Abstract

The crystal structure of yofortierite, (Mn2+,Mg,Fe3+,□)5Si8O20(OH,H2O)2(H2O)7, monoclinic, C2/m, Z = 4, a 14.1686(12), b 17.8583(16), c 5.2919(5) Å, β 105.878(1)°, V 1287.9(3) Å3, has been refined to R1 = 4.9 % for 1795 unique (Fo > 4σF) reflections collected on a Bruker D8 three-circle diffractometer equipped with a rotating-anode generator (MoKα X-radiation), a multi-layer optics incident-beam path, and an APEX-II CCD detector. Chemical analysis by electron microprobe plus Fe3+ determination by Mössbauer spectroscopy gave SiO2 51.78, Al2O3 0.05, TiO2 0.15, Fe2O3 1.84, MnO 22.97, ZnO 0.99, MgO 4.32, CaO 1.10, H2Ocalc 16.69, sum 99.89 wt.%. The resulting empirical formula is (Mn3.01Mg1.00Zn0.11Ca0.18Fe3+0.21 Ti0.02Al0.010.46) ∑=5Si8.00O20[(OH)1.34(H2O)0.66]∑=2(H2O)7. Yofortierite is a palygorskyite-group mineral. There are two tetrahedrally coordinated T sites occupied by Si with <Si–O> distances of 1.621 and 1.617 Å, and three octahedrally coordinated M sites, occupied primarily by Mn2+ and Mg with minor Fe3+ and □, with <M–O> distances of 2.147, 2.079, and 2.183 Å. The <M–O> distances indicate strong order of M cations over the three M sites, with the smaller cations and vacancies ordered at the M(2) site and Ca ordered at the M(3) site. The presence of vacancies at the M(2) site locally couple with the replacement of (OH) at the O(4) site by (H2O)o, giving rise to strong short-range order, and H2O is incorporated into the framework part of the structure by the substitution Mn2+ + (OH) = □ + (H2O)o which is coupled to the substitution Mn2+ = Fe3+ by the requirement of electroneutrality.

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