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Amphibolite () is a
metamorphic rock , a type of metamorphic rock Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock, in a process called metamorphism upright=1.35, Schematic representation of a metamorphic reaction. Abbreviat ...

metamorphic rock
that contains
amphibole Amphibole () is a group of inosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust 350px, Plates in t ...

amphibole
, especially
hornblende Hornblende is a complex inosilicate series of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

hornblende
and
actinolite Actinolite is an amphibole Amphibole () is a group of Silicate minerals, inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedron, tetrahedra, linked at the vertices and generally containing ions of ir ...

actinolite
, as well as plagioclase. Amphibolite is a grouping of rocks composed mainly of
amphibole Amphibole () is a group of inosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust 350px, Plates in t ...

amphibole
and
plagioclase . (unknown scale) Plagioclase is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagiocla ...
feldspar, with little or no
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
. It is typically dark-colored and dense, with a weakly foliated or
schistose Image:Schist detail.jpg, Schist specimen showing the characteristic "scaly" schistose texture, caused by platy micas Schist (pronounced ) is a medium-Metamorphism#Metamorphic grades, grade metamorphic rock formed from mudstone or shale. Schist has ...
(flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance. Amphibolite need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals entirely based upon the chemistry of the protolith, certain 'dirty marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing Dolomite (mineral), dolomite and siderite also readily yield amphibolite (tremolite-schist, grunerite-schist, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalt creates ''ortho-amphibolite'' and other chemically appropriate lithologies create ''para-amphibolite''. Tremolite, while it is a metamorphic amphibole, is derived most usually from highly metamorphosed ultramafic rocks, and thus tremolite-talc schist is not generally considered as 'amphibolite'. A holocrystalline plutonic igneous rock composed primarily of hornblende amphibole is called a hornblendite, which is usually a crystal cumulate rock. Igneous rocks with >90% amphiboles, which have a feldspar groundmass, may be a lamprophyre.


Ortho-amphibolite vs. para-amphibolite

Metamorphic rocks composed primarily of
amphibole Amphibole () is a group of inosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust 350px, Plates in t ...

amphibole
,
plagioclase . (unknown scale) Plagioclase is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagiocla ...
, with subordinate epidote, zoisite, Chlorite group, chlorite,
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
, titanite, and accessory leucoxene, ilmenite and magnetite which have a protolith of an igneous rock are known as ''Orthoamphibolite''. ''Para-amphibolite'' will generally have the same equilibrium mineral assemblage as orthoamphibolite, with more biotite, and may include more quartz, plagioclase, and depending on the protolith, more calcite/aragonite and wollastonite. 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 metasedimentary rocks, especially greywacke and other poorly sorted sedimentary rocks. If the amphibolite appears to transgress apparent protolith bedding surfaces it is an ortho-amphibolite, as this suggests it was a Dike (geology), dyke. Picking a Sill (geology), sill and thin metamorphosed lava flows may be more troublesome. Thereafter, whole rock geochemistry will suitably identify ortho- from para-amphibolite. The word ''metabasalt'' was thus coined, largely to avoid the confusion between ortho-amphibolite and para-amphibolite. While not a true metamorphic rock name, as it infers an origin, it is a useful term.


Amphibolite facies

Amphibolite as a rock defines 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 amphibolite alone. Firstly, for an orthoamphibolite or amphibolite to be classed as a metamorphic amphibolite, it must be certain that the amphibole in the rock is a Metamorphism, prograde metamorphic product, and not a retrograde metamorphic product. For instance,
actinolite Actinolite is an amphibole Amphibole () is a group of Silicate minerals, inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedron, tetrahedra, linked at the vertices and generally containing ions of ir ...

actinolite
amphibole is a common product of retrograde metamorphism of metabasalt at (upper) greenschist facies conditions. Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacing pyroxene is an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies. Actinolite schist is often the result of hydrothermal alteration or metasomatism, 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 less than 1.2 GPa, well within the ductile deformation field. Gneissic texture may occur nearby, if not then mylonite zones, foliation (geology), foliations 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 phenocrysts of pyroxene, olivine,
plagioclase . (unknown scale) Plagioclase is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagiocla ...
and even magmatic amphibole such as pargasite rhombohedra, pseudomorphed by
hornblende Hornblende is a complex inosilicate series of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

hornblende
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 facies assemblages including, notably, Chlorite group, chlorite * High-magnesia basalt; as ortho-amphibolite, but may contain anthophyllite, a Mg-rich amphibole * Ultramafic rocks; tremolite, asbestos, asbestiform amphibole, talc, pyroxene, wollastonite, prograde metamorphic olivine (rarely) * Sedimentary para-amphibolite; hornblende/actinolite +/- albite +/- biotite +/- quartz +/- garnet (calcite +/- wollastonite) * Pelite; quartz, orthoclase +/- albite, +/- biotite +/- actinolite +/- garnet +/- staurolite +/- sillimanite 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 eclogite 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 Metamorphic facies#Granulite facies, granulite facies conditions.


Uralite

Uralite is a particular hydrothermally altered pyroxenite; during autogenic hydrothermal circulation the primary mineralogy of pyroxene and
plagioclase . (unknown scale) Plagioclase is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagiocla ...
, etc. has altered to
actinolite Actinolite is an amphibole Amphibole () is a group of Silicate minerals, inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedron, tetrahedra, linked at the vertices and generally containing ions of ir ...

actinolite
and saussurite (albite + epidote). 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, especially in Europe, to refer to a metamorphosed ortho-amphibolite with a protolith of diorite, gabbro or other mafic intrusive rock. In epidiorite the original clinopyroxene (most often augite) has been replaced by the fibrous
amphibole Amphibole () is a group of inosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust 350px, Plates in t ...

amphibole
''uralite''.


Uses

Amphibolite was a favourite material for the production of adzes (shoe-last-celts) in the central European early Neolithic (Linearbandkeramic and Rössen cultures). Amphibolite is a common dimension stone used in construction, paving, facing of buildings, especially because of its attractive textures, dark color, hardness and polishability and its ready availability.


References

* Winter, John D., 2001. ''An introduction to Igneous and Metamorphic Petrology'', 695 pages, Prentice Hall, {{Authority control Metamorphic petrology Metamorphic rocks