above example, the cumulate of anorthite + enstatite is rich in calcium and magnesium, and the melt is depleted in calcium and magnesium. The cumulate rock is a plagioclase-pyroxene cumulate (a gabbro) and the melt is now more felsic and aluminous in composition (trending towards
andesite
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 predomi ...
compositions).
In the above example, the plagioclase and pyroxene need not be pure end-member compositions (anorthite-enstatite), and thus the effect of depletion of elements can be complex. The minerals can be precipitated in any ratio within the cumulate; such cumulates can be 90% plagioclase:10% enstatite, through to 10% plagiclase:90% enstatite and remain a gabbro. This also alters the chemistry of the cumulate, and the depletions of the residual melt.
It can be seen that the effect on the composition of the residual melt left behind by the formation of the cumulate is dependent on the composition of the minerals which precipitate, the number of minerals which co-precipitate at the same time, and the ratio of the minerals which co-precipitate. In nature, cumulates usually form from 2 mineral species, although ranges from 1 to 4 mineral species are known. Cumulate rocks which are formed from one mineral alone are often named after the mineral, for example a 99% magnetite cumulate is known as a magnetitite.
A specific example is the
Skaergaard intrusion
The Skaergaard intrusion is a layered igneous intrusion in the Kangerlussuaq area, East Greenland. It comprises various rock types including gabbro, ferro diorite, anorthosite and granophyre.
Discovered by Lawrence Wager
in 1931 during the Br ...
in
Greenland
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...
. At Skaergaard a 2500 m thick layered intrusion shows distinct chemical and mineralogic layering:
[Hall, Anthony, ''Igneous Petrology,'' 1987, Longman, p. 228-231, ]
* Plagioclase varies from An
66 near the base to An
30 near the top (An
xx = anorthite percentage)
** CaO 10.5% base to 5.1% top; Na
2O + K
2O 2.3% base to 5.9% top
* Olivine varies from Fo
57 near the base to Fo
0 at the top (Fo
xx =
forsterite
Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthor ...
percentage of the olivine)
** MgO 11.6% base to 1.7% top; FeO 9.3% base to 22.7% top
The Skaergaard is interpreted to have crystallised from a single confined magma chamber.
[
]
Residual melt chemistry
One way to infer the composition of the magma that created the cumulate rocks is to measure groundmass chemistry, but that chemistry is problematic or impossible to sample. Otherwise, complex calculations of averaging cumulate layers must be utilised, which is a complex process. Alternatively, the magma composition can be estimated by assuming certain conditions of magma chemistry and testing them on phase diagrams using measured mineral chemistry. These methods work fairly well for cumulates formed in volcanic
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates a ...
conditions (i.e.; komatiite
Komatiite () is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite wa ...
s). Investigating magma conditions of large layered ultramafic intrusions is more fraught with problems.
These methods have their drawbacks, primarily that they must all make certain assumptions which rarely hold true in nature. The foremost problem is that in large ultramafic intrusions, assimilation of wall rocks tends to alter the chemistry of the melt as time progresses, so measuring groundmass compositions may fall short. Mass balance calculations will show deviations from expected ranges, which may infer assimilation has occurred, but then further chemistry must be embarked upon to quantify these findings.
Secondly, large ultramafic intrusions are rarely sealed systems and may be subject to regular injections of fresh, primitive magma, or to loss of volume due to further upward migration of the magma (possibly to feed volcanic
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates a ...
vents or dyke swarms). In such cases, calculating magma chemistries may resolve nothing more than the presence of these two processes having affected the intrusion.
Though crystallized at high temperature, cumulate can remelt when later intruded by a sill or dyke of magma.[J. Leuthold, J. C. Lissenberg, B. O’Driscoll, O. Karakas; T. Falloon, D.N. Klimentyeva, P. Ulmer (2018); ''Partial melting of the lower oceanic crust at spreading ridges.'' Frontiers in Earth Sciences: Petrology: 6(15): 20p; https://dx.doi.org/10.3389/feart.2018.00015]
Economic importance
The economic importance of cumulate rocks is best represented by three classes of mineral deposits found in ultramafic to mafic layered intrusions.
* Silicate mineral cumulates
* Oxide mineral cumulates
* Sulfide melt cumulates
Silicate mineral cumulates
Silicate minerals are rarely sufficiently valuable to warrant extraction as ore. However, some 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 c ...
intrusions contain such pure anorthite
Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon.
Mineralo ...
concentrations that they are mined for feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
, for use in refractories
In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
, glassmaking, semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
s and other sundry uses (toothpaste
Toothpaste is a paste or gel dentifrice used with a toothbrush to clean and maintain the aesthetics and health of teeth. Toothpaste is used to promote oral hygiene: it is an abrasive that aids in removing dental plaque and food from the teeth, a ...
, cosmetics
Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protect ...
, etc.).
Oxide mineral cumulates
Oxide mineral cumulates form in layered intrusions when fractional crystallisation has progressed enough to allow the crystallisation of oxide minerals which are invariably a form of spinel
Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals.
Properties
S ...
. This can happen due to fractional enrichment of the melt in iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
, titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
or chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
.
These conditions are created by the high-temperature fractionation of highly magnesian olivine or pyroxene, which causes a relative iron-enrichment in the residual melt. When the iron content of the melt is sufficiently high, magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
or ilmenite
Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...
crystallise and, due to their high density, form cumulate rocks. Chromite
Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can s ...
is generally formed during pyroxene fractionation at low pressures, where chromium is rejected from the pyroxene crystals.
These oxide layers form laterally continuous deposits of rocks containing in excess of 50% oxide minerals. When oxide minerals exceed 90% of the bulk of the interval the rock may be classified according to the oxide mineral, for example ''magnetitite'', ''ilmenitite'' or ''chromitite''. Strictly speaking, these would be magnetite orthocumulate, ilmenite orthocumulate and chromite orthocumulates.
Sulfide mineral segregations
Sulfide mineral cumulates in layered intrusions are an important source of nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
, copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
, platinum group elements
The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered to ...
and cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
. Deposits of a mixed massive or mixed sulfide-silicate 'matrix' of pentlandite
Pentlandite is an iron–nickel sulfide with the chemical formula . Pentlandite has a narrow variation range in Ni:Fe but it is usually described as having a Ni:Fe of 1:1. It also contains minor cobalt, usually at low levels as a fraction of wei ...
, chalcopyrite
Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mo ...
, pyrrhotite
Pyrrhotite is an iron sulfide mineral with the formula Fe(1-x)S (x = 0 to 0.2). It is a nonstoichiometric variant of FeS, the mineral known as troilite.
Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it is ...
and/or pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic Luster (mineralogy), lust ...
are formed, occasionally with cobaltite
Cobaltite is a sulfide mineral composed of cobalt, arsenic, and sulfur, Co As S. Its impurities may contain up to 10% iron and variable amounts of nickel.Klein, Cornelus and Cornrlius Hurlbut, 1996, ''Manual of Mineralogy'', 20th ed., Wiley, p.2 ...
and platinum-tellurium sulfides. These deposits are formed by melt immiscibility between sulfide and silicate melts in a sulfur-saturated magma.
They are not strictly a cumulate rock, as the sulfide is not precipitated as a solid mineral, but rather as immiscible
Miscibility () is the property of two chemical substance, substances to mix in all mixing ratio, proportions (that is, to fully dissolution (chemistry), dissolve in each other at any concentration), forming a homogeneity and heterogeneity, homoge ...
sulfide liquid. However, they are formed by the same processes and accumulate due to their high specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...
, and can form laterally extensive sulfide 'reefs'. The sulfide minerals generally form an interstitial
An interstitial space or interstice is a space between structures or objects.
In particular, interstitial may refer to:
Biology
* Interstitial cell tumor
* Interstitial cell, any cell that lies between other cells
* Interstitial collagenase, ...
matrix to a silicate cumulate.
Sulfide mineral segregations can only be formed when a magma attains sulfur saturation. In mafic and ultramafic rocks they form economic nickel, copper and platinum group
The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered to ...
(PGE) deposits because these elements are chalcophile
The Goldschmidt classification,
developed by Victor Goldschmidt (1888–1947), is a geochemical classification which groups the chemical elements within the Earth according to their preferred host phases into lithophile (Rock (geology), rock-lovi ...
and are strongly partitioned into the sulfide melt. In rare cases, felsic
In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...
rocks become sulfur saturated and form sulfide segregations. In this case, the typical result is a disseminated form of sulfide mineral, usually a mixture of pyrrhotite
Pyrrhotite is an iron sulfide mineral with the formula Fe(1-x)S (x = 0 to 0.2). It is a nonstoichiometric variant of FeS, the mineral known as troilite.
Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it is ...
, pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic Luster (mineralogy), lust ...
and chalcopyrite
Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mo ...
, forming copper mineralisation. It is very rare but not unknown to see cumulate sulfide rocks in granitic intrusions.
See also
* Igneous differentiation
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of (u ...
* Igneous rocks
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
* Komatiite
Komatiite () is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite wa ...
* 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 to over and several hundred metres to ...
* List of rock textures
This page is intended to be a list of rock textural and morphological terms.
A
* Adcumulate
* Agglomeritic
* Adamantine a type of lustre
* Amygdaloidal
* Anhedral
* Antitaxial veins
* Aphanitic
* Aplitic; aplite
* Augen textured gnei ...
* Ultramafic
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 ...
rocks
References
Sources
* Blatt, Harvey and Robert J. Tracy, 1996, ''Petrology: Igneous, Sedimentary and Metamorphic,'' 2nd ed., pp. 123–132 & 194–197, Freeman, {{ISBN, 0-7167-2438-3
* Ballhaus, C.G. & Glikson, A.Y., 1995, ''Petrology of layered mafic-ultramafic intrusions of the Giles Complex, western Musgrave Block, central Australia''. AGSO Journal, 16/1&2: 69–90.
Plutonic rocks
Igneous petrology