I have read with great interest the paper of Emmons, Stockwell and Jones1 on the subject named above which forms an important contribution to our knowledge of the sulphides and of many sulphide ore deposits. I desire in what follows to offer a supplement to this paper by drawing attention to articles already published upon the subject which have apparently escaped the attention of the authors. In the first place, both Ramsdell2 and at the same time F. Rinne3 have made röntgenographic investigations of argentite and acanthite from Freiberg in Saxony and of artificial silver sulphide. The latter has proved that the röntgenograms of these three substances are alike and show an apparently orthorhombic, not isometric symmetry. I was, however, the first to show, in 1922,4 by an optical method, the nonisometric and apparently orthorhombic symmetry of argentite. I was able to show that when examined in reflected polarized light between crossed nicols, polished sections of argentite and acanthite did not remain uniformly dark in all positions. On the contrary the individual grains varied in color as the preparation was turned under the microscope from light gray-blue in the position of illumination to dark gray or dark slate-blue in the position of extinction. Argentite and acanthite, which behave optically quite alike, are then optically anisotropic. It could generally be observed that an individual grain or even what appeared to be a crystal individual was actually not homogeneous but consisted of a large number of lamellae and of feather-like or spindle shaped parts which crossed one another for the most part, at right angles and possessed parallel extinction. Their arrangement was very similar to that of the lamellae which may be seen in leucite or boracite between crossed nicols at normal temperatures. I concluded as early as 1922 from this optical property which may be seen in well-polished preparations of argentite, sufficiently illuminated between crossed nicols, that, in considering the substance Ag2S, we are dealing with a paramorph, stable at ordinary temperatures as an orthorhombic modification, after an isometric modification, stable only at higher temperatures (above 180°). Rinne, in 1925, in the article cited, confirmed this conclusion and showed further that there was a complete analogy between the inversion forms of argentite and chalcocite.

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