Crystals structures of carbonate phases with Mg in Triassic rocks, mineral formation and transitions

Abstract

Detailed data on carbonate minerals different in magnesium content including their crystal structures were presented in this article. The samples were collected from the formations of the Lower Muschelkalk and the Upper Muschelkalk. X-ray diffraction and electron microprobe analysis were used for carbonate phases determination. The following minerals with different Mg content were identified: low-Mg calcite, high-Mg calcite, proto-dolomite, ordered dolomite and de-huntite. The content of Mg in high-Mg calcite is higher than in low-Mg calcite but lower than that of proto-dolomite. Proto-dolomite is characterized by lower content of Mg than typical for stoichiometric value for dolomite—ordered dolomite. Due to the reduced Mg content in identified huntite, this carbonate phase was named as de-huntite. The research results are significant because they are a source of new data on carbonate phases with different Mg content which build studied Triassic rocks and crystal structures of these minerals.

Geochemical and Mineralogical Characteristics of Triassic Dolomites from Upper Silesia, Poland

“…Therefore, this mineral phase was named de-huntite. The reduced magnesium content in de-huntite is probably the effect of the process of advanced-stage diagenesis-dehuntization [46]. This cannot exclude the theory, which assumes that the determined de-huntite phase may represent a huntite-dolomite solid solution or mixture.…”

“…The results of studies in the sample micro-areas have shown that high-magnesium calcite has a Mg content higher than "pure" (low-Mg) calcite but lower than that of protodolomite [34,36,46]. The protodolomite is characterized by a lower content of magnesium in relation to the stoichiometric value for dolomite but higher than that of high-magnesium calcite [34,36,46]. Ordered dolomite has a content of Mg close to the stoichiometric value.…”

“…On the basis of the microprobe measurements results, the chemical formulas of the carbonate phases were calculated. The results of studies in the sample micro-areas have shown that high-magnesium calcite has a Mg content higher than "pure" (low-Mg) calcite but lower than that of protodolomite [34,36,46]. The protodolomite is characterized by a lower content of magnesium in relation to the stoichiometric value for dolomite but higher than that of high-magnesium calcite [34,36,46].…”

“…Ordered dolomite has a content of Mg close to the stoichiometric value. Huntite determined by microprobe measurements is characterized by a lower Mg content than the stoichiometric value for this carbonate phase [34,36,46]. Therefore, this mineral phase was named de-huntite.…”