Journal of Conference Abstracts

Growth and Recrystallization of Chrysotile Fibres

Barbara A. Cressey (b.cressey@soc.soton.ac.uk)

Department of Geology, University of Southampton, Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH.

New insight into the structures, mechanisms for growth and recrystallization of normal asbestiform chrysotile fibres, large-diameter fibres and polygonal fibres has been provided by synchrotron X-ray diffraction and high-resolution transmission electron microscopy. The high-quality diffraction data obtainable using synchrotron radiation has enabled the elucidation of subtle structural variations indicated by diffraction peak profiles. This has shown that some specimens consist of fibres which have inter-layer spacings of about 0.72nm together with that which is more usually found, 0.73nm. To investigate further these unexpected structural variants, cross-sections of the fibres have been examined by TEM. Images show several novel features which demonstrate that the serpentine group of minerals can form in various complex structural arrangements that may seem difficult to classify, as individual fibres can be composed of both flat and curved layers.

The combination of high-resolution XRD and TEM has enabled us to gain a better understanding of the nature of interlayer stacking and bonding in flat and curved serpentine layers. Without this we may not have appreciated the significance of images that showed directly, for the first time, that chrysotile posesses five-fold symmetry. This new perception has also suggested a mechanism for the transformation, under disequilibrium conditions, from silky, small-diameter cylindrical chrysotile fibres to splintery, large-diameter polygonal fibres. The mechanism involves an adjustment of the interlayer and intralayer strain inherent in all serpentine structures, by a rearrangement of layer stacking that increases the extent of hydrogen bonding between layers.

REE Ion Control of Growth Surfaces in Monazite

G. Cressey (g.cressey@nhm.ac.uk)

Department of Mineralogy, Natural History Museum, London.

Single crystal X-ray diffraction, backscattered electron imaging and microprobe analysis have been used to establish the relationship between the morphology and sector chemistry of a natural (La,Ce,Nd)PO₄ monazite (P2₁/n). Uptake of La by {011} sector surfaces is enhanced relative to that of {1­01} and {100} sectors; Ce shows no partitioning differences; and uptake of Nd is more easily facilitated on {1­01} and {100} surfaces relative to {011}. There appears to be a distinct relationship between the size of the REE ion and the probability of uptake via the different growth surfaces. Interpetation of this uptake behaviour, based on theories involving protosites, involves a model to explain how the variable geometry of protosites in monazite influences the element partitioning behaviour at different surfaces. The overall morphology and sector growth is shown to be a function of uptake chemistry. This leads to the conclusion that time-synchronous growth layers in sector-zoned crystals do not reflect thermodynamic equilibrium, but rather a set of partial local equilibria between fluid/melt and crystal.