X-ray and HRTEM study of sursassite: Crystal structure,stacking disorder,and sursassite-pumpellyite intergrowth |
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| Authors: | M Mellini S Merlino M Pasero |
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| Institution: | 1. Dipartimento di Scienze della Terra della Università di Pisa and C.N.R., C.S. Geologia Strutturale e Dinamica dell' Appennino, via S. Maria 53, 56100, Pisa, Italy
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| Abstract: | Sursassite is monoclinic, space group P2 1/m, a=8.70, b=5.79, c=9.78 Å, β=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. Examination by high resolution transmission electron microscopy (HRTEM) shows that frequent (001) pumpellyite-like lamellae are intergrown with thicker (001) sursassite lamellae. Usually, the guest lamellae are a few unit cells thick along 001] and continuous along the (001) plane, although also rare interrupted lamellae are found. |
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