Basalt is not a mineral, because is can not be expressed in a definite chemical equation, nor does it have a definite crystal structure. Basalt is a hard, black volcanic rock with less than about 52 weight percent silica (SiO2). Common minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 to 1250° C. When basalt is beneath the surface it is called lava, when it is outside the earth's crust, it is basalt.   Some information provided by USGS