O możliwości występowania minnesotaitu (talku żelazistego) w soli kamiennej Inowrocławia

Marek Bielski, Karol Prochazka

Abstract


On the occurrence of Minnesoitaite in the rock salt of the salt deposits at Inowrocław (Central Poland)

Iron-talc (minmesotaite) was found (to occur in admixtures contained in rock salt of the Zechstein salt deposit at Inowrocław. The rock Salt in question is stratigraphically assigned to the upper pant of the Younger Salt Horizon (Z3). The most characteristic authigenic minerals of Zechstein salt deposits are: talc, chlorites and muscovite (Breitsch, 1962). Talc is abundant in ainhydrite, post-anhydritic gyplsum and in anhydrite- and gypsten- -bearing rock salt. It 'displays plat y habit and occurs in poorly developed grains up to 0.5 mm iin size. Its sedimentary origin is manifested by- the presence of halite inclusions in cleavage fissuires and on translation planes. Growth zones of talc grains are often observed too. Minnesotaite-bearing salt is orange-pinikish in colour and consists of medium to coarse-grained halite. Its grains display tectonic deformations and are elongalted in shape. Colouration is due to haematite and goethite admixtuire. Anhydrite content amounts to 1.65 wt. pet cent. Gray clay admixture is much less abundant and generally concentrates on the boundaries of halite grains. Minnesotaite occurs in this rock in very negligible amounts (after dissolving 2.5 kg of salt it was possible to get only very small powder samples for the X-ray and microscopic study). Thus, it is not possible yet to define its distribution in the rock salt and to carry out the chemical analysis of this mineral, especially the determination of iron content. Minmesataite was identified by means of the X-ray powder method. Among, other minerals anhydrite, hematite, talc and chlorite were found to occur. The occurrence of minnesotaite was documented by the presence of three characteristic diffraction line: 9.51; 3.14 and 2.51 A (table 1). Their position is identical with that of talc lines but the intensity ratio is completely different. For minnesotaite this ratio was found to be. 10:5:3 (according to Gruner, 1944b — 10:5:2) whilst for talc — 10:9:5 (according to Joganse — 10:10:6 , and to Gruner, 1934 — 5:10:5), (table 1). Microscopic observations have shown that minnesotaite occurs in the form of fibrous grains and white-grayish flakes, displaying irregular shape. Most probably, this mineral is the product of alteration of talc. Iron which substituted magnesium in talc lattice originated either from admixtures contained in the rock salt or was introduced by Fe-rich K-Mg-bearing solutions, circulating within the deposit during tectonic movements. From the quantitative ratio of these minerals it appears that only small part of talc was altered into minnesotaite. It is supposed that after formation of the Inowrocław salt structure, there existed no longer favourable conditions for such substitution. Since the X-ray diffraction lines of other minerals occuring in admixtures of the Inowrocław salt under study do not overlap those of talc, the change of intensity can only be due to isomorphic substitutions (Fe2+ for Mg2+?). Though the intesity ratio of the three main reflections (table 1) corresponds to that reported by J. W. Grune r (1944 b) for minnesotaite, it is not possible yet to claim its identity with iron talc from the Mesabi Range. The mineral in question can represent an intermediate member between magnesium and iron varieties of talc.

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