A modulated 1:1 layer silicate with a continuous, planar octahedral sheet and a general chemical composition of M2+8T6O15(OH,Cl)10. Pyrosmalite is the M = Mn, Fe series, manganpyrosmalite is M for Mn > Fe, and ferropyrosmalite is M for Fe > Mn. Friedelite is the Mn end member and a disordered (polytypic) equivalent of mcGillite. In addition, mcGillite has several additional polytypes. The pyrosmalite structure has an equal number of tetrahedra coordinating to two adjacent octahedra sheets via tetrahedral apical oxygen atoms (Kato and Takéuchi, 1983). Each tetrahedral sheet is composed of 4-, 6-, and 12-fold tetrahedral rings linked laterally, with half of the tetrahedra in the 4- and 12-fold rings inverted. Schallerite and nelenite are polymorphs and similar to friedelite, but apparently with As3O6 molecules within the 12-fold rings. Arsenite analogues of pyrosmalite-type minerals (T = As) occur: manganarsite (analogue manganpyrosmalite), and unnamed arsenite equivalents of schallerite and friedelite. Phase assemblages and occurrences are complex. Pyrosmalite occurs in greenschist facies manganiferous rocks. A near Fe-rich end member was reported from low-grade Fe- and Mn-rich sulfide deposits near Mt. Isa, Queensland, Australia. Friedelite occurs in low-grade metamorphic rocks and is Cl bearing.