Abstract
THE formula of the rare mineral ralstonite is usually written (Na2,Mg)F2.3Al(F,OH)3.2H2O. This implies a replacement of Na2 by Mg. X-ray investigation of this mineral shows that it belongs to the space-group Oh7 — Fd3m and that the edge of the unit cube has a length of 9.87 A. By recalculation from the best available analysis (Penfield and Harper, 1886), it can be shown that a unit cube contains on the average 48 (F,OH), 16 (Al,Mg), 2.7 Na and 7 H2O. The structure is similar to that of the pyrochlore group (type E 81) for which the formula X2Z2(O,OH,F)7 has been written. In the case of ralstonite the X position (16c) is only partly filled by 2.7 Na. Water partly fills the 8b position which is occupied by O, OH or F in minerals of the pyrochlore group.
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PABST, A. A Fluoride with Pyrochlore Structure. Nature 143, 520–521 (1939). https://doi.org/10.1038/143520c0
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DOI: https://doi.org/10.1038/143520c0
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