Thin section of basanite, close-up, viewed in transmitted polarised light. Basanite is an extrusive igneous rock, meaning that it formed when the molten lava from a volcanic eruption cooled rapidly at the Earth's surface. It is mafic (potassium rich and silica deficient) and consists of feldspathoid minerals (minerals that resemble feldspar), amongst others, as are seen here in various colours. The main feldspath mineral is leucite, a rock-forming mineral composed of potassium and aluminium tectosilicate. Magnification: x3.1 when printed 10 centimetres wide.