Microstructural development of rock salt:

observations from in-situ heating experiments

Michel Bestmann1, Sandra Piazolo1, Chris Spiers2, Dave Prior1, Colin Peach2

1Department of Earth Sciences, 4 Brownlow Street, Liverpool, L69 3GP

2Utrecht University, Budapestlaan 4, 3584 TA Utrecht, the Netherlands

michelb@liv.ac.uk

We present the first in-situ observations of inter- and intracrystalline processes in a geological material during stepwise heating experiments in a scanning electron microscope. Dry deformed rock salt samples (axially compressed at 175° C to 30% strain), were heated to temperatures between 350 and 410ºC. Inter- and intracrystalline processes were observed using both secondary electron imaging and electron backscatter diffraction mapping techniques.

Two different types of grain interaction can be observed:

    1. Slow grain boundary migration (GBM) between substructured "old" grains due to low differential strain energy.
    2. Fast GBM between "old" and "new" grains due to high differential strain energy. These new grains appear from the 3rd dimension, grow abnormally fast and neither exhibit nor develop any intracrystalline substructure during GBM (see Piazolo et al., this volume).

The grains of the deformed rock salt sample show a high density of low angle boundaries (<15° ). During heating experiments between these "old" grains (grain size 20-400 mm) no obvious trend is evident for specific grains to grow on expense of neighbour grains, neither related to the size nor to the internal substructure of the grains. However, within grains coarsening of the subgrain structure and an increase of the misorientation of subgrain boundaries may occur. Subgrain boundaries move and new internal substructures develop. The angle of misorientation between adjacent grains may change. Nevertheless, so far no specific high angle boundary (>15° ) could be identified which exhibits significantly higher mobility than other high angle boundaries. GBM is accompanied by the following substructural features:

    1. The growing grain develops new low angle boundaries in the swept area.
    2. The existing substructure of the growing grain is continued in the swept area.
    3. Pre-existing subgrain structures of the swept grain are taken over.
    4. The pre-existing swept grain boundary develops into a low angle boundary within the growing grain.

(5) Only rarely, swept areas are substructure free.

During these processes the misorientation axis distribution of the internal substructure does not change significantly. The misorientation axes show a girdle distribution perpendicular to the compression direction. Thus it seems that the development of new subgrain boundaries is still controlled by the previous deformation kinematics.

According to these first results the existing model and interpretation of active processes related to static heating of a deformed rock microstructure may have to be revised.