A trioctahedral member of the true mica group. The ideal end-member formula is KMg2Al(Al2Si2)O10(OH)2, although such a chemical composition has not been reported. The original eastonite occurrence from Easton, Pennsylvania, USA, was shown to be a mixture of phlogopite and lizardite-1T with some “antigorite-like offsets”. The eastonite composition is useful to describe solid solution series where there are Mg + Al substitutions.
Cf., presiwerkite, siderophyllite