X-ray and HRTEM study of sursassite: Crystal structure, stacking disorder, and sursassite-pumpellyite intergrowth

Sursassite is monoclinic, space group P2₁/m, a=8.70, b=5.79, c=9.78 Å, ß=108.9°. The crystal structure was determined with X-rays and refined to R=0.065, obtaining Mn₂Al₃[(OH)₃(SiO₄)(Si₂O₇)] as ideal crystal chemical formula. Sursassite, isostructural with macfallite Ca₂Mn₃[(OH)₃(SiO₄)(Si₂O₇)], is closely related to pumpellyite Ca₂Al₃[(OH)₃(SiO₄)(Si₂O₇)]. In fact both sursassite and pumpellyite, apart from the different chemical composition, are built up by common structural layers, which are repeated by different stacking vectors. As a result, faulted stacking sequences are energetically possible.