examination; still we find coal possessing its full coking quality within 50 ft. of where the seam or part of it has been actually burned out. A fault or fracture line, however, may carry the effect of the burning coal a considerable distance farther; we have found places where the gases from combustion have condensed and penetrated all the fracture planes of the coal and left the most beautiful analine dye stains imaginable, arranged in streaks shading off irregularly one into the other.

ALEXANDER BOWIE (communication to the Secretary*).—The belief of Mr. Williams that the fire at the Somerset mine is in a coal bed above the one now being mined is possibly correct, but that phase of the question must remain a matter of opinion until we have more evidence than we now have on the subject.

His statement that "where we ran up against the burned coal in our mines in Utah there was no evidence of it until we were 25 or 30 ft. from it" coincides with the experience we had on the west side of the Juanita Co.'s King mine. My statement describing the peculiar condition of the coal next the outcrop should be modified by stating that it applied to the ground developed by the main entry and the region east of it, and not to the west side of the mine. The seam of coal smut on the east side of the mine which we found 5 ft. above the seam we are now working, and which we believe to be a partly burned coal bed that was responsible for the peculiar condition of the coal below it, does not exist on the west side of the mine.

Even if we assume that we are correct in the supposition that the heat generated by the coal bed of which the smut is the residue was the cause of the deterioration of the coal bed we are working, it does not necessarily follow that we should have the same result at Somerset from the burning of an upper bed, as it is quite possible that a coal bed might be on fire above the one being mined, and near enough to give it a great deal of heat, but too far away to affect the physical characteristics of the coal.

* Received June 4, 1914.

VOL. XLVIII.—13

To What Extent is Chalcocite a Primary, and to what Extent a Secondary, Mineral in Ore Deposits

A discussion at the New York Meeting, February, 1914

L. C. GRATON, Cambridge, Mass.-The subject of chalcocite occurrence and its geological significance has, of course, a very important commercial bearing, as shown by the recent remark of a hard-headed mining man: "This secondary enrichment business is one thing you geologists have put up which we can hang on to, and come out right.

Most enrichments of copper ores depend upon the formation of chalcocite from leaner sulphides. Recently it has been shown that not only does there exist this secondary or derived chalcocite, but that in certain ores the mineral is present as an initial or original constituent, which therefore we may call primary. The desirability of distinguishing between these two kinds of chalcocite is evident and has recently been emphasized. That constitutes the first problem in this connection. I believe many who are working upon secondary enrichment of mineral deposits are giving much attention to that particular question.

It is now possible to distinguish primary from secondary chalcocite in many cases. In such cases, what do we know of the conditions under which this secondary or derived chalcocite was formed? This is the second main question in connection with the subject. It is now plain, at least in many instances, that the formation of this derived chalcocite represents a phase of oxidation if we have in mind the primary ore as the thing acted upon. The next important question-and it is most important is: does such oxidation go on above the zone where the rocks are saturated with water-that is, above the water level or below, or indiscriminately above and below?

Many of you have, or have had, opportunities to study at first hand, day after day, occurrences that enable valuable conclusions to be drawn upon these and other important questions concerning the formation and significance of chalcocite. It is greatly to be hoped that your testimony will be made available and undoubtedly it will go far toward clearing up some of these doubtful points.

JOHN D. IRVING, New Haven, Conn.-A short time before this meeting was called, H. V. Winchell, who was unable to be present, requested me to exhibit and describe these two samples of pyrite. (Exhibiting specimens.) They consist of fragments of pyrite inclosed in two small bottles. The first bottle shows fragments of pyrite which were immersed in water carrying copper sulphate but without SO2 for a period of two years. They remained comparatively clear and bright. No chalcocite has been deposited on them. The second bottle shows fragments of

pyrite which were immersed in a solution in every way similar except that it was saturated with SO2. Immersion was for a period of one year. On removing the pyrite it was found to be coated with gray metallic chalcocite. The coating covers all parts of the pyrite grains, so that unless the grains are broken or scratched they appear to be solid chalcocite.

Members of the Institute may recall that in 1903 Mr. Winchell contributed an article1 in which he described certain experiments in the precipitation of chalcocite by pyrite from mine waters. The specimens which have been exhibited are those of the pyrite which was used in the original experiments described by Mr. Winchell. The portion of his paper which deals with these experiments is found on pages 272 to 275 of the article above mentioned. In order that the meaning of the specimens may be clear to the members the following paragraphs are quoted from Mr. Winchell's paper:

"Artificial Formation of Chalcocite"

"After considering the geological history and physical structure of these ore deposits, the writer came to the conclusion some three years ago that the copper glance was formed by a chemical reaction between copper sulphate in solution in descending waters and the iron pyrites and other primary sulphide minerals lying below. In order to ascertain the truth or falsity of this theory, laboratory experiments were undertaken by the writer and carried on by Messrs. S. J. Gormly and C. F. Tolman in the laboratories of the Anaconda Copper Mining Company.

"The first experiments were conducted with a relatively small amount of cupriferous pyrite and a dilute solution of copper sulphate. The results, as reported, show the formation first of SO, and then of H2SO4; the solution of both copper and iron and the precipitation of the iron as ferric hydrate, and the formation of copper sulphide. "Analyses of the mine waters showed no ferrous salt in the strong copper water, but disclosed the presence of quantities of cuprous salts, in acid solution.

"The experiments repeatedly showed that SO2 is formed by the action of pyrite and chalcopyrite upon CuSO4, and that the SO2 reduces some of the copper ions of the CuSO, to the cuprous form. According to theoretical chemistry, a relatively insoluble compound may be precipitated by very small amounts of a salt containing one of the ions of the insoluble compound, if a large amount of the salt containing the other ion is present. To test this, a solution of copper sulphate was treated with the sulphides of arsenic, lead, copper, iron, zinc, and with pyrite; and in each case copper sulphide was precipitated, proving that these sulphides may precipitate copper sulphides from a solution of a copper salt. It is probable also that the more insoluble the precipitating sulphide, the more concentrated must be the solution of copper sulphate.

"To produce a solution containing cuprous ions, the above mentioned sulphides were treated with a solution of copper sulphate (CuSO1) and SO2, and precipitates were formed in each instance. An analysis of the precipitate formed by copper sulphide showed a precipitation of 12 per cent. of the weight of the original CuS as Cu S, indicating the formation of chalcocite under these conditions.

1 Synthesis of Chalcocite and Its Genesis at Butte, Montana. Bulletin of the Geological Society of America, vol. xiv, pp. 269 to 276 (1902); also Enginering and Mining Journal, vol. lxxxiv, No. 23, pp. 1067 to 1070 (Dec. 7, 1907).

"It was not ascertained whether the iron salts will reduce enough copper to form Cu2S in presence of pyrite or other sulphides, or whether the SO2 formed by solution of the pyrite and other sulphides is the active agent.

"Knowing full well that it might be urged that the formation of a precipitate of a certain chemical composition is quite a different matter from the production of a mineral having the same composition, the experiment now about to be briefly described was undertaken and carried to completion with exceedingly gratifying and satisfactory results.

"In a slightly acid solution containing sulphurous anhydride (SO2) was digested pyrite (FeS2) at ordinary temperature and pressure for three months. The pyrite taken was ordinary 'jig concentrates,' about one-fourth of an inch in diameter, from the Parrot concentrator at Butte, and of the following composition:

"Dividing these results by the molecular weights, the molecular constitution is represented as follows:

"After standing for three months in an ordinarily well lighted room, inclosed in a sealed jar to exclude the atmosphere, the formerly yellow grains of pyrite were completely plated with a solid coating of a dark blue-black mineral, and so closely resemble grains of solid chalcocite that they can only be distinguished from the latter by breaking them open, while in another jar which stood alongside, similarly sealed and exposed to light and ordinary temperature, containing pyrite and copper sulphate solution (but no SO2), the grains of pyrite were just as bright and yellow as before they were immersed. Indeed, there has been no visible alteration on the surface of grains which have now been thus immersed in copper sulphate without SO2 for two years, while in an adjacent jar containing SO2 there has been formed what appears to be, and undoubtedly is, a fine coating of chalcocite.

"From the first jar there were taken some of the larger grains for analyses, with results as follows:

"The molecular constitution is now Fe, 0.716; Cu, 0.057, and S, 1.448. There are .016 equivalents of sulphur left over (after calculating the iron as FeS2) to unite with the copper. The exact theoretical amount to form Cu2S is .014, and the surplus sulphur may be combined with zinc, lime, or manganese. That the mineral coating thus formed on the jig-concentrates is clearly chalcocite can not be doubted from a mere inspection of the samples and comparison with ore from the mines."

So much has been said in discussions on secondary chalcocite concerning the prevalence of a black powder, or what is commonly known as "sooty chalcocite," that Mr. Winchell desires to call the attention of the members to the fact that the chalcocite secured in his experiments wast not of the sooty variety, but rather of the usual gray variety with metallic luster. It forms, as may be seen in the sample which stood for a long time in water carrying an excess of SO2, a thin, glistening, grayish coating on the outside of the pyrite and bears no resemblance to the usual “lamp black" variety.

While the "sooty" type is therefore to be considered as secondary, the gray massive variety may likewise originate in the same manner.

THOMAS T. READ, New York, N. Y.-In 1905 I did some experimental work along much the same lines as that which Mr. Winchell has done, because it was so obviously of great significance, and found that the presence of SO2 was apparently exceedingly effective in bringing about this condition. The action was not at all marked in the absence of SO2, but where the SO2 was present it was very marked.

A difficulty in drawing conclusions from this fact, however, is that W. H. Emmons reports that he has been unable to detect the presence of SO2 in mine waters. Geologists commonly regard SO2 as a reducing agent, possibly because SO2 is the agent in which we used to reduce gold chloride in the laboratory, in order to precipitate gold. But SO2 is, obviously, exceedingly rich in oxygen, and toward FeS2 might act as a fairly strong oxidizing agent. That induced me, at that time, in a paper presented to the Institute, to suggest that the action of copper sulphate on the sulphides is ascribable to ordinary oxidation and that secondary sulphides might be considered to be oxidation products, just as, for example, in the copper converter, from a sulphide which is quite low in copper you can, by oxidation, get one which is high in copper. The rich sulphides of copper may be and in some cases doubtless are due simply to the effects of oxidation on lean sulphides.

L. C. GRATON.-In connection with the synthetic production of chalcocite, it may be pointed out that results like those in which Messrs. Winchell and Reed produced coatings on the primary sulphides, are indeed positive, but that negative results, such as their failure to secure such reactions without the presence of SO2, cannot be relied upon with entire safety as indicating that in nature some reducing agent like SO2 is required for the operation of the secondary enrichment process. The