Amphibolite is a non foliated metamorphic rock that is mainly composed of mineral amphibole and plagioclase feldspar with little or no quartz. The amphibole are usually the member of the hornblende group. It can also contains other metamorphic minerals such as biotite, epidote, garnet, wollastonite, andalusite, staurolite, kyanite and sillimanite. It is typically dark coloured and heavy with a weak foliated or schistose structure. The small flakes of white and black minerals in the rock often give it a salt and pepper appearance.
How amphibolite forms?
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.
Protoliths forming amphibolite
Amphibolites need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals based entirely upon the chemistry of the protolith, Certain 'dirty marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolites 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.
Uses of amphibolite
Amphibolite is harder than limestone and heavier than granite so for this reason amphibolite is quarried and crushed for use as an aggregate in highway construction and as a ballast stone in railroad construction. It is also used as a dimension stone after cutting into specific size and shape. Higher quality of amphibolite is quarried for use as an architectural purposes and as a flooring tiles, facing stone on building exterior and panels indoors.