Several specimens of pisanite, the isomorphous mixture of cupric and ferrous sulphates with 7 H2O, were collected in the Rico mining district in southwestern Colorado during the 1931 field season. Following Schaller’s suggestion2 the mineral should be called cuprian melanterite, as it contains more iron than copper. The rapid dehydration of the specimens after collection suggested a laboratory study of the relation between water content and humidity in this mineral. The results, which are recorded in this paper, have proved of considerable importance in the preservation of specimens and in the analytical determination of water of crystallization. The writer wishes to acknowledge his indebtedness to his colleague Mr. T. S. Lovering, for many valuable suggestions regarding the laboratory investigation, and to Mr. P. G. Nutting, also of the Geological Survey, for notes on the dehydration method adopted.

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