ORIENTED TRANSFORMATION OF JOHANNSENITE TO BUSTAMITE
1966 年 5 巻 1 号 p. 44-64
詳細
1966 年 5 巻 1 号 p. 44-64
Details of the oriented transformation of johannsenite to bustamite, have been studied by the single crystal X-ray method, in order to elucidate the transformation mechanism for silicate chain structures in general. A single crystal of johannsenite was heated at 900°C in a silica tube. The product is a pseudomorph in which the johannsenite has been partly changed into bustamite. The latter is formed in a definite orientation relative to the johannsenite: the sheet of closed-packed oxygen atoms of johannsenite makes an angle of 4.5° with that of bustamite and [012] of johannsenite coincides with [11-1] of bustamite. This orientation shows that a dense zone of large cations and silicon atoms in johannsenite is preserved in the transformation.
These results are interpreted on the basis of the preservation of the close-packed framework of oxygen atoms along a dense zone of metal cations and silicon atoms in johannsenite in the transformation. The differences in the direction of the SiO3 chains and in the ordering of Ca and Mn atoms in the johannsenite and bustamite structures provide direct evidence of the migration of Ca and Mn atoms as well as of Si atoms during the transformation.
The oriented transformation of rhodonite to wollastonite, described by DENT GLASSER and GLASSER, is better explained by the same principle as the johannsenite-bustamite transition than by that of simple preservation of the close-packed framework of oxygen atoms.
