Phlogopite mica is a commonly known phenocryst and groundmass phase within ultrapotassic igneous rocks such as lamprophyre, kimberlite, lamproite, and other deeply sourced ultramafic or high-magnesian melts.

Phlogopite in this association is a primary igneous mineral present because of the depth of melting and high vapor pressures.

In other instances, metasomatism has resulted in phlogopite formation within large volumes, as in the ultramafic massif at Finero, Italy, within the Ivrea zone.

Phlogopite is encountered as a primary igneous phenocryst within lamproites and lamprophyres,[6] the result of highly fluid-rich melt compositions within the deep mantle.

The largest documented single crystal of phlogopite was found in Lacey mine, Ontario, Canada; it measured 10 m × 4.3 m × 4.3 m and weighed about 330 tonnes.