The Hiraki Mine is situated in the northwest of Sanda City, Hyôgo Prefecture. This district is composed of the Arima Group (Paleogene or Cretaceous) and the Kôbe Group (Middle Miocene).
The Arima Group is widely exposed in this area, and is composed of rhyolite lava, pyroclastics, such as tuff, tuff breccia and welded tuff; and is divided into upper and lower formations.
The lower formation is subdivided into three members. The Kôbe Group covers The Arima Group unconformably and is composed of sedimentary rooks, such as zeolitic tuff, mudstone, sandstone and congromerate.
Zeolite in the zeolitic tuff is identified as clinoptilolite by means of X-ray diffraction, D. T. A. and microscopic methods.
The ore deposit of the Hiraki Mine is massive kaolin deposit and was formed by hydrothermal alteration of tuff and tuff breccia of the lower fomation in Arima Group. Mineral constitution is identified by means of X-ray analysis, D. T. A., thermo-balance and electron micrographs; crystalline kaolinite composes about 30-60% of the ore and the remaining 70-40% consists of quartz along with small amounts of dickite, a long-spacing clay mineral, pyrite, iron oxide and others.
Dickite was found from veinlets, globular aggregates and walls of fissures.
Surrounding the ore body, there are zones of weak alteration which are characterized by quartz and green clay mineral.
The mineralization of the Hiraki Kaolin deposit took place immediately after the eruption of rhyolitic lava and pyroclastic of the lower formation, and ended before the deposition of rhyodacitic welded tuff of the upper formation.

View full abstract

The authors have shown that addition of a small amount of powdered frits of lithia containing glass to naturally occurring petalite powders increases the firing range of their mixture, improving the modulus of rupture and thermal shock resistance of its sintered product (J. Ceram. Assoc. Japan, 70, 8 (1962) and 71, 65 (1963)). The sntering method so far used in the authors' experiments was the conventional one, viz. cold-pressing followed by high-temperature sintering. In the present study, a hot-pressing technique has been applied to the same mixture for the fabrication of the product of the higher density and machanical strength.
The glass used has the composition of 10.7% Li₂O, 8.9% MgO, 8.9% Al₂O₃, 71.5% SiO₂ by weight. It was added in powder form to petalite powders in a weight ratio of 10:90. The mixture (20g), after ball-milled to a finess over 325 mesh, was introduced into a graphite mold with an inside diameter of 30mm, and hot-pressed in the temperature range of 1, 000 to 1, 150°C under pressures varying from 50 to 200kg/cm² for 60 min. The determinations of bulk density, modulus of rupture and thermal shock resistance were made on the hot-pressed bodies prepared under these conditions.
The results obtained are as follows:
The optimum firing temperature required to produce a dence petalite-glass body was markedly lowered by about 150°C in hot-pressing than in the conventional method. The body hot-pressed under the pressure of 150kg/cm² at 1, 150°C for 60 min. has a bulk density of 2.40 and a modulus of rupture of about 1, 000kg/cm². This body however, has a drawback of being darken in color by carbon as a result of the use of graphite mold at the higher temperature. The darkening was little for the body hot-pressed below 1, 100°C under the pressure less than 150kg/cm². The body hot-pressed at 1, 100°C under the pressure of 150kg/cm² has a density of 2.25 and a modulus of rupture of 800kg/cm², the latter being higher by about 300kg/cm² than that of the body having the same density but prepared by the conventional firing method. It did not show any change in modulus of rupture after taken out from the furnace at 700°C and dropped into water.

View full abstract