A trioctahedral member of the true mica group. The end-member formula is NaMg3AlSi3O10(OH)2 and it occurs most commonly as the 1M polytype. Aspidolite is rare and can occur in meta-evaporates, in chromite sequences of mafic/ultramafic layered intrusions, gabbraic xenoliths, and metapelites. In older literature, aspidolite is referred to as sodium phlogopite (a term now considered obsolete).