We present the result of the first deformation experiments at high-temperatures and high-pressures on synthetic magmatic suspensions of strongly anisometric particles. The results highlight the interplay between the rheological response and the development of microstructures and they demonstrate the critical importance of the shape of crystals on the mechanical behaviour of magmas. Plagioclase suspensions with two crystal fractions (0.38 and 0.52) were deformed both in compression and in torsion in a Paterson apparatus. With increasing crystal fraction, the rheological behaviour of the magmatic suspension evolves from nearly steady-state flow to shear weakening, this change being correlated with a microstructural evolution from a pervasive strain to a strain partitioning fabric. Magmatic suspensions of plagioclase have viscosities approximately five orders of magnitude higher than suspensions of equivalent crystallinities made of isometric particles such as quartz.