The crystal structure of philolithite, Pb12O6Mn(Mg,Mn)2(Mn,Mg)4(SO4)(CO3)4Cl4(OH)12, P42/nnm, a = 12.627(9), c = 12.595(9) Å, V = 2008(2) Å3, Z = 2 has been solved by Patterson difference-Fourier syntheses and refined to R = 0.053 for 814 Fo > 4σFo using MoKα X-ray data. In the structure, MnO6 octahedra form straight chains parallel to [110] and [11̅0] by sharing opposite octahedral edges. Octahedra within the chains are further linked by sharing free corners with MnO4 and SO4 tetrahedra and CO3 triangles. The MnO4 and SO4 tetrahedra also form bridging struts between octahedral chains, connecting them in the [001] direction into an open framework. The Pb, Cl, and non-framework O atoms occupy the open spaces within the framework. The 10- and 12-fold coordinations of the Pb atoms exhibit the lone-pair effect. Pb atoms link via short bonds to non-framework O atoms to form chains parallel to [110] and [1̅10]. When viewed down [111], [1̅1̅1], [11̅1̅], or [1̅11̅], the framework (less the CO3 groups) is seen to be based upon cubic closest-packing of anions. This open framework of composition [[6](Mn,Mg)122+[4](SO4)22–[4](Mn2+O4)26− O8(OH)24]32−, referred to as a closest-packed trellis, is the fundamental unit for the structure.

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