Igneous Rocks


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Igneous Rocks

 

(or magmatic rocks), rocks formed from molten magma as it solidifies and crystallizes. Based on solidification conditions, two principal types of igneous rocks are distinguished: effusive (volcanic, extrusive), which have solidified on the surface of the earth after magma has poured forth in the form of lava during volcanic eruptions, and intrusive (abyssal), which have solidified in a stratum of the earth’s crust between other rocks. Owing to their rapid solidification, effusive rocks are usually fine-grained and consist partially (sometimes completely) of glass. Larger crystals called phenocrysts are often found in them, giving rise to what is known as “porphyritic texture.” Intrusive rocks, which solidify slowly deep within the crust, have a holo-crystalline and more coarse-grained structure.

Igneous rocks are normally composed of silicates. Silica (SiO2) is their chief component and depending on silica content igneous rocks are divided into ultrabasic (less than 40 percent SiO2), basic (40-56 percent), intermediate (56-65 percent), acidic (65-75 percent), and ultra-acidic (more than 75 percent). Igneous rocks that do not contain silicates (for example, carbonatites) are very rare. The mineral composition of the different groups of igneous rocks changes correspondingly. Ultrabasic rocks (pyroxenites, dunites, olivines) are composed entirely of olivines and pyroxenes; basic rocks (gabbro, basalts) contain calcic plagioclase in addition to olivines and pyroxenes. In acidic rocks (granites, liparites, dacites) the content of ferromagnesian and calcic silicates decreases and alkali feldspars and quartz appear. The intermediate rocks include chiefly feldspars with small amounts of ferromagnesian minerals (diorites, andesites).

Rocks of the normal series and the alkalic series (alkali granites, nepheline syenites, phonolites) are distinguished on the basis of the alkali content of each group of igneous rocks. The alkali silicates (aegirines, alkali amphiboles, and feldspathoids) are placed in the alkalic series.

Various minerals are also associated with different types of igneous rocks. For example, tin, tungsten, and gold are associated with acidic igneous rocks; titanomagnetite and copper with basic igneous rocks; chromium, platinum, nickel, and other minerals with ultrabasic igneous rocks; and titanium, phosphorus, apatites, zirconium, rare earths, and other minerals with alkali igneous rocks.

Igneous rocks can be used as building materials (Artik tuffs and labradoritites, for example), abrasive materials (pumice), and thermal insulation materials (pumice, perlite). In addition, they can serve as raw material for the extraction of valuable components (for example, aluminum from nepheline syenites) and as foundations for hydraulic and other structures.

REFERENCE

Zavaritskii, A. N. Izverzhennye gornye porody. Moscow, 1955.

V. P. PETROV and T. I. FROLOVA

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