On Halloysites in Omura Clay

Abstract

Mineralogical descriptions along with modes of occurrence are given on halloysites composing Omura clay, which has been mined as refractory raw materials at the Masuragahara mine and the Chiwata mine near Omura city, Nagasaki Prefecture, Japan. The Omura clay may have been formed by a hydrothermal alteration of two-pyroxene andesite of the Pleistocene age. Most of the clay is gray in color and composed of 10Å-halloysite with small amounts of cristobalite and quartz. Numerous thin veins of white clay consisting purely of 10Å-halloysite occur in the grayish clay. Some parts of the grayish clay directly surrounding unaltered andesite are tinged with brown and include gibbsite, kaolinite, 14Å-mineral, illite, alkali-feldspar and allophane.
The x-ray basal reflections of halloysite in the white clay vein are relatively sharp, and its interlayer hardly dehydrates. Under the electron microscope, relatively large grains with spherical, spheroidal and tubular shapes are observed in the white clay.Properties of halloysite in the grayish clay are variable: some materials tend to hold the interlayer water firmly but the interlayer water in the material of relatively small grains giving broad basal reflections is lost more easily. The brownish clay containing kaolinite dehydrates easily.
The d (06) values of halloysite in the white vein fall in the range of 1.480-1.481Å, while those in the grayish clay are between 1.483 and 1.486Å. When the interlayer water is lost, these d (06) values increase by about 0.002Å. It has also been found that the basal spacing of about 7A after dehydration tends to be small in the material having larger bdimension the d (002) values range from 3.57 to 3.60Å.
With increasing b-dimensions of halloysite, the aluminum contents decrease and the iron contents increase. Accordingly, it is presumed that some parts of the octahedral aluminum ions are substituted by ferric ions. Under the electron microscope, considerable amounts of tabular particle are observed in the halloysite samples that have larger b-dimension.
The DTA curves and infrared absorption spectra also show subtle differences among these various halloysites.