Crystal Packing in a Mush - the Role of Grain Shape

Particle packing controls the physical properties of accumulations of grains. Grain size and shape in a crystal mush affects the amount of pore space and its interconnectivity, controlling permeability and mechanical strength during compaction. The spatial distribution and aspect ratio of grains also has an influence on mechanical strength during expansion, with application to the survival of entrained fragments of crystal aggregate during decompression-driven vesiculation in erupting magma. At present very little is known about the controls of grain shape, and in particular aspect ratio, on the physical properties of a crystal mush layer. We use packings of rice and beads as analogue models to investigate how the packing is affected by particle shape and the accumulation rate of particles at the top of the mush. Alignment factors, derived from eigenvectors, are used as a measure of the preferred grain orientations. Results show that grain anisotropy is an important factor when considering the structure and behaviour of a crystal mush. In addition, it is believed that high alignment factors seen in outcrop, which are sometimes attributed to dissolution and reprecipitation of material under compressive strain, may in fact be largely due to the original crystal shape. This means that estimates of the volume of fluid/melt liberated during compaction may have to be reconsidered.