3:30 PM - 5:00 PM
[SCG48-P02] Occurrence of the common opals and chalcedony in the Sakurajima-Kinkowan Geopark area;
preliminary petrographic results and applications for the preservation
Keywords:Common opal, Geothermal fluids dynamics, Sakurajima-Kinkowan Geopark, Conservation
We report on the occurrence of common opals from the Aira caldera margin which is part of the Sakurajima-Kinkowan Geopark (Kusumoto and Kichise, 2022). "Usonuki waterfall" in Kajiki town, Aira city is designated as a geological interest site that is manly comprised of andesitic intrusions of the pre-caldera stage(Otuka and Nishiinoue, 1980). The opal is a whitish blue milky that does not show any color oscillation, exhibited by rounded aggregates with diameter in the range of 1 to 10 mm.
It is generally thought that opals crystallize from silica-rich geothermal fluids (Deer et al., 2013). Therefore, opals can be expected to record about local material circulation (especially hydrothermal circulation) during the post-caldera period. In this presentation, we report on the occurrence and petrology of opal found in the Kajiki town.
Common opal and agate are found at the margin of the Usonuki waterfall. The facies of the cut surface of the samples show banded structure of agate. The common opal appears along the margin or on the surface of andesite (Fig. 1a). The opal and agate are embedded or grown on the andesitic host, where the contact areas in host rock are oxidized (Fig. 1b) and at places the opal is exposed on the surface of the rock (Fig. 1c).
A banded structure with repeated layers of common opal and agate is obvious through the microscope observation (Fig. 1c). The thickness of the opal in contact with the outside of the rock is a few mm. The agate has a petal-like structure with crystals radiating in the direction of their long axes (Fig. 1d).
As is generally accepted, opal is thought to crystallize from silica in hydrothermal fluids, as the temperature of fluid decreases or the fluid evaporates at the surface, opal transforms to agate (Deer et al. 2013). It is interesting to note the process of crystallization or phase transition recorded in the rock in specific environments. In this study, the case of opal transforming to agate, as shown in Fig. 1c, was observed over time, with changes in temperature, pressure or hydrothermal composition. A detailed study of these rocks may provide constraints on the environment at the time of crystallization and the causes of the appearance of oscillatory patterns on agate (Liesegang ring phenomenon).
The geoscientific value of the opals within the Geopark area could serve as excellent education for the local residents. The Usonuki waterfall is considered as a resource of the Sakurajima-Kinkowan Geopark. It is desirable to find a way how to utilize and how to preserve therefore academic research and surveys are needed at this stage to further understand their formation mechanisms and petrogenesis.
In the Aira caldera margin, other opal-bearing sites have been reported besides the Usonuki Falls, and their investigation and research will provide clues to understanding the evolution of the magma-hydrothermal system in the post-caldera stage, including the influence of magma chamber at the bottom of the Aira caldera.
It is generally thought that opals crystallize from silica-rich geothermal fluids (Deer et al., 2013). Therefore, opals can be expected to record about local material circulation (especially hydrothermal circulation) during the post-caldera period. In this presentation, we report on the occurrence and petrology of opal found in the Kajiki town.
Common opal and agate are found at the margin of the Usonuki waterfall. The facies of the cut surface of the samples show banded structure of agate. The common opal appears along the margin or on the surface of andesite (Fig. 1a). The opal and agate are embedded or grown on the andesitic host, where the contact areas in host rock are oxidized (Fig. 1b) and at places the opal is exposed on the surface of the rock (Fig. 1c).
A banded structure with repeated layers of common opal and agate is obvious through the microscope observation (Fig. 1c). The thickness of the opal in contact with the outside of the rock is a few mm. The agate has a petal-like structure with crystals radiating in the direction of their long axes (Fig. 1d).
As is generally accepted, opal is thought to crystallize from silica in hydrothermal fluids, as the temperature of fluid decreases or the fluid evaporates at the surface, opal transforms to agate (Deer et al. 2013). It is interesting to note the process of crystallization or phase transition recorded in the rock in specific environments. In this study, the case of opal transforming to agate, as shown in Fig. 1c, was observed over time, with changes in temperature, pressure or hydrothermal composition. A detailed study of these rocks may provide constraints on the environment at the time of crystallization and the causes of the appearance of oscillatory patterns on agate (Liesegang ring phenomenon).
The geoscientific value of the opals within the Geopark area could serve as excellent education for the local residents. The Usonuki waterfall is considered as a resource of the Sakurajima-Kinkowan Geopark. It is desirable to find a way how to utilize and how to preserve therefore academic research and surveys are needed at this stage to further understand their formation mechanisms and petrogenesis.
In the Aira caldera margin, other opal-bearing sites have been reported besides the Usonuki Falls, and their investigation and research will provide clues to understanding the evolution of the magma-hydrothermal system in the post-caldera stage, including the influence of magma chamber at the bottom of the Aira caldera.