"Microscopic" Cleavages in Bornite from the Jinmu Mine, SW Japan and their Implications

During microscopic and electron probe investigation of the copper ores from the Jinmu mine, Hiroshima Prefecture, SW Japan, we have found well-developed "microscopic" cleavages oriented in two or three directions in the bornite matrix. They are represented by the regularly arranged intergrowths, lattice or lamellar. According to their distribution density and size (width), the cleavages are divided into three types. (1) Type 1, densely developed and very fine (less than about 0.1 micron in width), visible only under higher magnifications (x 400 or greater); (2) Type 2, less densely developed and fine (about 1micron in width); and (3) Type 3, sparsely developed and coarse (up to about 10 microns in width). The Type 1 cleavages are less common in the Jinmu ores, while they are very rare in natural sulfides. The very fine and coarse lamellae are always chalcopyrite, while the fine ones are mainly wittichenite and/or some chalcopyrite. Under the electron beam only the image of chalcopyrite forms a dark set of lines (backscat-tered image) and appears as dented (topographic image). It is important to note that their abundances of the cleavage types differ significantly from one place to another, even within a polished section. This fact suggests that there might be a compositional heterogeneity in the original solid solution, that is, a difference in the degree of supersaturation, with the higher degree for the Type 1 and Type 2 assemblages than for the Type 3 assemblages. As experimentally confirmed by SUGAKI (1955), the crystallographic intergrowths described are concluded to have been formed by exsolution from the solid solution during cooling. In spite of the difference of the degree of supersaturation, the exsolution reactions under discussion are assumed to be of stepwise in such that the lamellae exsolved in the order of the Type 3→Type 2→Type 1 with decreasing temperatures.