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Five common magmatic rocks - rhyolite

Release date:2018-11-08  source:东盟  Browse times:285

Rhyolite is an acidic effluent rock. The composition is comparable to granite (intrusive and effusion rock formed by the same magma). Grayish white or light pink. Commonly, there are rhyolite structures and plaque structures, vitreous structures, spherulitic structures, fine structures, and microscopic image structures. The phenocrysts are often quartz, alkaline feldspar, and sometimes a small amount of plagioclase; the matrix is generally dense cryptocrystalline or vitreous. Most of the occurrences are rock mounds. The devitrified rhyolite is called rhyolite porphyry.


Figure 1 Rhyolite porphyry


Rhyolite is widely distributed in the southeastern coastal provinces of China. The northern Liaoning, Inner Mongolia, Hebei, Shanxi, Shandong, Jilin, and Heilongjiang are also distributed. The minerals associated with it are kaolinite, montmorillonite, pyrophyllite, alum and pyrite.

Rhyolite A volcanic eruption rock equivalent to granite. Most rhyolites have a porphyritic structure, indicating that crystallization has begun before the eruption. Sometimes when the magma is still buried deep, crystallization may have begun; in this case, the formed rock may consist mainly of a single large crystal (plaque) that has developed well at the time of ejection. The amount of microcrystalline matrix in the final product may be too small to be seen under the microscope. Such rocks (spotted rocks) are easily mistaken for granite from the hand specimens. However, most rhyolites have a relatively short crystallization period, so the rock is mainly composed of a microcrystalline matrix or a partially vitreous matrix containing a few phenocrysts. The matrix is ​​sometimes micro-like or motley-like. Among the vitreous rhyolites are obsidian, rosin, perlite and pumice.

The chemical composition of rhyolite is much like granite. This correspondence means that at least some of the granite, and perhaps most of the granite, is produced by magma. There may be quartz, alkaline feldspar, aragonite, biotite, hornblende or pyroxene in the phenocrysts of rhyolite. If sporadic pyroxene or sporadic hornblende is the main dark mineral, then aragonite is rare or absent, while feldspar porphyry is composed mostly or entirely of feldspar; this rock is called Turbulent rock. If both aragonite and feldspar have a prominent position in the phenocryst, then the dominant dark silicate mineral will be biotite, and there will be neither hornblende nor pyroxene, if any, It will be a sloping variant; such lava is the quartz porphyry or "real" rhyolite in most classifications.

Some differences between rhyolite and granite are noteworthy. In most granites, alkaline feldspar is a micro-plagioclase with little sodium (ie, micro-twill feldspar); however, in most rhyolites it is a perlite, often rich in sodium. The fact that potassium greatly exceeds sodium is uncommon in granite, and is not uncommon in rhyolite unless it is the result of hydrothermal alteration. Rhyolites are found all over the world and in all geological ages. Rhyolite is mainly confined to the mainland or close to the edge of the continent, but it is also found elsewhere. It has been reported that there are also a few rhyolites (or quartz trachyte) on the oceanic islands far from any continent.

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