X-ray and HRTEM structure analysis of orientite

Orientite is orthorhombic, a 9.044(10), b 6.09(7), c 19.031(20) A, with ideal crystal chemical formula: 265Ca (sub 8) Mn (sub 10) (super 3+) ((SiO (sub 4) ) (sub 3) (Si (sub 3) O (sub 10) ) (sub 3) (OH) (sub 10) ).4H (sub 2) O in the unit cell. A structural model with space group P2mm was proposed on the basis of chemical data, X-ray and HRTEM (High Resolution Transmission Electron Microscopy) analyses. The model was refined to R = 0.069, using 650 independent reflections collected with MoKalpha radiation. The structure is characterized by the regular alternation of the c direction of two distinct structural modules and by the extensive occurrence of microvoids, elongated in the c direction located at the borders between misfitting crystal regions.