Phlogopite mica has crystallized from blast-furnace slags at atmospheric pressure. Its chemistry is characterized by an excess of Na and K over their normal stoichiometric content, which is attributed to a composite association between phlogopite and glass. The diffraction pattern indicates that the phlogopite has a preferred orientation around the a-axis. Scanning electron microscopy shows that this phlogopite consists of numerous small platelets, which probably have their a-axes aligned.