Research Results For 'Rhyolite'

IGNEOUS ROCK

Igneous rock is rock that has originated from molten or semi-molten magma. Igneous rocks are composed almost entirely of silicate minerals. Of many different types, they can be classified in various ways: by composition, crystal size, or mode of occurrence. One method is to divide them according to their silica content into three main groups: acid rocks, basic rocks, and intermediate rocks. There is, however, continuous gradation from acid to basic. Acid rocks are characterized by the presence of 10 per cent or more of quartz, usually accompanied by lighter- coloured minerals such as orthoclase feldspar and muscovite. This acid group includes rocks such as granite and rhyolite. Molten acid lava is very viscous, and its viscosity prevents the molten lava flowing very far and produces tall, conical volcanoes of the Vesuvius type. Intermediate rocks have less than 10 per cent quartz; diorite and andesite are typical of this group.

Basic igneous rocks are characterized by the absence of quartz and the predominance of dark minerals such as ferromagnesian minerals (amphiboles, pyroxenes). Common basic igneous rocks include gabbros and basalts. When extruded on to the Earth's surface these low-viscosity basic lavas form flat plateaux such as the Columbia Plateau, USA. Those igneous rocks that crystallize below the Earth's surface are termed intrusive or plutonic, according to whether they were formed at shallow depths, like dolerite, or deeper down, like granite. All these rocks have relatively large crystals produced by slow cooling of the molten magma. Igneous rocks that are extruded at the Earth's surface are termed extrusive or volcanic: basalt is an example. The rapid cooling that occurs at the surface does not allow large crystals to form, and these rocks are glassy or contain only small crystals.
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PIEMONTITE

Piemontite (piedmontite) is a rare reddish-brown coloured mineral first discovered in New Zealand and confirmed as a distinct species in 1853. It is a hydrous silicate of calcium, aluminium and ferric iron and usually contains between five and twenty percent manganese oxide. Piemontite is found in metasomatic manganese ore deposits in andesite, rhyolite and low-grade schist. Rarely it also occurs in pegmatite. Piemontite has a relative hardness of 6.
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PUMICE

Pumice is a light igneous volcanic rock used as a scourer, abrasive, polisher and in the manufacture of soundproofing tiles. Pumice can occur in any type of lava, but is more common in rhyolite and trachyte than in basalt. Pumice is formed from molten lava that cools so quickly that it doesn't have time to form either crystals or glass. Pumice may be heated until it melts, and will then form obsidian. Pumice is full of tiny vesicles which are the solidified remains of bubbles that were in the molten rock. These vesicles may pumice very light and porous - pumice will float and absorb water until saturated.
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RHYOLITE

Rhyolite is a fine grained extrusive igneous rock similar to granite. Rhyolite forms from particularly viscous molten lava and is grey, brown or reddish in colour, darkening with age. Rhyolite is essentially composed of feldspar, mica and quartz with numerous accessory minerals present.
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SANIDINE

Sanidine is a clear, glassy, often cracked variety of orthoclase feldspar, which is confined to modern volcanic rocks, such as trachyte, rhyolite and phonolite. It is found in the Rhine, Italy and Hungary.
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