NOTES OF THE ORIGIN OF GLAUCONITE. The origin of glauconite is still imperfectly understood. According to Murray and Renard,19 organic matter inclosed in the shells and present in the mud itself transforms the iron in the mud into sulphide, which may be oxidized into hydrate, sulphur being at the same time liberated. The sulphur becomes oxidized into sulphuric acid, which decomposes the fine clay setting free colloid silica, alumina being removed in solution. The colloid silica and hydroxide of iron are in a condition most favorable for their combination and, in the presence of potash salts in sea water, these substances form glauconite. Collet20 suggests in modification of these views three stages for which he cites evidence: (1) the formation of gray casts composed exclusively of clay; (2) brown casts of different shades representing different stages in the replacement of clay by peroxide of iron, no potash being present; (3) "glauconization," the introduction of potash and probably also of the water of constitution. Murray and Renard21 regard it as improbable that any minute grains of glauconite are formed in a free state in the mud. They think this mineral is formed in the cavities of calcareous organisms, though they admit that some grains appear to be highly altered fragments of ancient rocks, or coatings of glauconite on these rock fragments. Cayeux22 cites a variety of evidence to show that organic matter is not essential to the origin of glauconite grains. He ascribes an important share in their genesis to the so-called pigmentary glauconite and shows that grains of glauconite have originated or have continued to increase in size after all the elements of the accompanying sediments were in place as a consolidated deposit. He concludes that organic matter may be more often the primordial condition of the production of glauconite, but that it is very 19 Murray, J., and Renard, A. F., “Challenger Rept., Deep Sea Deposits," p. 389, 1891. 20 Collet, L. W., op. cit., pp. 176–178. 21 Murray, J., and Renard, A. F., op. cit., pp. 387-388. 22 Cayeux, L., op. cit., pp. 176–184. certain in many cases that organic matter has had no part in the genesis of this mineral. Collet23 also recognizes the important part that pigmentary glauconite may play in the origin of glauconite grains and adds that the absence of cement in microscopic sections of glauconite may be explained by the fact that both the glauconite grains and the glauconitic cement are cryptocrystalline and composed of particles differently oriented so that suture lines would be masked. On p. 555 the fine material washed from the composite samples of New Jersey greensand beds is shown to contain approximately three to seven per cent. of potash. It consists largely of greenish to yellowish colloidal matter that may be classed as pigmentary glauconite. It may have been part of the original deposit, in which for some reason grains of glauconite did not develop, or it may have been formed by the mechanical wear or disintegration of previously existing grains. It may, however, have originated by the chemical action of circulating waters. Possibly grains of glauconite may be forming in it today, but of this no direct evidence was observed. With reference to celadonite, a mineral closely related in composition to glauconite, Clarke 24 remarks: If, now, we assume that celandonite and glauconite are at bottom the same ferri-potassic silicate, differing only in their impurities, we may begin to see that the several modes of its formation are not absolutely different after all. Probably, in all their occurrences, the final reaction is the same, namely, the absorption of potassium and soluble silica by colloidal ferric hydroxide. In the ocean these materials are prepared by the action of decaying animal matter upon ferruginous clays and fragments of potassium-bearing silicates. In the sedimentary rocks, where glauconite appears as a late product the action of percolating waters upon the hydroxide would account for its formation. In igneous rocks the hydroxide is derived from augite, or perhaps from olivine. Thus the various productions of glauconite and celadonite become the results of a single process, which is exactly equivalent to that in which potassium compounds are taken up by clays. The observation of L. 23 Collet, L. W., op. cit., pp. 154-155. 24 Clarke, F. W., "The Data of Geochemistry," 4th ed., U. S. Geol. Survey Bull. 695, p. 514-515, 1920. Cayeux that glauconite is frequently present in arable soils, in all conditions from perfect freshness to complete alteration into limonite, suggests that perhaps the formation of the species is one of the modes by which potassium is withdrawn from its solution in the ground waters. This statement tends to confirm the suggestions presented above in the discussion of enrichment. SUMMARY. Separation of New Jersey greensand by washing shows an average of 19.5 per cent. of fines in samples representing the entire thickness of the marl at six localities, favorably situated for commercial enterprise. Magnetic separation of the residues shows that they contain on the average 89.9 per cent. of glauconite, and that the beds of unaltered greensand average 73 per cent. of glauconite grains. Much glauconitic material is also included in the fines. The most common size of the glauconite grains is between one twentieth and one fortieth of an inch; of quartz, one twentieth to one sixtieth of an inch. The shapes of glauconite grains are irregular, suggesting mechanical aggregation. The smaller grains resemble the larger but are more worn as if transported. Chemical analyses of greensand show a silica content of about 50 per cent. and a relatively high content of ferric iron. The potash content of the Hornerstown marl is about double that of the Manasquan. Only small amounts of the carbonate and phosphate of lime are present but the Manasquan marl contains more of these substances than the Hornerstown or the Navesink. Analyses of the coarse and fine products of wet separation show that these products have much the same composition. The potash content of the fines is about 0.1 to 1.5 per cent. lower than that of the unaltered marl. Comparison of analyses of recently formed glauconites with those of glauconites from older sedimentary formations shows that the older glauconites though still ferric have a greater percentage of ferrous iron than the others and contain considerably more potash. 566 The better New Jersey greensand beds are composed of nearly pure glauconite. The maximum amount of water soluble potash obtained in te analyses of 18 samples was .06 of 1 per cent. The waters that circulate through the marl are very dilute b probably contain far less potassium than the amount required i saturation. The potassium is probably being withdrawn iron solution. This action may produce enrichment throughout the marl beds but especially in the more clayey layers. Thin sections of glauconite grains show them as masses ci fine crystalline flakes without core, skeleton, or other structure. Glauconite is not all formed through the agency of organit matter. The greenish colloidal matter in greensand may be part of the original deposit, or may have originated by mechanical wear or disintegration of existing grains. It may have been formed by the chemical action of circulating waters. Clarke suggests the formation of glauconite is one of the means by which potas sium is withdrawn from solution in ground waters. U. S. GEOLOGICAL SURVEY, WASHINGTON, D. C. THE R AND S MOLYBDENUM MINE, TOAS COUNTY, NEW MEXICO. ESPER S. LARSEN AND CLARENCE S. Ross.1 While engaged in geologic reconnaissance in the southern part of the San Luis Valley for the United States Geological Survey under the direction of Whitman Cross during the fall of 1919, and incidental to this work the authors visited the interesting molybdenum mine near Questa in the north part of New Mexico. The following brief description of the mine and its geology is a result of this visit. Only one day was devoted to the area, and for this reason it was not found possible to study all of the important details and many problems concerning the origin of the ore remain unsolved. LOCATION AND TOPOGRAPHY. The R and S molybdenum mine is located in the western part of the Culebra Range in Toas County, New Mexico, on Sulphur Gulch, a northern branch of Red River, a stream that flows in a westerly direction until it empties into the Rio Grande. It lies about 8 miles up the river from Questa, N. Mex., and about 27 miles from Jarosa, Colo., on the San Luis Southern Railway. The mine lies at an elevation of about 8,700 feet above the sea level. The topography is sharp and near the mine the slopes on either side of Sulphur Gulch form almost shear cliffs. The streams have steep grades and erosion is proceeding at a rapid rate in the soft, altered rocks. HISTORY. The yellow molybdic ocher that formed as an alteration product at the outcrop of the veins was long regarded as sulphur and gave 1 Published with the permission of the Director of the U. S. Geol. Survey. |