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<TD><H1><A HREF=http://store.unconventionallapidarist.com/Category667>Designer Cabochons</A> / Rhyolite</H1></TD>
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<P><p><b>Rhyolite</b> is an igneous, volcanic (extrusive) rock, of felsic (silicon-rich) composition (typically >69% SiO<sub>2</sub> — see the TAS classification). It may have any texture from <span class="mw-redirect">aphanitic</span> to <span class="mw-redirect">porphyritic</span>. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2 — see the QAPF diagram). Biotite and hornblende are common accessory minerals.</p>
<p>Rhyolite can be considered as the extrusive equivalent to the <span class="mw-redirect">plutonic</span> granite
rock, and consequently, outcroppings of it often bear a resemblance to
granite. Due to their high content of silica and low iron and magnesium
contents, rhyolite melts are highly polymerized and form highly viscous
lavas. They can also occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra or of ignimbrites.</p>
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-- Rhyolite Designer Cabs ( 39.9mm x 30.5mm x 6.3mm )</P>
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-- Rhyolite Designer Cabs ( 57.2mm x 25.7mm x 4.5mm )</P>
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