A modulated 1:1 layer silicate with octahedral sheets interlayered by a continuous tetrahedral sheet with double 6-fold tetrahedral rings (Heinrich et al., 1994). The 6-fold rings are interconnected and have tetrahedra pointing up and down, and adjacent octahedral sheets are connected with 5- and 7-fold tetrahedral rings. The ideal chemical composition is M7Si6O15(OH)8, where M are medium size cations, such as Mn, Fe, Mg, Zn. Small amounts of Al are known to enter the tetrahedral site for Si. Important localities include Franklin, New Jersey, USA in a metamorphosed, zinc stratiform ore body, and the Olympic peninsula, Washington, USA.
Cf., armbrusterite, parsettensite, pyrosmalite, innsbruckite, varennesite