A dioctahedral member of the true mica group. The end-member formula is KAl2□BSi3O10(OH)2, where □ = vacancy. Boromuscovite occurs as 2M1 and 1M polytypes and has been found in pegmatites at the Řečice pegmatite dikes, Czech Republic, where it occurs asfine-grained masses, and Little Three Mine pegmatite dike, San Diego County, California, USA, where it occurs as coatings from late-stage hydrothermal fluids.
Cf., borocookeite, manandonite, muscovite