High resolution electron microscopy of chloritoid minerals from different geological melieu
- 14 Downloads
Structural defects of three chloritoid minerals from distinet geologic melieu have been investigated by high resolution electron microscopy. X-ray powder and electron diffraction patterns indicate that the chloritoid from one geological source (A) is2M 1+2M2 monoclinic variant while those from another geological source (B) are 2M 2 monoclinic variants. In a typical one-dimensional lattice image of a crystal from sourceA, the 2M 2 matrix is broken by insertion of triclinic inter-growths. Another crystal with the 2M 2 matrix showed single, triple, quadruple and quintuple layers displaying an unusually high degree of disorder. Lattice images of 2M 2 monoclinic variants from sourceB yielded more homogeneous micrographs.
The important finding from the present studies is that the chloritoid from sourceA is a severely disordered low-temperature intermediate phase in the conversion of the triclinic chloritoid to the high-temperature ordered monoclinic variants of sourceB. Severely disordered chloritoids, marking the beginning of low grade metamorphism, are generated as intermediates between the state of complete disordered arrangement towards the end of low grade metamorphism within the narrow stability range of 400°–500°C.
KeywordsChloritoid mineral electron microscopy lattice images different geological melieu
- Anderson J S 1978Proc. Indian Acad. Sci. A87 295Google Scholar
- Buseck P R 1979Proc. EMAG 93 Google Scholar
- Buseck P R and Iijima S 1974Am. Mineral. 59 1Google Scholar
- Halferdahl L B 1957Carnegie Inst. Washington, Pap. Geophys. Lab. p. 200Google Scholar
- Hanscom R H 1980Am. Mineral. 65 534Google Scholar
- Hutchison J L, Jefferson D A and Thomas J M 1977 InSurface and defect properties of solids Vol. 6 (London: The Chemical Seciety) p. 420Google Scholar
- Jefferson D A 1980Proc. EMAG 113 Google Scholar
- Thomas J M, Jefferson D A, Mallinson L G, Smith D J and Crawford E S 1979Chem. Scipt. 14 167Google Scholar
- Winkler H G F 1974Petrogenesis of metamorphic rocks (New York: Springer-Verlag)Google Scholar