High resolution electron microscopy of chloritoid minerals from different geological melieu

Abstract

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.