Abstract

The crystal structure determination of hellandite was made on a specimen from Predazzo, Italy (a = 18.99, b = 4.715, c = 10.30 Å, β = 111.4°, space group P2/a), through Patterson and Fourier methods, with data collected on a single crystal diffractometer. The least squares refinement led to a final discrepancy index R = 0.05 for 1480 independent reflexions.

The most interesting features in the crystal structure are the infinite silicoborate chains [Si4B4O20(OH)2]−14; two such chains are parallel to c in the (010) plane, with silicon and boron atoms at y ≃ ½ and are related to each other by the glide plane. Iron and aluminum in octahedral coordination and calcium, yttrium, and rare earths in square antiprismatic coordination are all located at y≃0 and connect the silicoborate chains. The structural investigation indicated the following formula for hellandite:  
[Ca5.5(Y,RE)5.01.5](Al1.1Fe0.93+)(OH)4[Si8B8O40(OH)4]

The relation between the structures of hellandite on the one hand and melilite and melilite-like minerals on the other hand is stressed. The crystal chemistry of hellandite is clarified, and its close relationship with tadzhikite is discussed.

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