Abstract
Magnesium calcite, which is usually observed in many natural low-temperature environments, was identified in Triassic limestones of the Polish part of the Germanic Basin. The occurrence of unstable magnesium calcite in Triassic limestones is remarkable. High-Mg calcite was identified in all beds of the Muschelkalk Karchowice Formation: Lower Crinoidea Beds, Lower Biohermal Beds, Upper Crinoidea Beds and Upper Biohermal Beds. The general chemical formulas of magnesium calcite of mentioned unites are: (Ca0.87–0.80,Mg0.13–0.20)CO3, (Ca0.83–0.75,Mg0.17–0.25)CO3, (Ca0.81,Mg0.19)CO3, (Ca0.78–0.77,Mg0.22–0.23)CO3 respectively. This mineral originated probably during the early stage of diagenesis, during compaction of limestones deposited in a warm, shallow marine environment. Some amounts of magnesium come from high-Mg skeletons of marine organisms (such as echinoderms). Diagnenetic processes and the conditions in seawater, especially higher salinity, influence the preservation of high magnesium calcite in the Muschelkalk limestone of the Polish part of the Germanic Basin.
Highlights
The area of the Polish part of the Germanic Basin is built of Muschelkalk limestones which include carbonate phases varying in magnesium content
Results of FTIR indicate that high magnesium calcite is present in limestones of all investigated strata
The values of the V3 absorption band show a higher content of Mg in high magnesium calcite of upper Biohermal limestone in comparison with this carbonate phase of other limestone formations
Summary
The area of the Polish part of the Germanic Basin is built of Muschelkalk limestones which include carbonate phases varying in magnesium content. Analyses of low-Mg calcite and high-Mg calcite show that these two carbonate phases differ, in stability, solubility and crystal structure. It is connected with different cationic size of Ca and Mg, ionic radii length and the strength of ionic bonds. The structure of a calcite crystal with Mg substitutions has weaker stability than the calcite without substitution. The solubility of high-Mg calcites is the same as that of low-Mg calcite and aragonite at temperatures below 42 °C. Between 25 and 42 °C, calcite with 15 mol% of MgCO3 content is stable in relation to low magnesium calcite but unstable with respect to aragonite (Bertram et al 1991).
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