Clean magnetic separations of mixtures of rock minerals are uncommon for several reasons including two that are important, but these have not received much attention. First, as most magnetic separators are designed, they attract larger grains more strongly than smaller grains; apparently even a difference in size of 0.03 mm. is great enough to cause trouble. Second, the different members of several isomorphous series are differently attracted by strong magnets; the series from Mg to Fe compounds shows a wide range in attractability. Magnets can probably be designed to reduce or eliminate the effect of grain-size, but one can hardly expect zoned crystals which have a range of compositions to be cleanly separated from other minerals.

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