Geochemical Characterization of Subaqueous Molten Sulfur at Active
Crater Lakes
Bokuichiro Takano1
2Ko-ichi Nakamura

1 Graduate School of Arts and Sciences, The University of Tokyo
Komaba 3-8-1 Meguro-ku, Tokyo 153-8902, Japan
2 National Institute of Advanced Industrial and Science and Technology
Institute of Geology and Geoinformation
AIST Tsukuba central 7, Tsukuba, Ibaraki 305-8567, Japan

1. Introduction
Considerable numbers of molten sulfur flows and pools have been reported at active
volcanoes and hydrothermal areas in the world. However, studies on their geochemical implications
have been very few, although some works on their isotopic characteristics have been reported forsome volcanoes. On the other hand, elemental sulfur deposits along the volcanic gas-rising network
underground, and is very likely to occur through interactions between volcanic gas and liquid
elemental sulfur. In this paper we discuss the chemical interactions between liquid sulfur and H2S,
SO2 gases, and intend to extract some geochemical implications from the published as well as nonpublished data.

2. Gas Solubility
Molten sulfur has been known to dissolve some gases such as H2S, SO2, CS2 etc. Amongthese gases H2S dissolves in molten sulfur in a peculiar way as demonstrated by Fanelli et al. (1949).
Wiewiorowski and Touro (1966) reasonably interpreted the solubility in terms of sulfane (H2Sx)
formation in the liquid. Dissolved H2S excluding H2Sx follows Henry’s law as normal gas
dissolution in liquids.

ln (C/p)H2S = 412/T − 4.13

(1)

On the other hand, SO2 gas in liquid sulfur obeys Henry’s law (Touro and Wiewiorowski, 1966),

ln (C/p)SO2 = 931/T − 5.32

(2)

The relationships between temperature and the reciprocal of the Henry’s constant for the H2S andSO2 gases reveal that in the temperature range of >167°C H2S is more soluble than SO2 in liquidsulfur if the partial pressures of both gases are equal. This suggests a likely process that (SO2/H2S)
ratios of rising volcanic gas may increase through interaction with pre-deposited sulfur liquid along
the conduit. Sulfane formation in the liquid sulfur may partly contribute to raise the ratio at elevatedtemperature, although rates of its formation and decomposition in the liquid are relatively slow
compared with the H2S solubility process.

3. Homocyclic sulfur in liquid elemental sulfur: Does hydrogen sulfide change thedistribution of homocyclic sulfur?
Elemental sulfur once melted has been known to contain the Sx allotrope (x=6~20) in various
concentrations. Among them S7 is most abundant, often up to a few % (w/w) at ca. 160°C. Takano etal. (1994) found that the S7/S8 ratios of the solidified sulfur are considerably lower than those ofpure sulfur melts at the same temperature. In addition, however, the sulfur samples from the Yugama
lake showed the S7/S8 ratio similar to the pure sulfur melt when they were re-melted in theatmosphere. Where does this difference come from? We think that the difference between pure sulfur
and the molten sulfur in nature is likely to come from the coexistence of H2S gas with the naturalmolten sulfur. The mechanism resulting in the difference in the S7/S8 ratio could be explained by thefollowing reactions:

nS8 = -(S8)n-(S)n- = -(S7)m- + -(S9)n-m-(S7)m- = mS7-(S7)m- + H2S = H2S7(3)
(4)
(5)
(6)

Hydrogen atoms from H2S dissolved in liquid sulfur terminates polymerization of smaller sulfur
chains, resulting in lowering of the liquid viscosity. The equations from (1) to (4) may give othersulfanes together with homocyles other than S7 and S8. Blockage of the terminal S radicals (Poulis etal., 1962) in chainlike sulfur molecules resulting from cleavage of S8 rings (Equation 1) depends
upon partial pressure of H2S (PH2S) in rising volcanic gases through the conduit, resulting in
reduced populations of the cyclic S7 in the melt. Thus the linear relationship between temperatureand S7/S8 ratio presented by Takano et al. (1994) is significantly lowered than that found for the databy Steudel et al. (1985). Practically the linear relationship between the ratios and temperature isdependent on partial pressure of H2S in the environment considered. Isobars of (S7/S8) ratios should
be constructed for temperature estimation by this method.
A similar mechanism with dissolved SO2 gas is likely to proceed in the molten sulfur, forming
polythionic acids in it. However, we think that SO2 is not likely so much to reduce the formation ofsulfur homocycles.

4. Conclusion
Elemental sulfur found in volcanic area, particularly molten sulfur from active crater
lakes can provide useful information about the in-situ volcanic gas, when we getproperly quenched sulfur samples, and quick chemical analysis is applicable.