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The presence of small crystals (microlites) is expected to influence the rheological properties of magmas by increasing the effective viscosity and causing non-Newtonian behaviour, especially at high concentrations of crystals. To investigate the effect of microlites on lava flow rheology, viscosity determinations were performed on natural microlite-bearing rhyolitic obsidians of calc-alkaline to peralkaline compositions containing 0.1 - 0.4 wt% water, and with average crystal contents ranging from 0 - 45 vol%.

The glass transition temperatures were determined from calorimetric measurements on the melts for a range of cooling/heating rates. As the presence of crystals does not affect the heat capacity peak in calorimetry (the glass transition, T_g), differences in T_g for samples with identical thermal treatment may be interpreted solely as a result of differences in melt chemistry. Consequently, the physical effect of suspended crystals can be determined by difference from the calculated viscosity of the melt obtained from scanning calorimetry and from the viscosity of the melt plus crystals from parallel-plate measurements. For rod-like microlites, < 8 vol% crystals do not have a significant physical effect (within ± 0.2 log units) on the effective viscosity of the magma at low stresses and strain rates. Exceptions to the rule include a crystal-connectivity influence for branching "spider-like" networks of crystals (~ 1 vol%) - a new and surprising result. Results for a microlite-rich/-poor pair of samples from the same lava flow with an almost identical bulk composition, show that the effect of changing melt chemistry during microlite crystallisation in nature significantly outweighs any physical effect of these crystals on effective viscosity. In contrast, for samples containing crystal-rich flow bands with at least 45 % microlites, deformation using our parallel-plate methods does not occur owing to the formation of a tetrahedral close-packing crystal framework.