Amphibolite is the name given to a
rockIn geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
consisting mainly of hornblende
amphiboleAmphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
, the use of the term being restricted, however, to
metamorphic rockMetamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s. The modern terminology for a holocrystalline plutonic
igneous rockIgneous rock is one of the three main rock types . Igneous rock is formed by magma being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks...
s composed primarily of hornblende amphibole is a
hornblenditeHornblendite is a plutonic rock consisting mainly of the amphibole hornblende. Hornblende rich ultramafic rocks are rare and when hornblende is the dominant mineral phase they are classified as hornblendites with qualifiers such as garnet hornblendite identifying a second abundant contained...
, which are usually crystal cumulates. Rocks with >90% amphibole which have a feldspar groundmass may be a
lamprophyreLamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...
.
Amphibolite is a grouping of rocks composed mainly of
amphiboleAmphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
(as
hornblendeHornblende is a complex inosilicate series of minerals . Hornblende is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an...
) and
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
s, with little or no
quartzQuartz is the second most abundant mineral in the Earth's continental crust . It is made up of a continuous framework of SiO
4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO
2.There are many different varieties of...
. It is typically dark-colored and heavy, 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.
Amphibolites need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals based entirely upon the chemistry of the protolith, certain 'dirty
marlMarl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
s' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing
dolomiteDolomite is the name of a sedimentary carbonate rock and a mineral, both composed of calcium magnesium carbonate CaMg
2 found in crystals....
and
sideriteSiderite is also the name of a type of iron meteorite.----Siderite is a mineral composed of iron carbonate FeCO
3. It takes its name from the Greek word sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
also readily yield amphibolites (tremolite-schists, grunerite-schists, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalts create
ortho-amphibolites and other chemically appropriate lithologies create
para-amphibolites.
Tremolite, while it is a metamorphic amphibole, is derived most usually from highly metamorphosed ultramafic rocks, and thus tremolite-talc schists are not generally considered as 'amphibolites', because it is abundantly clear that one could just as easily say 'ultramafic schist'.
Because
hornblendeHornblende is a complex inosilicate series of minerals . Hornblende is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an...
, as a mineral, is essentially a mineralogical 'garbage bin' and is stable across a very wide range of compositions and chemistries, as well as temperature and pressure conditions, it is suggested that the reader make use of the entries on
amphibole chemistryAmphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
.
Ortho-amphibolites vs. para-amphibolites
Metamorphic rocks composed primarily of
amphiboleAmphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
,
albiteAlbite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi
3O
8. It is a tectosilicate...
, with subordinate
epidoteEpidote is a calcium aluminium iron sorosilicate mineral, Ca
2Al
2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry...
,
zoisiteZoisite is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca2Al3O...
,
chloriteThe chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3...
,
quartzQuartz is the second most abundant mineral in the Earth's continental crust . It is made up of a continuous framework of SiO
4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO
2.There are many different varieties of...
, sphene, and accessory
leucoxeneLeucoxene is a fine granular alteration product of titanium minerals. It varies in color from yellow to brown.It is not a valid mineral species and consists mainly of rutile or anatase...
,
ilmeniteIlmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
and
magnetiteMagnetite is a ferrimagnetic mineral with chemical formula Fe
3O
4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name ferrous-ferric oxide...
which have a
protolithProtolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
of an igneous rock are known as
Orthoamphibolites.
Para-amphibolites will generally have the same equilibrium mineral assemblage as orthoamphibolites, with more biotite, and may include more quartz, albite, and depending on the protolith, more
calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 470°C, and vaterite is even less stable....
/
aragoniteAragonite is a carbonate mineral, one of the two common, naturally occurring polymorphs of calcium carbonate, CaCO3. The other polymorph is the mineral calcite. Aragonite's crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic system with...
and
wollastoniteWollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...
.
Often the easiest way to determine the true nature of an amphibolite is to inspect its field relationships; especially whether it is interfingered with other sediments, especially
greywackeGreywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly-sorted, angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally-immature sedimentary rock generally found...
s and other poorly sorted sediments. If the amphibolite appears to transgress apparent protolith bedding surfaces it is an ortho-amphibolite, as this suggests it was a
dykeA dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
. Picking a
sillIn geology, a sill is a tabular pluton that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
and thin metamorphosed
lavaLava is molten rock expelled by a volcano during eruption. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at temperatures from 700 °C to 1,200 °C...
flows may be more troublesome.
Thereafter, whole rock geochemistry will suitably identify ortho- from para-amphibolites.
The word
metabasalt was thus coined, largely to avoid the confusion between ortho-amphibolites and para-amphibolites. While not a true metamorphic rock name, as it infers an origin, it is a useful term.
Amphibolite Facies
Amphibolites define a particular set of temperature and pressure conditions known as the
amphibolite facies. However, caution must be applied here before embarking on metamorphic mapping based on amphibolites alone.
Firstly, for an (ortho)amphibolite to be classed as a metamorphic amphibolite, it must be certain that the amphibole in the rock is a
progradePrograde can refer to:*prograde or direct motion, in astronomy, is a type of motion of astronomical bodies* Prograde metamorphism, in geology, describes mineral changes in rocks under increasing pressure and/or temperature conditions...
metamorphic product, and not a retrograde metamorphic product. For instance,
actinoliteActinolite is an amphibole silicate mineral with the chemical formula .-Mineralogy:Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, , and iron-rich ferro-actinolite, . Mg and Fe ions can be freely exchanged in the crystal structure...
amphibole is a common product of retrograde metamorphism of basalts at (upper)
greenschistGreenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered mafic volcanic rock. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the rock. However, basalts may remain quite black if primary pyroxene...
facies conditions. Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacing
pyroxeneThe pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
is an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies.
Actinolite schists are often the result of hydrothermal alteration or
metasomatismMetasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
, and thus may not, necessarily, be a good indicator of metamorphic conditions when taken in isolation.
Secondly, the microstructure and crystal size of the rock must be appropriate. Amphibolite facies conditions are experienced at temperatures in excess of 500 °C and pressures in excess of 1.2 GPa, well within the ductile deformation field.
GneissGneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated and largely recrystallized but do not carry large...
ic texture may occur nearby, if not then
myloniteMylonite is a fine-grained, compact rock produced by dynamic crystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
zones,
foliationsFoliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the typical sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and gneiss...
and ductile behaviour, including stretching lineations may occur.
While it is not impossible to have remnant protolith mineralogy, this is rare. More common is to find
phenocrystthumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
s of pyroxene,
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
,
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
and even magmatic amphibole such as
pargasitePargasite is a complex inosilicate mineral with formula NaCa24AlO222....
rhombohedra, pseudomorphed by
hornblendeHornblende is a complex inosilicate series of minerals . Hornblende is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an...
amphibole. Original magmatic textures, especially crude magmatic layering in layered intrusions, is often preserved.
Amphibolite facies equilibrium mineral assemblages of various protolith rock types consist of:
- Basalt ortho-amphibolite; hornblende/actinolite +/- albite +/- biotite +/- quartz +/- accessories; often remnant greenschist
Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered mafic volcanic rock. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the rock. However, basalts may remain quite black if primary pyroxene...
facies assemblages including, notably, chloriteThe chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
- Sedimentary para-amphibolite; hornblende/actinolite +/- albite +/- biotite +/- quartz +/- garnet (calcite +/- wollastonite)
- High-magnesia basalts; as ortho-amphibolite, but may contain anthophyllite
Anthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
, a Mg-rich amphibole
- Ultramafic rocks; tremolite
Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite...
, asbestiform amphiboleAsbestos is a naturally occurring silicate mineral with long, thin fibrous crystals. The word asbestos is a borrowed Greek adjective meaning inextinguishable. The Greeks termed asbestos the miracle mineral because of its soft and pliant properties, as well as its ability to withstand...
, talcTalc is a mineral composed of hydrated magnesium silicate with the chemical formula H
2Mg
34 or Mg
3Si
4O
102. In loose form, it is the widely used substance known as talcum powder...
, pyroxeneThe pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, wollastoniteWollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...
, prograde metamorphic olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
(rarely)
- Pelites; quartz, orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. An alternate name is alkali feldspar...
+/- albite, +/- biotite +/- actinolite +/- garnet +/- stauroliteStaurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and a rather complex chemical formula: 2Al94O204...
+/- sillimaniteSillimanite also called Bucholzite is an alumino-silicate mineral with the chemical formula Al
2SiO
5. Sillimanite is named after the American chemist Benjamin Silliman ....
Amphibolite facies is usually a product of
Barrovian Facies Sequence or advanced
Abukuma Facies Sequence metamorphic trajectories. Amphibolite facies is a result of continuing burial and thermal heating after greenschist facies is exceeded.
Further burial and metamorphic compression (but little extra heat) will lead to granulite facies metamorphism; with more advanced heating the majority of rocks begin melting in excess of 650 to 700 °C in the presence of water. In dry rocks, however, additional heat (and burial) may result in eclogite facies conditions.
Uralite
Uralites are particular hydrothermally altered pyroxenites; during autogenic hydrothermal circulation their primary mineralogy of
pyroxeneThe pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
and
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, etc. has altered to
actinoliteActinolite is an amphibole silicate mineral with the chemical formula .-Mineralogy:Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, , and iron-rich ferro-actinolite, . Mg and Fe ions can be freely exchanged in the crystal structure...
and
saussuriteSaussurite is a mineral aggregate which is formed as a hydrothermal alteration product of plagioclase feldspar. It appears very similar to zoisite with a green or grayish-green color, it has been used as a substitute or simulant for jade....
(
albiteAlbite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi
3O
8. It is a tectosilicate...
+
epidoteEpidote is a calcium aluminium iron sorosilicate mineral, Ca
2Al
2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry...
). The texture is distinctive, the pyroxene altered to fuzzy, radially arranged actinolite pseudomorphically after pyroxene, and saussuritised plagioclase.
Epidiorite
The archaic term
epidiorite is sometimes used to refer to a metamorphosed ortho-amphibolite with a
protolithProtolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
of
dioriteDiorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
,
gabbroGabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
or other mafic intrusive rock. In epidiorite the original clinopyroxene (most often
augiteAugite is a single chain inosilicate mineral described chemically as SiO
3 or calcium magnesium iron silicate. The crystals are monoclinic and prismatic. Augite has two prominent prismatic cleavages, meeting at angles near 90°....
) has been replaced by the fibrous
amphiboleAmphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
uralite.
Uses
Amphibolite was a favourite material for the production of
adzeAn adze or adz is a tool used for smoothing rough-cut wood in hand woodworking. Generally, the user stands astride a board or log and swings the adze downwards towards their feet, chipping off pieces of wood, moving backwards as they go and leaving a relatively smooth surface behind...
s (shoe-last-celts) in the central European early
NeolithicThe Neolithic Age, Era, or Period, or New Stone Age, was a period in the development of human technology, beginning about 9500 BCE in the Middle East that is traditionally considered the last part of the Stone Age...
(Linearbandkeramic and Rössen cultures). In the VSG, it was used to produce bracelets as well.
Amphibolite is a common dimension stone used in construction, paving, facing of buildings, etcetera especially because of its attractive textures, dark colour, hardness and polishability and its ready availability.
See also
- Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey.On Earth, most...
and other igneous rockIgneous rock is one of the three main rock types . Igneous rock is formed by magma being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks...
types
- List of rock types
- Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Both mineralogical, chemical and crystallographic changes can occur during this process.Three types...
- Mineralogy of amphibole
Amphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
- Pyroxene
The pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...