High resolution electron microscopic and X-ray studies of non-random disorder in an unusual layered silicate (chloritoid)

Abstract

The power of high resolution electron microscopy as a technique for the direct study of non-random disorder in layered minerals is illustrated by the discovery, reported herein, of two new polytypes (one of monoclinic, the other of trigonal symmetry) in the orthosilicate chloritoid. These new variants had hitherto escaped detection in previous X-ray studies of chloritoid. Several specimens of this layered mineral, which cleaves on {110} as well as the expected {001} planes, were also analysed in parallel by X-ray diffraction, the limitations of which are demonstrated by these investigations. The X-ray diffraction procedures, unlike the real-space electron microscopic approach, yield no information concerning the inhomogeneity of intergrowths of one polytype within others. Apart from the existence of new polytypes, unexpected structural faults, in which one polytypic variant terminates within another, have also been identified by electron microscopy. A plausible structural model, which describes the atomic reorganization and the implied change in chemical composition, is proposed for this fault.

Topotactical dehydration of chloritoid

“…Two groups of single crystals were examined by X-ray precession methods, six levels of reciprocal space, namely (hOl), (hhl),(hhl),(Okl),(3h,h,l) and (3h,h,l) being recorded. These confirmed the original unit cell of Harrison & Brindley (Table 1), the polytype exhibited being designated 2M 2 (Jefferson & Thomas, 1978 complete dehydration, and subsequently re-examined, all six reciprocal-lattice levels being again recorded. Examples of the second set were given identical heat treatment, but in a vacuum furnace at a partial oxygen pressure of approximately 1.3 x 10 -5 Pa.…”

“…The possibilities for polytypism in chloritoid have been enumerated by Jefferson & Thomas (1978), and only a brief discussion will be given here. As with the structures, polytypism in chloritoid shows similarities to that in micas, but with distinct differences.…”

“…More recently, a second twolayer monoclinic form and a three-layer structure have been noted during an electron-optical investigation (Jefferson & Thomas, 1978). The possibility of further, more complex polytypes cannot be ruled out.…”