T H E LIESEGASG I’HENOMEh’OT\T APPLIED TO THE LAKE: SUPERIOR IRON FORMATIONS* -
B Y ROBERT J . HARTMAN AND ROBERT McCULLOUGH DlCKEY
Introduction From an economic standpoint, the rock st.rata known as the Lake Superior iron formations are perhaps the most important natural deposits on the North American continent. To the geologist they afford innumerable and fascinating problems of structure and stratigraphy, of secondary ore concentration, and ultimately of origin. I t is with the origin of this formation that this paper proposes to deal, but it, will concern itself primarily with a single phase of a hypothetical process of origin, one which has either been completely overlooked heretofore, or which has been passed over with scant notice. The study lies within the legitimate province of the chemist, but his experimental work must be constantly limited by the geologic conditions either known definitely or strongly believed to have existed at the time of formation of these remarkable deposits. If it, is possible for the structure and composition of the rocks comprising the iron formations to be synthetically reproduced in the laboratory, within the limitat,ions imposed by the necessity of using only those elements in the reactions which could easily have existed in nature according to the most widely accepted geologic theories of origin, then a considerable advance will have been made. I t is not proposed t o enter into an involved discussion of the merits of the various hypotheses advanced by geologists to account for the deposition and occurrence of these rocks, except insofar as the results of the work described tend to support or disprove one or the other. K h a t are generally referred to as the iron formations are not in theniselves iron ores. The iron formations proper were probably originally cherty iron carbonate and gremalite (FelIgSiOs.nH20) which have undergow varying amounts of oxid:ition, leaching by circulating meteoric and magmatic waters, and metamorphism, leading to the production of minerals characteristic of these processes during the millions of years which have elapsed since their deposition. They vary from a few feet to one thousand feel in thickness, and contain roughly about 2 57’ iron. The iron ores of the region have developed under favorable geologic conditions through the leaching out of the amorphous or finely crystalline silica of the iron formations and oxidation of the ferrous salts to hematite, limonite, and other of the varied forms of the higher oxides of iron. In short, the development of iron ore in these instances is the result of secondary concentration, through various agencies, of the iron in the primary iron formations. *Contribution from the Laboratory of Colloid Chemistry of Indiana University.
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ROBERT J. HARTMAN AND ROBERT McCULLOUGH DICKEY
A peculiar and generally well-marked characteristic of these iron formations is their definite banding, which is particularly apparent in the rocks of some localities. Even a cursory examination suggests to the chemist the possibility of reactions in a silica gel having been of some importance in the formative processes. The iron carbonate, oxidized to different degrees, occurs in the iron formations in bands of varying thicknesses separated by other bands of chert, or silica. These bands exhibit no uniformity in thickness. Many attempts to explain this banding have been made, but all fall somewhat short of satisfying the conditions. It is true that no one explanation will suffice for the whole of the iron formations, since they were laid down over a n area thousands of square miles in extent and were doubtless subject to the greatest differences in conditions. Nevertheless, it is entirely possible that one t,ype of process may have been dominant, and the attempt to discover this certain type,of process has occupied many geologists for a number of years. Van Hise and Leith' assert that most of the iron salts and silica were contributed directly to marine waters by submarine basic lavas, that is, by direct contact of heated igneous material possessing a high ferromagnesian content, with the sea water. They show by a series of laboratory experiments that such a process is adequate to provide all the elements present in the iron formations, and their contention is supported by the close association in the Lake Superior region of vast amounts of weathered basic igneous rocks, greenstones and basalts, with the iron formations. Subordinat,e to this, they believe, wtzs the weathering of the igneous rocks when cooled, leading to transportation to settling basins by meteoric waters of iron in the form of the ferrous salts of silicic, carbonic, sulphuric, hydrochloric, and other acids. They mention t,he fact that when iron silicates make contact with calcium carbonate, ferrous carbonate is precipitated, tending t o agglomerat,e into bands separated by free silica. Gruner,2 on the other hand, contends that weathering of large land areas covered with greenstones and basalts was the predominant process. He postulates a humid climate, tropical or subtropical in nature, and abundant land vegetation of a low form affording rapid decay and solution of iron and silica as being of fundamental importance. According to his theory, most of the iron was carried to depositional basins as organic colloids or adsorbed by organic colloids, and was precipitated primarily through the influence of living algae and bacteria. Thus he subordinates inorganic reactions to organic. An elaboration of the Gruner theory is offered by Maynard and Moore,3 wherein they enter more thoroughly into the colloidal aspects. They contend that the banding is due to differential rates of precipitation of iron and silica, combined with seasonal changes causing varying quantities of these substances to be brought into the basins of deposition a t different times of the C. R. Van Hise and C. K. Leith: C. S.Geol. Sur., Mon. 52. 'Gruner: Econ. Geol., 17, 407 ( 1 9 2 2 ) ; 18, 612 ( 1 9 2 3 ) . E. S.Moore and J. E. Maynard: Econ. Geol., 24, 272, 365, 506 (1929).
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year. The emphasis of the banding, according to their idea, is due to metamorphic processes, although the distinctness of the banding for the most part may be regarded as primary. They have produced banding of a rough order by differential settling in sea water of well-agitated ferric hydroxide and silica. IlIention is made of the possibility of Liesegang’s rings having been of some importance. They prepared a gel of equal volumes of 1.16 density sodium silicate and 3 Ts sulphuric acid which was made 0.1N with respect to ferric chloride. The gel formed in a day or two, and was covered with ammonium hydroxide. In a month, ferric hydroxide bands developed. However, the authors claim that this process is inadequate to account for great thicknesses of the iron formation.
Experimental The experimental work detailed in this paper tends to support the theories of Van Hise and Leith. It demonstrates conclusively that reactions in a silica gel will produce, under proper conditions, well-defined bands of ferrous carbonate, separated by silica bands. The banding peculiar to t’he iron formations is faithfully reproduced by this method. Briefly, the geologic considerations entailed are the presence of ferrous iron, non-oxidizing conditions in the gel tending to keep the iron in its ferrous form, sodium silicate, carbon dioxide (hence some form of life), and relatively quiescent conditions. All these can be reconciled with the direct submarine magma contribution theory. The sodium silicate and iron could readily have been supplied from the magma itself. Van Hise and Leith’ found that when fresh basalt is heated to 1200’ Centigrade, so that the surface is fused, and the mass is plunged into salt water, sodium silicate is produced, together with small amounts of iron salts. Nitrogenous material and carbon dioxide may have been furnished by organisms; however, some of the carbon dioxide may have been supplied through action of the magma itself. Chamberlin and Salisbury? assert that carbon dioxide increases greatly in relative abundance with expiration of volcanic activity. If such is the case, most of this substance could have been supplied at a time when the period of violent igneous activitybad passed and quiescent, conditions had set in. When the Simplon tunnel was dug through the Alps, the workmen cut through a vein of silicic acid soft enough to dig out with the hand.3 I t is the contention of the authors that this iron formation was no doubt at one time in similar gelatinous form and that while in this state a reaction, nnalogous to the Liesegang phenomenon, took place, giving rise t o the ferrous carbonate bands. Dehydration, pressure, etc., through the ages leaves the iron formation in its solid state with its ferrous iron having been trapped and isolated from air, preventing it from further oxidation. I C . R. Van Hise and C. I
