Roeblingite, Pb2Ca6(SO4)2(OH)2(H2O)4[Mn(Si3O9)2], monoclinic holosymmetric, a = 13.208(4), b = 8.287(2), c = 13.089(9)Å, β = 106.65(6)°, space group C2/m, possesses a ∞2[Mn(Si3O9)2]10− corner-linked sheet oriented parallel to c{001}, the plane of perfect micaceous cleavage. R = 0.066 for 2479 nonequivalent reflections.

The large cations are tucked between the [Mn(Si3O9)2] sheets. All vertices of the MnO6 octahedron link to the (Si3O9) radicals which are oligosilicate three-membered rings and are geometrically similar to the rings in paragenetically related margarosanite, PbCa2[Si3O9], Coordination polyhedra include Pbϕ7 (maximal point symmetry mm2); Ca(1)ϕ8 distorted square antiprisms; Ca(2)ϕ7 polyhedra similar to Pbϕ7; MnO6 octahedra; SiO4, SO4 tetrahedra. Mean bond distances are: Pb–O 2.82 (2.22 to 3.42), Ca(1)–O 2.53, Ca(2)–O 2.43, Mn–O 2.22, Si(1)–O 1.64, Si(2)–O 1.62 and S–O 1.47Å.

The packing efficiency, defined as the volume of the unit cell divided by the total number of anions in that cell, is usually close to the values of hep or ccp oxysalt structures for most minerals which don’t have channels. This value, VE, is unusually large for Pb(II) oxysalts. By including the number of lone pair cations for that cell, the value, VE+L, is more reasonable and approximates the packing efficiencies for oxysalts with those cations of similar size but stripped of all valence electrons.

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