Amphibolite
Encyclopedia
Amphibolite is the name given to a rock
consisting mainly of hornblende
amphibole
, the use of the term being restricted, however, to metamorphic rock
s. The modern terminology for a holocrystalline plutonic igneous rock
s composed primarily of hornblende amphibole is a hornblendite
, which are usually crystal cumulate
s. Rocks with >90% amphibole which have a feldspar
groundmass may be a lamprophyre
.
Amphibolite is a grouping of rocks composed mainly of amphibole
and plagioclase
feldspars, with little or no quartz
. 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 marl
s' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing dolomite
and siderite
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 basalt
s 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'.
, albite
, with subordinate epidote
, zoisite
, chlorite
, quartz
, sphene, and accessory leucoxene
, ilmenite
and magnetite
which have a protolith
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 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 sediments, especially greywacke
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 dyke
. Picking a sill
and thin metamorphosed lava
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.
Firstly, for an (ortho)amphibolite to be classed as a metamorphic amphibolite, it must be certain that the amphibole in the rock is a prograde
metamorphic product, and not a retrograde metamorphic product. For instance, actinolite
amphibole is a common product of retrograde metamorphism of basalts 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 schists are 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 in excess of 1.2 GPa, well within the ductile deformation field. Gneiss
ic texture may occur nearby, if not then mylonite
zones, 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 phenocryst
s of pyroxene, olivine
, plagioclase
and even magmatic amphibole such as pargasite
rhombohedra, pseudomorphed by 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:
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.
s; during autogenic hydrothermal circulation their primary mineralogy of pyroxene
and plagioclase
, etc. has altered to actinolite
and saussurite
(albite
+ epidote
). The texture is distinctive, the pyroxene altered to fuzzy, radially arranged actinolite pseudomorph
ically after pyroxene, and saussuritised plagioclase.
of diorite
, gabbro
or other mafic intrusive rock. In epidiorite the original clinopyroxene (most often augite
) has been replaced by the fibrous amphibole
uralite.
s (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.
Rock (geology)
In geology, rock or stone 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
Hornblende
Hornblende is a complex inosilicate series of minerals .It 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....
amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
, the use of the term being restricted, however, to metamorphic rock
Metamorphic rock
Metamorphic rock is 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 rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s composed primarily of hornblende amphibole is a hornblendite
Hornblendite
Hornblendite 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 cumulate
Cumulate rock
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.-Formation:Cumulate rocks are the...
s. Rocks with >90% amphibole which have a feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
groundmass may be a lamprophyre
Lamprophyre
Lamprophyres 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 amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
and plagioclase
Plagioclase
Plagioclase is an 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...
feldspars, with little or no quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
. 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
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
rocks. Because metamorphism creates minerals based entirely upon the chemistry of the protolith
Protolith
Protolith 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...
, certain 'dirty marl
Marl
Marl 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 dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
and siderite
Siderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
also readily yield amphibolites (tremolite
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. Pure magnesium tremolite is creamy white, but the color grades...
-schists, grunerite
Grunerite
Grunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O222. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The luster is glassy to pearly with colors...
-schists, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalt
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...
s 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'.
Ortho-amphibolites vs. para-amphibolites
Metamorphic rocks composed primarily of amphiboleAmphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
, albite
Albite
Albite 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 NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
, with subordinate epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, 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 faces are often...
, zoisite
Zoisite
Zoisite is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca2Al3O...
, chlorite
Chlorite group
The 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....
, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, sphene, and accessory leucoxene
Leucoxene
Leucoxene is a fine granular alteration product of titanium minerals. It varies in color from yellow to brown.It consists mainly of rutile or anatase...
, ilmenite
Ilmenite
Ilmenite 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 magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
which have a protolith
Protolith
Protolith 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 calcite
Calcite
Calcite 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 380-470°C, and vaterite is even less stable.-Properties:...
/aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
and wollastonite
Wollastonite
Wollastonite 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 greywacke
Greywacke
Greywacke 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 dyke
Dike (geology)
A 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 sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion 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 lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. 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...
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 prograde
Prograde
Prograde can refer to:*Prograde or direct motion, in astronomy, 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, actinolite
Actinolite
Actinolite is an amphibole silicate mineral with the chemical formula .-Etymology:The name actinolite is derived from the Greek word aktis , meaning "beam" or "ray", because of the mineral's fibrous nature...
amphibole is a common product of retrograde metamorphism of basalts at (upper) greenschist
Greenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
facies conditions. Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacing pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate 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 metasomatism
Metasomatism
Metasomatism 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. Gneiss
Gneiss
Gneiss 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.-Etymology:...
ic texture may occur nearby, if not then mylonite
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
zones, foliations
Foliation (geology)
Foliation 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 standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
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 phenocryst
Phenocryst
thumb|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, olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
, plagioclase
Plagioclase
Plagioclase is an 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 pargasite
Pargasite
Pargasite is a complex inosilicate mineral of the amphibole group with formula NaCa24AlO222.It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality....
rhombohedra, pseudomorphed by hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It 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....
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 greenschistGreenschistGreenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
facies assemblages including, notably, chloriteChlorite groupThe 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 anthophylliteAnthophylliteAnthophyllite 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; tremoliteTremoliteTremolite 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. Pure magnesium tremolite is creamy white, but the color grades...
, asbestiform amphiboleAsbestosAsbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...
, talcTalcTalc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
, pyroxenePyroxeneThe pyroxenes are a group of important rock-forming inosilicate 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...
, wollastoniteWollastoniteWollastonite 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 olivineOlivineThe mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
(rarely) - Pelites; quartz, orthoclaseOrthoclaseOrthoclase 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. Alternate names are alkali feldspar and potassium feldspar...
+/- albite, +/- biotite +/- actinolite +/- garnet +/- stauroliteStauroliteStaurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak.-Properties:It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and a rather complex chemical formula: 2Al94O204...
+/- sillimaniteSillimaniteSillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....
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 pyroxenitePyroxenite
Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...
s; during autogenic hydrothermal circulation their primary mineralogy of pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate 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 plagioclase
Plagioclase
Plagioclase is an 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 actinolite
Actinolite
Actinolite is an amphibole silicate mineral with the chemical formula .-Etymology:The name actinolite is derived from the Greek word aktis , meaning "beam" or "ray", because of the mineral's fibrous nature...
and saussurite
Saussurite
Saussurite 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....
(albite
Albite
Albite 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 NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
+ epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, 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 faces are often...
). The texture is distinctive, the pyroxene altered to fuzzy, radially arranged actinolite pseudomorph
Pseudomorph
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form , resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another...
ically after pyroxene, and saussuritised plagioclase.
Epidiorite
The archaic term epidiorite is sometimes used to refer to a metamorphosed ortho-amphibolite with a protolithProtolith
Protolith 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 diorite
Diorite
Diorite 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...
, gabbro
Gabbro
Gabbro 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 augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
) has been replaced by the fibrous amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
uralite.
Uses
Amphibolite was a favourite material for the production of adzeAdze
An adze is a tool used for smoothing or carving rough-cut wood in hand woodworking. Generally, the user stands astride a board or log and swings the adze downwards towards his 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 Neolithic
Neolithic
The Neolithic Age, Era, or Period, or New Stone Age, was a period in the development of human technology, beginning about 9500 BC in some parts of the Middle East, and later in other parts of the world. It is traditionally considered as the last part of the Stone Age...
(Linearbandkeramic and Rössen cultures).
Amphibolite is a common dimension stone
Dimension stone
Dimension stone is natural stone or rock that has been selected and fabricated to specific sizes or shapes. Color, texture and pattern, and surface finish of the stone are also normal requirements...
used in construction, paving, facing of buildings, especially because of its attractive textures, dark color, hardness and polishability and its ready availability.