Microcline mineral. Microcline is a potassium-rich (sodium-poor) alkali feldspar (K.Al.Si3.O8), a tectosilicate mineral that is an important constituent of igneous rocks such as granites and pegmatites. It is triclinic and the crystal habitat is often characterized by crosshatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline. Microcline may be clear, white-green, red, pale-yellow or (as here) pale orange. This sample, from Mystic in Connecticut, USA, is 33 centimetres across.