EXPERIMENTS SEEKING TO EXPLAIN THE ORIGIN OF MOLAR TOOTH CALCITE AND DOLOMITE IN CYCLES OF THE PROTEROZOIC HELENA FORMATION, BELT SUPERGROUP, MONTANA

Molar tooth structures in the Proterozoic Belt Supergroup were formed by gas expansion bubbles and cracks filled by calcite. They are common in the dolomitic upper parts of Helena Formation cycles. Eroded pockets at the bases of these cycles contain molar tooth chips and only occasional dolomite chips, revealing the early lithification of molar tooth and some dolomite. We revisited the "dolomite problem" with experiments seeking to precipitate unstable Mg:Ca carbonate minerals that have the potential to re-crystallize to calcite and dolomite. The reagents CaCl and MgCl*6H₂O were mixed at different concentrations and added to solutions of NaHCO₃. The precipitates were analyzed by SEM and XRD methods. At a pH of 6.9 carbonate with a Mg:Ca molecular ratio of 26:74 was precipitated that produced the X-ray pattern of the dolomite group mineral kutnohorite. When experiments used excess NaHCO₃ the reaction expelled CO₂ gas at a pH close to 6 and precipitated calcite despite the Mg ions in the system. Calcite forming conditions similar to those created in the lab may be responsible for the synthesis of molar tooth calcite. We propose that CO₂ released from microbial decomposition of organic matter produced both the molar tooth voids and the inorganic chemical reaction that precipitated the void-filling calcite. The release of CO₂ into sediment pore water may have caused a drop in pH with the formation of HCO₃^-. Then the reaction Ca(HCO₃)₂ ® CaCO₃ + H₂O + CO₂ may have precipitated the calcite that filled the molar tooth voids before dolomite formation. Dolomite of the Helena Formation may have formed by Ostwald Ripening of an unstable CaMg(CO₃) mineral, such as kutnohorite.