Mesoperthite mineral, light micrograph. This is a thin section, imaged with transmitted light. Perthite refers to rocks characterized by the intergrowth of two feldspars as they crystallize. In the case of this mesoperthite, sodium feldspars (albite and oligoclase) forms first, while the potassium feldspars (microcline and orthoclase) form later as thin strands (blue) within the mineral. This sample is 3 centimetres across.