Andesite

ANDESITE (from the name of the Andes mountain range within thAndesitee South America continent * EN: andesite; DE: Andesit, FR: andesite; ES: andesita; RU: андезит) is the cenotypal rock, effusive analog of diorite. The colour of andesite is from gray to black, sometimes with the green tint, the structure is porphyritic.

Andesites are formed during the process of the volcanic eruptions during the solidification of lava, which has arrived onto the daylight surface, or has remained within the deep of the earth near the surface. The major bulk mass of andesite consists of the microlites of plagioclase and of the subordinated quantity of pyroxene, which are immersed into the volcanic glass. Phenocrysts are represented by plagioclase, monoclinic or rhombic pyroxene, hornblende, biotite. Sometimes there exists olivine. Average chemical composition of andesite (% by mass) according to Daly is: SiO2 is 59.59; TiO2 is 0.77; Al2O3 is 17.31; Fe2O3 is 3.33; FeO is 3.13; MnO is 0.18; MgO is 2.75; CaO is 5.80; Na2O is 3.53; K2O is 2.04; H2O is 1.26; P2O5 is 0.26.

Andesitic lavas commonly contain large quantity of volatile components (water, carbon dioxide, hydrogen sulfide, and others), which leads to the catastrophic eruptions with explosions (island of Martinique, the Krakatoa volcano). Andesite together with basalts are common within the limits of the modern island arcs (Kamchatka, Kuril islands), and also of the Alpine orogenic belts (Carpathians, Caucasus, partially Crimea). Commonly andesites form covers and flows of lava, and also the extrusive forms, namely, domes and obelisks. The density is 2280-2680 kilograms per cubic metre, the porosity is 0.8-9.6%, rarely up to 14%, the compressive strength is 80-237 megapascals, the coefficient of softening is 0.8-1.0.

They use it as the crushed gravel for concrete, for the road construction (crushed gravel, paving block stone, bridge block stone), less often as the acid-resistant material and facing stone. Within the USSR, there are explored for the crushed gravel 21 deposits of andesite with industrial reserves of approximately 250 million cubic metres (1976); the deposit of the andesitic porphyrite (Akbastau, Kazakh SSR) has been explored for the facing stone. Within the USSR, the deposits of andesite are mainly concentrated within the Transcarpathian province of the Ukrainian SSR, within the Sakhalin island and Primorsky Krai, and also within Armenia. One of the large deposits is Rokosovsky (Transcarpathian province): the reserves are 42.4 million cubic metres, the annual extraction is more than 0.8 million cubic metres of the crushed gravel.

Annual extraction of andesite within the USSR is approximately 4 million cubic metres. Abroad, large deposits of andesites are known on the western coast of the USA, within France and Belgium.