metmorphic rock

8/6/2019 Metmorphic Rock

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MARBLE

Marble is a metamorphic rock, formed by

the alteration of limestone under hightemperature and/or high pressure.

Marbles formed from pure calcite

limestones are white, with a sugary

texture, and they effervesce when testedwith dilute (~10%) hydrochloric acid.

Impurities in the limestone may lead to

the formation of new minerals, giving the

marble a variety of colours. Stonemasonsoften give the name marble to any rockthat takes a good polish, but this is

incorrect in geological terminology.

A famous white marble comes from Carrara in the Italian Appenines where it hasbeen quarried for two thousand years. It is probably the finest in the world for

sculpture, and has been used by famous sculptors such as Donatello,

Michelangelo and Canova for their masterpieces. The Taj Mahal in India is built of

white marble from Makrana.

Colour: may be uniformly white and glistening, light brown or grey; alsovariegated with green, red or black.

Mineralogy: 95% calcite (CaCO3) or dolomite Ca,Mg(CO3)2. Impurities may give

rise to new minerals such as olivine.

Composition: calcareous.

Texture: medium to coarse grained, often showing a sugary texture.



SLATE

Slate is a metamorphic rock formed from

a mudstone or shale. Its maincharacteristic is that it splits very easily

into very thin sheets, a property known

as cleavage. This is caused by the

alignment of the very fine grains thatform the slate, particularly the platy

mineral mica, when the rock is

metamorphosed under moderate

temperatures and quite high pressures.

Slate was commonly used a roofing

material on old houses because it splitsso readily. Slates from the quarries of

North Wales were once widely used for roofing in Britain. Slate was also used forbilliard tables and blackboards, and decorative fireplaces made from enamelled

slate can sometimes be found in nineteenth Century houses. Nowadays, it is rare

to see a new slate roof, probably because alternative materials are cheaper.

Colour: mostly grey to black, but may be purplish or greenish.

Mineralogy: micas, quartz, chlorite and graphite.

Composition: pelitic.

Texture: fine-grained, grains too small to see with the naked eye, alignment of

minerals leads to a very well developed cleavage.



QUAR TZITE

Quartzites are formed through the

metamorphism of quartz-richsandstones. They are formed mostly of

the mineral quartz. Quartzite may

sometimes look like marble, but can be

distinguished because quartzite cannotbe scratched with a knife, unlike marble.

Nor does it react with weak acid, as does

marble. It is very hard and weather

resistant. Quartzite is used in the glass

and ceramic industries, and builderssometimes use it for flooring and facing.

Colour: white, grey or reddish.

Mineralogy: mostly quartz, may contain some feldspar or mica.

Composition: siliceous.

Texture: medium-grained, grains equidimensional.



GNEISS

Gneiss is a rock formed during

regional metamorphism. It isgenerally a coarse-grained

granular textured rock which can

develop from a wide variety of

igneous and sedimentarymaterial. Gneisses consist of

alternating dark and light bands

of minerals which can vary in

thickness, from milimetres up to

a metre and can be highlycontorted. Varieties are distinguished by characteristic minerals, texture,

structure or the parent rock.

Colour: commonly formed of alternating dark and light (white,pink or grey)

bands.

Mineralogy: quartz and feldspar, may contain biotite mica.

Composition: varied, may be igneous or sedimentary.

Texture: medium-to coarse grained.



SCHIST

Schist is often of silvery appearance,

and it shows a tendency to split intosheets, although not nearly as well

as the finer grained slate. It is rich in

the silvery-white mineral mica,

which is platy in shape itself. Themicas are arranged roughly parallel

to each other, which is why the rock

shows this tendency to split, a

phenomenon known as slatey

cleavage in slates, and as schistosityin coarser grained rocks such as

schist.

This rock type is very widespread in metamorphic regions, such as Scotland,Norway or the Alps.

Colour: greyish and sparkling, white grains of quartz may be visible.

Mineralogy: white mica (muscovite) and black mica (biotite) common, alsoquartz. It may contain other minerals as well, such as garnet, which form as therock undergoes metamorphism.

Texture: umedium or coarse-grained, may contain new minerals of larger size

then the matrix, known as porphyroblasts (e.g. garnet, kyanite). Characterizedby schistosity, may show folding of layers.

metmorphic rock

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