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This study presents the first crystal-structure determination of natural MgCO3·5H2O, mineral lansfordite, in comparison with previous structural works performed on synthetic analogues. A new prototype single-crystal X-ray diffractometer allowed us to measure an extremely small crystal (i.e. 0.020 mm × 0.010 mm × 0.005 mm) and refine anisotropically all non-hydrogen atoms in the structure and provide a robust hydrogen-bond arrangement. Our new data confirm that natural lansfordite can be stable for several months at room temperature, in contrast with previous works, which reported that such a mineral could be stable only below 10°C.

The relevance of the work is conditioned by the need for a comprehensive study of the mineralogical and microbiological parameters of the Shulgan-Tash (Kapova) cave, which is an archeological monument of universal importance. The purpose of the work: to describe findings of subaquatic stalactites in the Shulgan-Tash cave and to solve mineralogical and genetic issues. Research methodology: hydrochemical, electron microscopic and microbiological techniques of research were used in the present work. Results. This article describes subaqueous stalactoids for the first time in Russia. In the foreign literature they are called "Pool Fingers". They are the carbonate forming with the origin, which is not yet definitely installed. They are emerging in the coastal zones of the cave waters. Subaqueous stalactoids were found in the Dal'nee Verkhnee lake on the second floor of the Shulgan-Tash cave 700 meters from the entrance. Subaqueous stalactoids are widespread along the Eastern and Western shores of the lake. They are thin and elongated aggregates of cream-colored up to a length of 30 cm, composed of calcite. They were formed in the lake of calcium bicarbonate type water with total mineralization of 390-510 mg/l, their hydrogen index is close to neutral (pH 6.8-7.3) and minor (not more than 5 mg/l) magnesium, sulfates and chlorides. Stalactitical surfaces are covered with the biofilm of the extracellular polymeric substances (EPS) and microorganisms. In their environment, the crystallization of calcite takes place. The "filaments", whose taxonomic affiliation is not clear are found inside the units in the flattened strands of biological origin. Calcite is a block crystal subindividuals, which possess the elements of splitting. Their size is about 200-300 µm. The manifestation of skeletal crystal growth happens rather often. Researches prove the participation of microorganisms in the origin of subaqueous stalactoids. Apparently, filamentous bacterial mats play the role of the primers and a "framework", which provides a gravity-oriented growth of the aggregates. Extracellular polymeric substance (EPS) has the ability to form complex compounds with ions of Ca 2+ . It may indirectly lead to the precipitation of calcite. Conclusions. The microbial mediated genesis, similar to that described in the literature of the North American counterparts, was substantiated for the subaqueous stalactoids (Pool Fingers).Keywords: subaqueous stalactoids, Pool Fingers, secondary mineral formation, the carbonates, the calcite skeletal crystals, bacteria, Shulgan-Tash cave. IntroductionThe relevance and purpose of the work The subject of our research is the microclimate and gas composition of the atmosphere [1, 2]. The subject consequently includes modern mineral formation, the study of rare mineral species [3], and the isotopic Geochemistry of elements in the minerals of caves. One of the objects of our research is the cave Shulgan-Tash in Bashkortostan. It is famous for its Paleolithic paintings. To preserve them, the monitoring and study o...

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