栃木県鹿沼市堂ノ入鉱山産ソーダ南部石

概要

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Natronambulite from the Dohnoiri mine was found as the second occurrence in Japan. It is an essential constituent of bedded manganese ores formed by very weak metamorphism. The other manganese minerals are braunite, rhodochrosite and rhodonite. They occur as minute grain aggregates forming fine bands with or without quartz. After the calculation of Li_2O and H_2O contents, EDS analyses are idealized as (Na,Li,Ca)(Mn,Ca,Mg)_4[Si_5O_<14>(OH,O)], where Na>Li, Mn>Ca, Mg and OH>O, substantiating the original interpretation of essential nature of Ca substituting Mn. Natronambulite and nambulite in Japan have been found in weakly to moderately meta-morphosed bedded manganese ore deposits which are principally developed in such a siliceous rock as chert in Jurassic melange. The ore minerals therefrom include braunite without exception. It is thus very likely that the oxidizing condition might have helped the concentration of lithium. Two natronambulites in Japan were found from a member of low grade siliceous bedded manganese ores, whereas nambulite are found as veinlets cutting low to high grade manganese ores in all the known Japanese localities. These two lithium-manganese silicates might have been confused due to the color with rhodonite or pyroxmangite, especially high iron materials of the former, suggesting the more common occurrences.