Występowanie, diageneza i oznaczanie kalcytu magnezowego

Irena Iwasińska, Marek Narkiewicz, Andrzej Pisera



This paper reviews the pertinent literature concerning mineralogy. occurrence diagenesis and identification of magnesian calcite. Numerous investigations of recent carbonate facies have revealed the potential importance of this mineral for interpretation of ancient calcareous and dolomitic deposits. Moreover, several useful petrological and geochemical criteria have been developed that permit some direct conclusions about a considerable Mg-calcite content in fossil sediments (e.g. 36. 14. 48). However, there is still a need for a key to the identification of a former magnesian-calcite micrite, including a micritic cement in particular.
The geological importance of Mg-calcite is related to its strong diagenetic activity being, in turn. caused by its metastable behaviour in eogenetic environment. Herein, magnesian calcite undergoes a relatively rapid nondestructive transformation into its stable low-magnesium variety, detailed mechanism of this process being still a matter of speculation (34, 39). In the course of a transformation vast amounts of free Mg ions are released and may form an important agent of local and distant dolomitization. The present experimental investigations have confirmed the applicability of widely used X-ray powder diffractometry for Mg-calcite identification (9). The Mg content of the recent and ancient carbonate samples has been determined using an experimental graph (Fig. 2) based upon the X-ray (Fig. 1) and emission spectroscopy data (Table 1). In the investigated fossil organic structures no magnesian calcite has been detected. even though some structures did contain aragonite. Slightly raised Mg content has been only found in the loose starfish plates (Korytnica clays, Miocene) and in the cryptoalgal laminite from the Middle Devonian of the Holy Cross Mts. (3.5 and 2.5 mol% respectively).

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