Пустовалов Л.В., Новые данные о происхождении липецких и тульских железных руд

In the environs of Lipetsk and Tula the ore-bearing bed is often almost wholly consisting of such geodes. These geodes present more or less rounded hollow bodies, from 2,3 cm. to 1 1 m. in diameter. The inner cavity is usually completely or only in part filled up by sand and clay, the material inclosed in the geode corresponding to that in which the geode is embedded; in cases where the geode lies in a sandy me­ dium it encloses loose sand, in the case of a clayey medium, it includes clay. Sometimes the geodes contain some turbid water. The Avails of the geodes present a more or less clayey limonite (if they are embedded in clay), or ferruginous sandstone (if they are disposed in sand). In the Avails of the geode a concentric structure is nearly always discernible, the clayey and arenaceous concentres alternating Avith concentric zones of clayey limonite or of ferruginous sandstone. Figs. 1 and 2 represent geodes, showing Avith especial prominence the alternation of the concentres of ferruginous sand­ stone and of white, loose sand. The upper and side surfaces of the inner cavity of geodes are often coated by a thin layer of hydrogothite 3F20 3 -47/20 ; the latter has in fracture a stellate structure, and a budlike, encrusting surface. Sometimes upon the hydorgothite, also Avithin the geode, gothite Fe20 3 H 2О is deposited. Thus in proceeding from the surface of the geode tOAvards its center, we are first passing a layer of arena- ceous or clayey limonite, then, a layer of hydrogothite and lastly, in some cases, — of gothite. Consequently, inside the geodes are deposited ferric hy­ droxides with a lower content of water, than on their outer side. Basing on his field observations the author explains the process of formation of geodes in the following way: the colloidal ferric hydroxides, being more or less evenly distributed in the arenaceous or clayey rock of hog origin acquire mobility; they begin to move from a certain centre; due to this at some distance from this centre a ferruginous ring is formed. Towards this ring (or belter to say— closes sphere) colloidal ferric hydroxides are moving also from the surroun­ ding rock (fig. 1,p. 56). In that way a hollow body is gradually formed enclosing both inside of it and within its walls that material (clay, sand) among which it has arisen. During the formation of the Avails of geodes the colloidal ferric hydroxides undergo a process of recrystallization accompanied by a reduction of volume. Therefore a cavity is formed Avithin the geodes, its size being the greater, the larger has been the amount of ferruginous material contained in the surroun­ ding rock. The passage of the colloidal ferric hydroxides into a crystalline slate is accompanied by the separation of colloidal water Avhich, in absence of cracks in the Avails of the geodes may be preserved in their inner cavity. Moreover, inside the geodes there goes on a process of dehydration, namely, the conversion of limonite into hydrogothite and further, — into gothite. The water separated during there processes may also be concentrated Avithin the geode. Thus, Avithin the geodes we are finding both «colloidal» and «constitutional» water. The process of dehydration of limonite may go on exclusively in the upper part of the inner cavity of geodes, for the moisture which is ahvays present heloAv hinders the process of dehydration. Due to this, the hydrogothite and gothite are always disposed Avithin the geode in its upper parts while at base these minerals, as a rule, are absent. The above exposed scheme of formation of geodes of limonite is confirmed by that in cases Avhere the geodes are confined to an arenaceous medium, they alvvays include a snOAV-xvhite sand; for some distance about the geodes, the sand also shoAvs a distinct decoloration, Avhereas the main mass of the sand has a more or less intense ferruginous or brown colour. Directed by the author, N. N. S h a p o s h n i k o v realized some experiments 5*