Bornite found in Red Bed types of deposits has often been observed to exsolve chalcopyrite upon being heated to less than 200 degrees C, and to homogenize again as temperature was increased up to 500 degrees C. An attempt has been made to explain this phenomenon. The theory is based on the assumptions that the deposition of bornite took place at low temperatures; that the solid solution of chalcopyrite in the low temperature tetragonal bornite is extensive at temperatures below 150 degrees C; and that a more limited solid solution of the two minerals occurs at moderate temperatures. These conditions are considered probable for the two minerals are likely to be isostructural at low temperatures, but not so at moderate temperatures. This hypothesis, when experimentally verified, could reconcile the occurrence in the Red Beds of bornite containing up to 25 percent of chalcopyrite in solid solution with the low temperatures suggested by the environment of the deposits.

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