Basalt

Basalt, dark, often black, basic extrusive igneous rock, chemically and mineralogically identical to the intrusive rock "gabbro". The main minerals of basalt: basic plagioclases, augite and often olivine. Distribution in the USSR: Caucasus, Transbaikal, Altai, Kamchatka. Basalt - dense, uniform, heavy rocks, occurring most often in the form of flows and sheets, in which this rock appears as characteristic columnar, separating from other rocks, breaking down into 5-6-sided columns. In the chemical composition of basalt, the low content of silica (up to 40%) is characteristic; Al₂O₃ - from 10 to 20%; high content of MgO and CaO; sometimes significant iron content - from 10 to 25% and a small amount of alkalis (Na₂O > K₂O). Basalt serves as a building material, is used, mainly, for pavements, as well as for the facing of foundations and construction of monuments.