Abstract
Ralstonite of the following composition
has been found in the cryolite deposit in Ivigtut, South Greenland. Various cubic minerals of ralstonite type have been registered from this deposit. These and ralstonite from other localities as well as from Ivigtut can be arranged in a series where the new compound represents a member close to one end of the series, NaMgAl(F,OH)6H2O. The other members of the series show decreasing amounts of Na and Mg. Among Ivigtut samples a formula content of 0.34 Na and Mg represents the lowest value; a specimen from Kazakhstan is registered in literature with a content of Na and Mg in the formula at 0.29. Basic aluminium fluoride Al2 (F,OH)·H2O may be regarded as the other end member, as its structural data correspond closely to those of the ralstonite series.
Examination of optical data, x-ray data and specific gravity for a number of samples together with corresponding data from literature indicates certain regularities in the series of ralstonite with the mentioned compositional variation. Low refractive index occurs together with high specific gravity, d~2.6, n~1.37, and a large unit cell, a about 10.02 Å. Samples with high lefractive index, n~1.43, have low specific gravity, d~2.4, and cell dimension about 9.90. The first group has a high content of Na and Mg, whereas the last group has a low amount of these elements.
