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Amphibolite

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Amphibolite is a coarse-grained metamorphic rock, predominantly composed of mineral amphibole and plagioclase feldspar. It can also contain minor amounts of other metamorphic minerals such as biotite, epidote, garnet, wollastonite, andalusite, staurolite, kyanite, and sillimanite. Amphibolite is found around metamorphic and igneous rock intrusions that solidify between other rocks that are located within the Earth. Also, amphibolite has significant components found in both volcanic and plutonic rocks that range in composition from granitic to gabbroic. The formation of amphibolite took place millions of years ago and is found in various countries around the world today.

Name: Amphibole, originates from the Greek word amphibolos, meaning “ambiguous,” and was named by the famous French crystallographer and mineralogist Rene’-Just Hauy (1801)

Colour: Mainly of green, brown, or black

Group: Metamorphic rock

Texture: Coarse grain,gneissose or granofelsic metamorphic rock

Major minerals: Amphibole and plagioclase feldspar

Accessory minerals: Biotite, epidote, garnet, wollastonite, andalusite, staurolite, kyanite, and sillimanite

Amphibolite Classification

The Amphibolite classification is based on the following statements:

1) The modal compositions of amphibolites show that most of them contain more than 50% of amphibole, but those with 50 to 30% are not unusual. The content of amphibole and plagioclase together is mostly higher than 90%, and may be as low as 75%.

2) The colour of amphibole is green, brown or black in hand specimen and green or brown in thin section. The common varieties are tschermakitic and magnesio- and ferro-hornblende.

3) Plagioclase is the prevalent light-coloured constituent, the quantity of quartz or epidote or scapolite should be lower than that of plagioclase.

4) Clinopyroxene, where present, should be less abundant than amphibole (hornblende). When pyroxene prevails, the rock should be named hornblende-pyroxene rock or calc-silicate rock, depending on its composition and on the composition of the clinopyroxene.

5) The presence of other major mineral constituents (>5%) is expressed by the corresponding prefix according to general SCMR rules (e.g. garnet amphibolite, pyroxene amphibolite, quartz amphibolite, etc.).

6) The amphibolite is characterised by the presence of hydroxyl-bearing minerals (amphibole, biotite), which prevail over the hydroxyl-free ones (garnet, diopside). The boundary with the higher grade, granulite-facies metamorphic rocks, is determined by the appearance of orthopyroxene.

Chemical Composition of Amphibolite

Amphibolites define a particular set of temperature and pressure conditions known as the amphibolite facies, with temperature of 500 to 750 °C and pressures of 8-7 kbar. Changes in mineralogy depends very much on protolith, however, production of abundant garnet and hornblende are most characteristic. Sodic feldspars are oligoclase rather than the albite that dominates at lower T. Biotite and muscovite are both abundant in pelitic rocks of amphibolite facies. Kyanite and sillimanite are often produced by reaction of muscovite and quartz.

Typical assemblages for different protoliths include:

Mafic Protolith: hornblende + oligoclase ± epidote ± almandine garnet ± titanite ± quartz ± chlorite ± biotite.

Pelitic Protolith: biotite ± muscovite ± oligoclase ± almandine garnet ± cordierite (low-P) ± andalusite (low-P) ± kyanite (high-P) ± sillimanite (moderate-P, and/or high-T) ± staurolite (high -T) ± graphite ± titanite.

Quartz-feldspathic Protolith: oligoclase + alkali feldspar + muscovite + biotite ± hornblende.

Calc-silicate Protolith: calcite, dolomite, quartz, diopside, tremolite, forsterite, grossular garnet, hornblende, clinozoisite.

Formation of the Amphibolite Rock

Amphibolite is a rock associated with the convergent plate boundaries where heat and pressure cause regional metamorphism of mafic igneous rocks such as basalt and gabbro or from the clay rich sedimentary rocks that can be either marl or greywacke. The metamorphism sometimes also flattens and elongates the mineral grains which produces schistocity in the rock.

Ortho-amphibolites vs. para-amphibolites

Metamorphic rocks composed primarily of amphibole, 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.

Uralite

Uralites are particular hydrothermally altered pyroxenites; 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 pseudomorphically after pyroxene, and saussuritised plagioclase.

Epidiorite

The archaic term epidiorite is sometimes used 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 uralite.

Where is It Located

This common metamorphic rock is found around the world, with variable chemical makeups from deposit to deposit. It originally begins as an igneous rock such as basalt, although all original materials cannot be determined due to the metamorphic process. During this process, the base material is exposed to water-borne minerals, which combine to form the new rock.

Amphibolite (or hornblende) can also be found as inclusions in moss agate, dendritic agate and zoisite. Amphibolite is commonly found in areas where mountains have formed. Deposits have been found on every continent except Antarctica.

Uses of The Rock

Amphibolite was a fave material for the production of adzes (shoe-ultimate-celts) in the imperative European early Neolithic (Linearbandkeramic and Rössen cultures).

Amphibolite is a not unusual size stone utilized in production, paving, dealing with of homes, specially due to its appealing textures, darkish coloration, hardness and polishability and its equipped availability

Amphibolite has a variety of uses in the construction industry. It is harder than limestone and heavier than granite. These properties make it desirable for certain uses. Amphibolite is quarried and crushed for use as an aggregate in highway construction and as a ballast stone in railroad construction. It is also quarried and cut for use as a dimension stone.

Higher quality stone is quarried, cut, and polished for architectural use. It is used as facing stone on the exterior of buildings, and used as floor tile and panels indoors. Some of the most attractive pieces are cut for use as countertops. In these architectural uses, amphibolite is one of the many types of stone sold as “black granite.”

Gemologists and lapidary workers have discovered that some amphibolite rock produces a shimmer effect when it is polished. They use rounded and polished pieces of amphibolite for various pieces of jewelry.

There are many options to amphibolite as dimension stone. Marble, granite, and quartzite, for instance, can all be polished and used as facing on the interior and exterior of buildings. In some environments even sandstone can be used for building construction. In the end, amphibolite is chosen for the particular color, texture and overall look it gives to a building. Substitutes that provide a similar look include plastics and some varieties of other dark rock like dark granite.

Facts About The Rock

  • Metamorphic rocks are formed by the heating of pre-existing rocks. The heat provided to a rock changes the mineralogical and physical changes which are called metamorphic rocks.
  • Amphibolite erodes over a long period of time. Wind erosion, sea erosion, glacier erosion and chemical erosion are all types of erosion that effect amphiboles.
  • The highest quality of amphibolite is quarried for specific uses in architectural design
  • Amphibolite often has features that are smooth to the touch, matrix variable, and shiny looking.
  • Because amphibolite is harder than limestone and heavier than granite, it is quarried and crushed and used for highway and railroad construction.
  • According to a variety of features like texture, appearance, hardness, streak, toughness, and resistance, an amphibolite is used for various antiquity uses such as artifacts, sculpture and small figurines.
  • Amphibolite is often used commercially in cemetery markers, commemorative tablets, and creating artwork
  • Amphibolite is used for exterior building stones, facing stones, curbing, and paving stone.
  • Amphibolite is used for interior countertops, entryways, floor tiles, and in hotels and kitchens.
  • When the presence of hydroxyl groups is found in the structure of amphiboles, it decreases their thermal stability relative to the more refractory (heat-resistant) pyroxenes.
  • Amphiboles have hydroxyl groups in their structure and are considered to be hydrous silicates that are stable only in hydrous environments where water can be found and incorporated into the structure
  • Most often, amphiboles form as asbestiform (fibrous) aggregates, radiating sprays, and long prismatic crystals.
  • Amphibolite can crystallize in igneous and metamorphic rocks with a wide range of bulk chemistries because of the large range of chemical substitutions allowed in the crystal structure.
  • According to the British mineralogist Bernard E. Leake, there are 5 major groups of amphibole that leads to 76 chemically defined compositions.

References

Cite this article as: Geology Science. (2019). Amphibolite. [online] Available at: http://geologyscience.com/rocks/metamorphic-rocks/foliated-metamorphic-rocks/amphibolite/ [9th December 2019 ]
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