日本海新礁産の重晶石団塊について

抄録

Barite nodules have been collected at the Sin Guri Bank in the Japan Sea by a survey ship of the Hydrographic Office of Japan. The Sin Guri Bank is situated at about 30km southwest of the Oga peninsula, and the rocks consist of Miocene yellowish siltstone, diatomaceous mudstone, two pyroxene andesite and trachybasalt. Barite nodules, occurring with diatomaceous mudstone, were collected in latitude 39°38.4'N, longitude 139°31.3'E and at a depth of 152m. Those barite nodules have well-rounded shapes being 1-20cm. in diameter and are widely distributed in the Sin Guri Bank. Diatom contained in diatomaceous mudstone is correlated to the basal part of the Funakawa formation and the upper part of the Onnagawa formation of the Middle Miocene.
In the present study, two specimens, ellipsoidal and orbiculate, weighing 362g and 142g respectively as shown in Photo. 1, were used. Barite nodules are compact and homogeneous, but have mudstone-like texture. Their individual mineral particles are not ob, served megascopically. A microscopic observation reveals that a large amount of diatom and radiolaria are contained in barite aggregates consisting of about 0.02-0.1mm particles, and a few other minerals. Specific gravity of the nodules is 3.59-3.74 (by a Berman balance) and X-ray diffraction patterns are in good agreement with that of typical synthetic barium sulphate and no other mineral is detected. DTA curve does not show any characteristic thermal reaction, and after heating at 1, 000°, no change in X-ray diffraction pattern was observed compaing with the unheated materials. The result of chemical analyses indicates 73.08-84.84% BaSO₄ and a small amount of SrSO₄, PbSO₄, and CaSO₄.
The content of BaSO₄ in this barite nodules is almost the same as that already reported barite nodules off Colombo, Indian Ocean, and off the San Clement Island, California.
The present authors consider the genesis of this barite nodules as follows: The SO₄ ion in the diatomaceous mudstone reacted with the ascending Ba-bearing hydrothermal solution resulting in the precipitation of barite.