Crystal structure of 2M and 1A polytypes of balangeroite

Balangeroite, with ideal crystal chemical formula (Mg,Fe)42O6(Si4O12)4(OH)40, displays two MDO polytypes, balangeroite-2M (a = 19.179, b = 9.601, c = 19.218 Å, β = 90.52°; space group P2/n) and balangeroite-1A (a = 9.602, b = 13.891, c = 14.012 Å, α = 86.99°, β = 76.79°, γ = 76.67°; space group P-1). Both polytypes are built up by octahedral walls 3 × 1, octahedral bundles 2 × 2 and four-repeat silicate chains. All these modules run parallel to the 9.6 Å axis (b and a in the monoclinic and triclinic polytype, respectively). The distinguishing features between the two polytypes are in the different positioning of the silicate chains, which gives rise to distinct cells and space group symmetries.