Phlogopite

Iron substitutes for magnesium in variable amounts leading to the more common biotite with higher iron content. For physical and optical identification, it has most of the characteristic properties of biotite. Phlogopite is an important and relatively common end-member composition of biotite. Phlogopite micas are found primarily in igneous rocks, although it is also common in contact metamorphic aureoles of intrusive igneous rocks with magnesian country rocks and in marble formed from impure dolomite (dolomite with some siliclastic sediment). The occurrence of phlogopite mica within igneous rocks is difficult to constrain precisely because the primary control is rock composition as expected, but phlogopite is also controlled by conditions of crystallisation such as temperature, pressure, and vapor content of the igneous rock. Several igneous associations are noted: high-alumina basalts, ultrapotassic igneous rocks, and ultramafic rocks. The basaltic occurrence of phlogopite is in association with picrite basalts and high-alumina basalts.