鉱物の光学異常

その150年間の謎と謎解き

抄録

Since the early 19 th century it has been recognized that many minerals show optical phenomena which are not in harmony with the apparent symmetry of their external form. The phenomena are embraced under the common term of optical anomalies. The problem of optical anomaly was mystery for 150 years.
In low temperature aluminosilicates such as zeolites and adularia, various Al, Si orderings are produced by electron charge balance between alkali ions and Al, Si atoms on the crystal surface during growth. If Al, Si tetrahedron forms after alkali ions on crystal surface, the tetrahedron will be preferentially occupied with Al ion. On the other hand, if the tetrahedron forms prior to the alkali ion, it will be preferentially occupied by Si. If growth steps are inclined to mirror plane, Al, Si structure will be ordered with respect to the mirror, and the mirror will change into twin plane. If the steps are normal to the mirror, the structure will be disordered, and the mirror will be maintained during growth.
The ordered arrangements of the Fe³⁺, Al in garnet and the F, OH in topaz are produced on the side faces of the growth steps, because of differences in ionic radii.