COMPOSITION CHANGES IN SILURIAN SHALES – CONCLUSIONS BASED ON CALCITE CONCRETIONS RESEARCH
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The subject of the study were calcite concretions collected and studied from eight localities of the Barrandian Silurian shales.
The comparison of the results of detailed laboratory research of concretions and their host shales is as follows: In the concretions, calcite as the sole carbonate mineral, represents 60 - 90 wt % (arithmetic mean 78 + 16 wt %), while in the host shale its content is only about 7 to 26 % (arithmetic mean 15,4 + 8,7 wt %).
In insoluble residue of concretions the amount of Na-feldspar (occasionally also potassium feldspar) and quartz are distinctly higher than in the insoluble residue of the shales, which are richer in K2O and MgO. In the latter, beside quartz, dioctahedral mica – illite / muscovite, 14 Å chlorite and smectite constitute a substantial portion of the rock.
The semiquantitative X-ray analyses of insoluble residues carried out on XRD as well as on Guinier chamber, supported by results of chemical, thermal (DTA – GTA) and infra-red analyses were used for calculation of contents of albite and dioctahedral illite / muscovite. Both these minerals were used as internal standards for fixing the content of further minerals, quartz, K-feldspar, chlorite and smectite.
Mutual comparisons of the composition of particular concretions and their host sediments revealed a decrease of clastic quartz and feldspars grains in insoluble residue of shales by 10 – 20 per cent. This dissolution was compensated by the formation of authigenic illite, chlorite and montmorillonite. The decrease in amount of clastic quartz and feldspars is inversely proportional to the present calcite content in the host shale.
A possible wider exploitation of carbonate concretions as an appropriate instrument for the study and research of diagenetic changes of the original composition and structure of shales can be recommended. However, it has been debased by the fact that in insoluble residue of many carbonate rocks (e.g.limestones) we can often find a diagenetic / authigenic growths of clastic quartz and feldspars grains.
As the growth of a calcite concretion proceeds quickly (the difference in the content of calcite between the central part and the outskirts of the concretion is less then 3 %) and, as the residual porosity in concretions is minimal, the difference between insoluble residue of concretion and host shale can be counted as a result of the conservation of clastic minerals in concretions on one side and their dissolution in host sediments on the other side. In shales, formation of sheet silicate (illite, chlorite, and smectite) proceeds.