—Since ferrous iron usually colors minerals green, and ferric iron yellow or brown, it may seem rather remarkable that the presence of both together should give rise to a blue color, as in the case described in the above paper. It may be pointed out, however, that this is by no means a unique instance of such an effect. Even apart from the artificial substances, “Prussian” and“Turnbull’s” blues, which are complex cyanides containing both ferric and ferrous iron, there are several blue minerals in which the color seems explainable only on this basis. The most noteworthy of these are: crocidolite and related amphiboles; iolite; and blue tourmaline or indicolit'e. Other instances may perhaps be discovered, should this subject ever be investigated as it deserves to be.

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