パイロスマライトグループの構造,双晶及びポリティピズム
1983 年 16 巻 Special 号 p. 333-344
詳細
1983 年 16 巻 Special 号 p. 333-344
A review has been presented on the HRTEM and electron diffraction studies of the pyrosmalite group of minerals, particularly mcGillite and friedelite. These ‘mineral species’ have been found to basically have the same one-layer monoclinic structure. The rhombohedral cell of the twelve-layer structure originally described for mcGillite (Donnay et al. 1980) corresponds to the cell of a twinned lattice of the monoclinic structure. The stacking disorder chracteristic of this mineral group is in fact interpreted as a twin by TLQS (twin-lattice quasisymmetry) with twin index n=4 and twin-obliquity ω=0. The frequency of twinning in friedelite is much higher than that in mcGillite; the reflections of the former that characterize the manclinic structure are completely diffuse out. The mineral species of ‘friedelite’ may thus be regarded as a disordered equivalent of mcGillite. Possible polytypic structures of this mineral group can be systematically studied in terms of the two kinds of unit layers, P and M, that characterize the pyrosmalite and mcGillite structures, respectively. They are classified into four categories.
