The structure of a Ca-dominant ganophyllite from Val Graveglia mining district, Italy, with an ideal formula (Ca,K,Na)x Mn6 (Si9Al) O24 (OH)4 . nH2O, has been solved and refined (R=0.0786), in space group Pnma, with a=16.8146(6) Å, b=25.2036(9) Å, c=13.3866(5) Å. The 3x "superstructure" along a, long observed but never successfully measured, was obtained using a CCD-detector. Ganophyllite is a 2:1 modulated layer silicate; the modulation results from the structural misfit between the layers of Mn(II)O6 octahedra and silicate tetrahedra. The atomic arrangement consists of corrugated T-O-T layers, with inverted tetrahedra in the tetrahedral sheets connecting adjacent layers along b. The inverted tetrahedra exist as four-member rings, and incorporate substituent Al, with a maximum Al occupancy of Si 2.00 Al 2.00 in the four-member rings, by the rule of Al-Al avoidance. Charge balance for the substituent Al is maintained by adding cations (Ca > K > Na) or [6]Al sufficient to balance the charge lost by the Al - Si substitution in the interlayer tetrahedra. Zeolitic H2O molecules also exist in the interlayer channels. Contrary to earlier speculation, the 5.6 Å "subcell" along a, observed in all ganophyllite-group phases, is not a true subcell. Numerous earlier attempts at solution of the ganophyllite structure failed because of the assumption of a subcell-supercell relationship among structural components.