A study of crystallization from water of compounds in the system Fe²⁺ - Cu²⁺ - So₄²⁻.

Abstract

The possible solid solution series between melanterite (FeSO(4)∙7H(2)0) and boothite (CuSo(4)∙7H(2)0) was investigated to ascertain the extent to which the series exists. Sy-ritheses of these compounds and an intermediate minera1 pisanite [(Fe,Cu) SO(4)∙7H(2)0], were attempted by introducing copper sulfate into an aqueous solution of ferrous sulfate. Temperature, concentration, and time of crystallization were varied in an effort to crystallize the desired products. Under various conditions, it was possible to crystallize all three of the compounds, but no intermediate mixtures or compounds other than pisanite could be isolated. Thus, a solid solution series between melanterjte and boothite does not exist, but rather is prepresented. by crystals of the end members arid one intermediate compound pisanite for which the new formula of (Fe(0.80)Cu(0.20)) SO(4)∙7H(2)0 which represents 2O.O3% Cu++ is proposed. Compounds formed were checked for composition by X-ray diffraction and polarography. Whereas crystal growth was attempted for many varied intermediate compositions, all samples representing intermediate compositions proved to be pisanite-type material. This leads to the conclusion that the melanterite structure cannot support substitution of copper for iron except in the amount stated above. Cell dimensions of the artificial boothite formed were determined by single crystal X-ray methods and were found to agree closely with those of the isostructural artificially formed nielanterite. Boothite dehydrates easily and is converted to chalcanthite (CuSo(4)∙7H(2)0) at room temperature and low humidity. For this reason, boothite as a pure compound cannot exist in nature except under very special conditions.