On Variation of Some Characteristics of Granitic Rocks through Weathering

For utilization of the weathered granitic rocks as industrial mineral resources

Abstract

Weathered granitic rocks were classified into five layers according to their occurrences and the degree of weathering at Iizuka, Kyuragi and Yasu in the northern part of Kyushu Island. The writers investigated chemical component, mineral constituents, degree of liberation of mineral particles, particle size distribution and porosity of these weathered granitic rocks. The results obtained are summarized as follows :
1) The total H₂O, porosity (bulk density) and particle size distribution can be used as properties to indicate the degree of weathering.
2) Owing to the difference of stability of the individual constituents against weathering, there is a concentration trend of the constituents according to the particle size. As weathering proceeds, plagioclase disappears most rapidly among the main rock-forming minerals, though potash feldspar which is more desirable for ceramics than plagioclase remains even in the extremely weathered layers. Kaoline clay is present in a range of fine paticle size.
3) Degree of liberation of mineral particles increases gradually as the particle size becomes finer and weathering proceeds. And almost all particles are liberated into each mineral in a range of particle size finer than 0.25 -0.4 mm.
4) The weathered and decomposed granitic rocks except the surface layer may be used as industrial materials. Especially, it is able to produce ceramic clay from the extremely weathered but not contaminated layers through scrubbing and sizing of it, and also able to obtain the aggregate for construction in good yield from the decomposed granitic rock at the early stage.
5) The most useful kind of granitic rocks as industrial mineral resources is the one that is rich in quartz and potash feldspar, such as Sawara, Saga, Kaho and Masaki granites in the northern parts of Kyushu Island.