A modulated trioctahedral 2:1 phyllosilicate with an ideal chemical composition of RM13T18O42(OH)14, with R = Na and K, M = Mg, Mn, Fe2+, Fe3+, Al, and Ti, and T = Si and Al. The structure of zussmanite (Lopes-Vieira and Zussman, 1969) has a continuous octahedral sheet with islands of tetrahedral 6-fold rings on both sites of the octahedral sheet. Each island of 6-fold rings is laterally linked to other islands of six-fold rings by inverted three-fold rings, and these three-fold rings are linked also to the six-fold island tetrahedral rings of the adjacent 2:1 layer. The 6-fold rings align across the interlayer region and form a mica-like interlayer site where the large R cation can reside. Coombsite is the Mn analogue of zussmanite. Zussmanite occurs in blueschist facies metamorphic regimes at the Laytonville Quarry, Mendocino County, California.