Rare hydrated magnesium carbonate minerals of the kimberlite pipe Obnazhennaya, The Yakutian Kimberlite Province

The first discovery of hydrated magnesium carbonates, dypingite and nesquehonite, in the kimberlite pipe Obnazhennaya of the Kuoyka field, the Yakutian kimberlite province is described. The pipe is composed of kimberlite breccia with abundant diverse xenoliths of practically intact mantle rocks. Olivine in phenocrysts and mantle rock is generally intact. The main body of the rock is carbonate-serpentine. Nesquehonite and dypingite are rare minerals and have first been observed in relation to kimberlites. The minerals were found in the bedrock outcrop of the Obnazhennaya pipe as white crusts up to 5 mm thick scattered over an area of a few tens of square meters. To identify and study the crusts we used the following methods: powder X-ray diffraction, electron microscopy, and Raman scattering spectroscopy. A comprehensive study suggests that the main minerals of these epigenetic formations are hydrated carbonates: nesquehonite MgCO₃□3H₂O and dypingite Mg₅(CO₃)₄(OH)₂□5H₂O. Also, Raman scattering spectroscopy revealed a small proportion of hydromagnesite Mg₅(CO₃)₄(OH)₂□4H₂O. Hydrated magnesium carbonate minerals we found make a significant contribution to the collection of kimberlites. They are epigenetic in nature, with their origin being related to weathering of silicates, in particular serpentine. Mechanisms of carbonate formation appear to be close to that suggested by Wilson et. al., 2009, with CO₂ being trapped from the atmosphere to form nesquehonite. In the case of the Obnazhennaya pipe, mineral solutions form when rainwater filters through the talus at the top of the outcrop. They are enriched in Mg from minerals and trap CO₂ from the atmosphere. After filtering, solutions reach the vertical wall of kimberlite breccia where modern precipitation of nesquehonite upon evaporation occurs. Further, dypingite and hydromagnesite form via decomposition of nesquehonite. A lip extending over the rock wall significantly contributes to the development and stability of nesquehonite and dypingite aggregates. Crusts of nesquehonite and dypingite are not found on rock outcrops without lips at the top. Thus, despite the fact that intrusion of the kimberlite pipe occurred during the Jurassic (Zaitsev, Smelov, 2010), formation of nesquehonite and dypingite in association with kimberlite rocks continues in the modern time due to favorable environmental factors, first of all, a unique natural outcrop of kimberlite.