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Tal como a maioria das rochas metamórficas, o xisto apresenta aspecto nitidamente cristalino, e tem foliação mais ou menos nítida como resultado das fortíssimas pressões a que a rocha é sujeita. Esta foliação é fina em rochas holocristalinas, por via de regra de grão médio a fino, por vezes sendo tão pequeno que não se distingue macroscopicamente. Em geral, as «folhas» têm composição sensivelmente igual. Podem ser definidos vários grupos de xisto, conforme o grau dexistosidade (foliação) e os minerais que predominam na sua constituição: nos micaxistos predominam o quartzo e as micas (biotite/moscovite), nos anfiboloxistosa anfíbola e o quartzo, nos cloritoxistos a clorite, e nos talcoxistos o talco. ... "
in http://pt.wikipedia.org/wiki/Xisto
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Photo from:
Autor(a) Guilherme Limas
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" The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellarminerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. By definition, schist contains more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. Schist is often garnetiferous.
The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen by the naked eye. Schist is characteristically foliated, meaning the individual mineral grains split off easily into flakes or slabs. The word schist is derived from the Greek word σχίζειν meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie.
Most schists have been derived from clays and muds which have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists have been derived from fine-grained igneous rocks such as basalts and tuffs. Most schists are mica schists, but graphite and chlorite schists are also common.
Schists are named for their prominent or perhaps unusual mineral constituents, such as garnet schist, tourmaline schist,glaucophane schist, etc.
Schists are frequently used as dimension stone. Dimension stone is stone that has been selected and fabricated to specific shapes or sizes. ... "
