Molybdenum Minerals - Molybdenum Blue and Molybdenite - Formed as Volcanic Sublimates at Satsuma-1wO-Jima Volcano

Abstract

A beautiful blue volcanic sublimate is found around the high temperature fumaroles (400°C) of Satsuma-iwei-jima Volcano. The identification of the mineral by the usual methods is difficult because it is amorphous (Fig.1) and is deposited as very thin films on alteration products. It is readily soluble in water giving a colloidal solution and can be extracted with organic solvents such as 1-butanol, ethyl methyl ketone, and ethyl acetate etc. The maximum absorption of the aqueous solution was found around 700 ma (Fig.2). The chemical properties agree well with those of synthetic molybdenum blue. From these facts, it is concluded that the blue sublimate is molybdenum blue. The 0/Mo atomic ratio of specimens purified by means of solvent extraction are within a range between 2.77 and 2.84 (synthetic: 2.84, 2.89).Molybdenite is found inside the low temperature fumaroles (-200°C). It was identified by the X-ray diffractometry (Fig.3). The chemical analysis of a specimen shows Mo 52.7 %, S 35.3 %, and Si02 (impurity) 10.9 %.Further, molybdenum contents of some volcanic rocks and volcanic gases in Japan were determined. The condensed water of the high temperature (>550°C) fumarolic gases of Satsuma-iwo-jima contains 0.3-7 mg Mo// (Table 5). On the other hand, molybdenum can not be detected in the gases of lower temperature fumaroles (Table 5) and in those of the other volcanoes (Table 6). Molybdenum contents of the volcanic rocks (17 samples) fall within a range between 0.5 and 3.4 pgig (Table 7).From the above data and the results of the thermodynamic calculations (Table 9), the mechanisms for the formation of these minerals are presumed as follows: Molybdenum is transported by high temperature volcanic gas (>800°K) as its hydroxide or oxihalides to the oriffice of the fumaroles. Molybdeum blue is formed there by cooling and by increasing partial pressure of oxygen caused by mixing of air. Molybdenite is formed as a result of the reaction of hydrogen sulfide in the fumarolic gas with molybdenum blue which was dissolved and transported into the lower temperature fumaroles by rain water.