SAPONITE, THALITE, GREEN AL1TE, GREENSTONE1 (Read By

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BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

VOL. 28, PP. 329-332 JULY 15, 1912

SAPONITE, THALITE, GREEN AL1TE, GREENSTONE1

BY N. II. WINCHELL

(Read by title before the Society December 27, 1911)

CONTENTS P age Saponite...... 329 T ha lite...... 330 Greenalite...... 330 Serpentine...... 331 Conclusions...... 331

S a p o n i t e A soft, soapy earth, varying in color from nearly white to greenish and bluish colors, has been known for more than a hundred and fifty years. It has been called soapstone and porcellanous earth. It was found to fill cavities in rocks, and especially in those rocks that contain little or no potassium, such as basic trap rocks. It is a hydrous silicate of alumina and magnesia, essentially, but with a little iron and some times a little lime, and its optical elements have not been ascertained (System of Mineralogy). To this mineral Dana referred several species that were studied later and whose optical characters were determined, at least in part, and which had a similar origin, such as bowlingite, thalite, and some glauconite. Bowlingite was found to be derived directly from an alteration of olivine, one of the commonmagnesian minerals of basic igneous rock.2 Thalite was found to have an internal vermicular struc ture and definite crystalline elements,8 while glauconite as a term is divisible into true glauconite, carrying some potassium, and a potash- free variety which fills cavities in igneous rocks, and can easily be affili

1 Manuscript received by the Secretary of the Society December 13, 1911. 2 American Geologist, vol. xxiii, 1899, p. 43. 8 American Geologist, vol. xxiii, 1899, p. 41. Downloaded from gsabulletin.gsapubs.org on January 30, 2016

330 N. H. WINCHELL----SAPONITE, THALITE, GREENALITE, GREENSTONE

ated with saponite or with some of the species based 011 variations in “'serpentine.”

T h a l i t e Thalite was discovered and named by D. D. Owen when he examined the north shore of Lake Superior.4 It was found to be essentially a hy drated silicate of magnesia and alumina, formed along the shore of the lake, from the alteration of basic igneous rock where the breaking waves dashed over the rock. It occupies amygdaloidal and all shapeless cavi ties, some of the masses being several inches across, and sometimes it is disseminated in fine granules through the mass of the decaying diabase. Its color is usually dirty white or gray, but within the rock it frequently is light green.5 By Dana this mineral was placed under saponite, to which it has a chemical and physical likeness and a similarity of origin. It appears to have as much right to recognition as an independent species as several other species of a green color and soft and greasy feel, which are of like origin and composition, derived from the decay of basic igne ous rocks, several of which have been embraced under the general term serpentine.

G r e e n a l i t e Greenalite is a similar mineral having almost the same composition and an identical origin. It is found to result from alteration of basaltic glass, or obsidian, an original constituent of the rocks of the Mesabi iron range.“ It was named by Leith in a report on the Mesabi district in 1903,7 but had been discovered by J. E. Spurr several years before, who regarded it as a non-potash form of glauconite. It is not only sprinkled through the original rock, where considerable alteration has taken place and where the iron and the silica also have become segre gated into individual masses, but it also serves as a general matrix, sur rounding the other secondary ingredients. The original basic rock in this case was in the form of more or less rounded fragmental grains of obsidian, and the greenalite retains quite frequently the subglobular shapes of the fragmental grains. Leith has supposed the greenalite to have been a chemical oceanic precipitate, in the form of a ferrous silicate of magnesia and iron, from the waters of the cotemporary ocean, and to

* Geological report on Iowa, Wisconsin, and Minnesota, 1852, p. 600. 6 Pinal report, Geological Survey of Minnesota, vol. v, pp. 162, 168, 232, 238. 6 N. H. W inchell: “A diamond drill-core section of the Mesabi rocks.” Proceedings of the Lake Superior Mining Institute, 1910, 1911. 7 Monograph xliii, U. S. Geological Survey. Downloaded from gsabulletin.gsapubs.org on January 30, 2016

CONCLUSIONS 3 3 1

have been the source, through alteration and segregation, of the iron ore of the region.

S e r p e n t i n e Serpentine, according to the latest determinations,8 is not worthy of ' perpetuation as a name of a mineral species. It is rather a rock, and embraces mixtures of various greenish and usually ferrous, silicates of magnesia, or magnesia and alumina, combined with a large percentage of water, such as steatite (or talc), chrysotile, picrolite, antigorite, clino- chlore, and sometimes pennine,9 with other forms of chlorite. Serpen tine is abundantly produced by the decay of the Archean greenstones, whether the greenstones were of igneous and crystalline nature and massive in structure or fragmental and stratified, in which latter case they should rather be called greenwacke.

C o n c l u s i o n s From the foregoing it is apparent that the decay of a basic igneous rock gives rise characteristically to a group of green minerals, the com position of which varies from the silicates of iron and magnesia to sili cates of alumina, iron, and magnesia, all of them hydrated and rather soft, and it is evident that the prevailing green color of the Keewatin greenstones is due to the predominance of these secondary minerals rather than to the existence of amphiboles and pyroxenes. The am phiboles indeed are plainly secondary after these greenish products, and can be seen to have been formed in microscopic spicules in the midst of the yellowish green isotropic field or to form directly by crystalline change from the original pyroxene. If the question arises as to the whereabouts of the lime and soda, which were the alkaline elements in the original feldspars of the dia base, it can be answered by stating that they entered into the waters of the ocean, being more soluble, where they still remain, and that the existence of accessory quantities of lime in several of the secondary green minerals mentioned accounts for that portion of the lime which escaped such removal. While a green product is characteristic of a change of basic igneous rocks undergoing weathering, it appears to be true also that the different insoluble elements when present in too large quantity for the secondary minerals are sometimes segregated by themselves. Thus were formed

8 Lacroix : Min. de France et de les colonies, part 1, p. 417. »Final report, Minnesota Geological Survey, vol. v, p. 329, Downloaded from gsabulletin.gsapubs.org on January 30, 2016

332 N. H. WINCHELL----SAPONITE, T[IALITE, GREENALITE, GREENSTONE beds of iron oxide, and perhaps of iron carbonate, of silica, kaolin, and occasionally of marble. The idea that these hydrous silicates of iron and magnesia or any of them may be formed by chemical sedimentation from the oceanic waters is apparently unnecessary and impossible. If it were proven that they are soluble at oceanic temperatures the question arises, Why would they not be carried away by the currents of the ocean along with the soda and most of the lime ? Also, Why do they now have fragmental and, as on the Mesabi range, globular forms? Also, Why are they not found in distinct sedimentary sheets like marble and kaolin? And why do the associated chemical products, such as iron oxide and silica, present the same globular shape? It may be admitted that from an alkaline ocean there may have been chemical deposition of silica and iron oxide under certain conditions, but it is hard to understand why such deposit should in any case take the shapes which they and the greenalite exhibit on the Mesabi range! Since the iron ore and the silica, having the same origin and date, assumed identically the same shapes—that is, forms of detrital grains—it seems much more probable that a common cause controlled them all. There appears to be no possible hypothesis that will answer all the conditions but to assume that they took the shapes of preexisting detrital grains, and it follows from this that those grains were of such a nature that these secondary substances could be injected into them or produced by them. No detrital substance in the form of sand is known which is so easily altered as volcanic glass, or basaltic sand; and, in the light of the foregoing, it seems warrantable to infer that the original grains which gave shape to the greenalite and to the iron ore and to the rounded secondary quartz grains were grains of volcanic sand. Further, this hypothesis will not exclude the same alteration of adjacent sheets of lava and more or less crystallized trap, cotemporary with the produc tion of these minerals in the volcanic sand; and in the case of consider able quantities of obsidian, which was not broken and distributed as sand, such masses would have been liable to the same change, namely, they would be likely to lose all their natural ingredients, maintaining their shape and their fluidal structure, and would present the banded structure seen in jaspilyte. Such masses are found not only on the Mesabi range, but in the ore bodies of the Vermilion and Cuyuna ranges. Downloaded from gsabulletin.gsapubs.org on January 30, 2016 Geological Society of America Bulletin

Saponite, thalite, greenalite, greenstone

N. H. WINCHELL

Geological Society of America Bulletin 1912;23, no. 1;329-332 doi: 10.1130/GSAB-23-329

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