Charakterystyka krystalochemiczna glaukonitu ze skał ordowiku obniżenia podlaskiego na tle litologii i genezy

Maria Turnau-Morawska, Bożena Łącka, Andrzej Wiewióra

Abstract


CRYSTALLOCHEMICAL CHARACTERISTIC OF GLAUCONITE FROM ORDOVICIAN ROCKS OF THE PODLASIE DEPRESSION IN RELATION TO LITHOLOGY AND ORIGIN

Summary

The authors performed a detailed crystallochemical analysis of glauconite from the Dobre borehole in the western part of the Podlasie Depression. The examined glauconite is a component of an Ordovician glauconitite belonging – after W. Bednarczyk (1969) – to the Lawer Arenigian. This mineral deserved a detailed examination as presenting an almost pure accumulation, easy to separate and as the relation to rocks neighbouring in the profile is well marked and characteristic. The authors took also into account the fact that the glauconite occurring in the Ordovician of Middle and north-eastern Poland often suggested some conclusions concerning conditions of sedimentation during Ordovician and sometimes helped to fix the stratigraphic position of the glauconite rock. However, no detailed crystallochemical investigation of Ordovician glauconite of Poland has been performed, whereas a fairly rich analytical material has been published abroad. The glauconite of the Dobre borehole was submitted to microscopic, X-ray, thermal and chemical analyses. The results of analyses were interpreted on diagrams which allowed conclusions on the position of the investigated glauconite in relation to other clay minerals, in relation to glauconites of different age, and also some conclusions as to the probable origin of this mineral. The glauconitite from which the glauconite was separated for analysis reposes on a conglomerate composed of phosphorite pebbles. Towards the top the glauconitite grades into a layer of clay minerals and hematite, whereas the Upper Arenigian is developed as a calcareous dolomite with nests of iron ores. Determined in a thin section the mineral composition of the glauconitite is as follows: glauconite 70.9, calcite 12.0, hematite with admixture of clay minerals 12.9, quartz 4.5% by volume. The glauconite has an intensive green colour and shapes characteristic for autochthonous glauconites. The cement of, the glauconitite is composed of calcite, hematite and clay minerals of optical character approaching that of glauconite. The X-ray analysis (Fig. 1) showed a well ordered structure of glauconite, a lack of larger amount of swelling layers in crystallites of the investigated glauconite and a low content of divalent octahedral cations. The DTA curve (Fig. 2) demonstrates a simple composition of the investigated sample and also a lack of larger amounts of swelling layers. The chemical analysis performed by classical methods gave a following formula of glauconite:

(K0.70Na0.01Ca0.04)[(Al0.76Fe3+0.78Mg0.31Fe2+0.15)(Si3.64Al0.36)O10(OH)2]

These results show that the glauconite from the Dobre borehole does not remarkably differ in its chemical composition from many other glauconites and its formula is fairly close to many chemical formulas of glauconites from various geological formations. It differs, however, from average chemical formulas of glauconites in having a higher amount of aluminium and lower amounts of divalent cations. It contains an inconspicuous number of swelling layers and represents an arrangement which may be treated as a monomineral aggregate in which it differs from most known glauconites which are very often heterogenous systems. As shown in Figs. 5 and 6 the data obtained place the glauconite from the Dobre borehole within the field of Ordovician glauconites. The crystallochemical characteristic of the investigated glauconite shows a full agreement between structure, composition and geological age of glauconites defined by V. D. Shutov et al. (1973). However, the authors emphasize the fact, that the origin of glauconite remains in many aspects uncertain. They express the view that the glauconite could not be formed directly from a solution because the necessary concentration of K+ does not exist in natural conditions. Consequently one must adopt a hypothesis of a glauconitization process of argillaceus material during sedimentation and especially during diagenesis (K. Smulikowski, 1954; J. F. Burst, 1956; J. Hover, 1961). V. D. Shutov et al. (1973) adopted a hypothesis of a diagenetic transformation of dioctahedral smectites, especially nontronite and also Fe-Al-montmorillonite. An alkaline environment favours the increase of fixation of Al3+ in the tetrahedral positions and the presence of K+ is a condition for transformation of smectites into micaceous minerals through an intermediate stage of mixed-layered phases. A lack of larger amounts of mixed-layers in the glauconite from the Dobre borehole, a high content of K+ and a well ordered structure demonstrate a remarkable advancement of the diagenetic transformation of the primary argillaceous weathering material deposited in the Ordovician sedimentation basin. The remarkably large amount of Al3+ in the octahedral position, lower than average content of Mg2+ and high content of K+ may be treated as an evidence that the transformation process attained the stage of illitization, which is in agreement with the view of V. D. Shutov et al. (1973), concerning the connection between the aluminium content especially the octahedral Al3+ in glauconite and its age and late diagenetic recrystallization.


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