Development of aragonite crystals in the Neoproterozoic Nikol’skoe formation of the Ura uplift (East Siberia)

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It is established that organogenic limestones of the Neoproterozoic (Upper Riphean) Nikol’skoe Formation contain aragonite crystals surrounding filamentous calcified algae, which take part in the formation of stromatolites. They indicate the possibility of the development of fine-fibrous, acicular, and prismatic aragonite crystals in the calcareous mud during the calcification of red algal thalluses (for example, genus Chaptchaica Kolosov) enveloped by a common thick gelatinous (during the organism life) sheath. The group of red algae, which concentrate lime in the form of aragonite crusts appeared not later than the Neoproterozoic (Late Riphean). A high-quality image of the cell nucleus was obtained for the first time from the Neoproterozoic red algae Chaptchaica viva Kolosov, 1975.