Abstract
Siliceous shale of the Miocene Onnagawa formation in the Oga Peninsula,
Akita, Japan, contains a large number of silica minerals. This formation,
which is composed of rhythmic alternation of siliceous mudstone and smectite
clay stone, was originally deposited in a deep marine, and therfore has
virtually homogeneous composition primarily. As a result of diagenesis, at the present time, this formation divided into three zones in terms of phase
transition of silica minerals, namely opal-A, opal-CT, and quartz zones. Both opql-A/opal-CT and opal-CT/quartz boundaries are gradational. Of these two
transition zones, the latter can be seen at one outcrop, ranging in width 50m, along the Shinzan forestry road of the Oga Peninsula, and like this place is
especially invaluable. Phase identification and chemical analyses for silica minerals of siliceous shale were made by X-ray diffraction (XRD), X-ray fluorescence (XRF), and
inductively coupled plasma (ICP) respectively. The result of X-ray powder diffraction show the silliceous shale from the
boundary between opal-CT and quartz zone of this formation is including two
kinds of quartz, specifically detritus quartz and autogeneous quartz. The
intensity of X-ray diffraction of primitive autogeneous quartz showed
synchronized change with chemical compositions. The elements, accompanied
synchronized change of chemical composition, are Li, Be, Mg, Al, K, Ti, Mn, and known as trace impurity of quartz. The formation process of autogeneous quartz is divided into three steps from behavior of these. In the first step, all
elements above incorporate into quartz structure (maybe these elements
concentrate on surface of quartz). The second step, with the advance of quartz crystal growth, increasing in volume of quartz crystal surpass rising of
surface area. Finally, as a result of ripeness of quartz, almost of the
elements are expelled from quartz structure except Li. This Li is the only
element which is able to make solid solution with Al in the quartz crystal
structure as β-eucryptite (LiAlSiO4).
