Laboratory investigation of crystallisation in annealed amorphous MgSiO

Daresbury Laboratory, Synchrotron Radiation Department, Warrington, Cheshire WA4 4AD, UK

Corresponding author: S. P. Thompson s.p.thompson@dl.ac.uk

Received: 11 October 2000
Accepted: 22 December 2000

Abstract

In situ high resolution synchrotron X-ray powder diffraction measurements are used to probe the structural changes in a sample of amorphous MgSiO₃ during annealing. Temperatures cover the astrophysically significant range 1000 K to 1173 K and prior to annealing the sample showed typical amorphous structure. Following ~2 hours annealing at 1000 K an initial crystallite had formed that was coexistent with a modified amorphous component. Increases in temperature coupled with long annealing times produced no further change to the amorphous component and only very minor changes in the appearance of the crystalline phase. This we identify as a structural stall in the crystallisation process and is likely to be the origin of the spectral stall reported for similar silicate materials. Finally at 1173 K, following a cumulative annealing time of ~79 hours, the amorphous background began to collapse coinciding with the development of further crystalline features. The stall in structural development arises because the crystallisation process is in part regulated by the amorphous phase during annealing. At low-end temperatures it is likely that the annealing process itself strengthens the amorphous phase allowing it to survive up to relatively high temperatures. This finding suggests that some lower temperature information may be preserved in certain cosmic dust grains that were initially annealed at lower temperatures.