Namsan karst cave is located in the northeastern edge of the Pyongyang lift, Phyongnam trough, Korean peninsula. In the study area, the rocks of Paleozoic Mandal Formation are distributed and the karst cave is formed in the limestone of Mandal Formation lower layer.
The mineralogical characteristics of the ultrafine calcite(UFC) in the Namsan karst cave were studied. The UFC formed in the cave comes in the form of a gel with white or light yellow very fine mineral particles containing water. Chemical composition is CaO 55.62%, MgO 0.3%, SiO2 0.09%, Al2O3 0.01%, Fe2O3 0.03% and ignition loss 43.78%.
As a result of laser particle size analysis, the minimum particle size of UFC was 0.12µm, the maximum size was 5.25µm, and it is characterized in that the average particle size(1.39µm) is very small and the size distribution is narrow. The pyrolysis temperature(630 ∼ 900◦C) is much lower than the theoretical that of calcite. Infrared absorption curves with a wavelength of 711.8, 873.9, 1423.6 and 1797.9cm−1 agree very well with the curve of typical carbonate minerals. Microscopic observation shows that almost round individual particles are formed into a net shape by being formed into a chain shape. The crystal particle size of UFC by X-ray diffraction (XRD) analysis is 149.3µm. As a result of XRD, the crystal structure of UFC is in agreement with the crystal structure of the chain-like calcium carbonate produced by the synthetic method and is confirmed as a chain-like UFC not known in nature hitherto.
The pH of UFC is 7.3, the covering power is 56.8g/m2, and the predicted specific surface area is 1989.19m2/kg. In Namsan karst cave, the UFC is basically divided into 20 ∼ 30cm thickness on the ceiling of the cave and 30 ∼ 50cm thickness on the middle of the cave bottom sediment layer. UFC in the Namsan karst cave is very important in not only mineralogy but also resources.
Our results were published in "Carbonates and Evaporites" under the title of "Ultrafine calcite formed in Namsan karst cave of DPR of Korea" (https://doi.org/10.1007/s13146-020-00645-x).
