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Abstract In this work, 99.999% pure magnesium (Mg) single microcrystals have been deformed in [0001] compression (c-axis compression) and [10 1¯ 0] tension (c-axis contraction) conditions at room temperature and under loading rates ranging from 5× 10− 4 up to∼ 590 s− 1. The strain rate sensitivity and apparent activation volume of prismatic and pyramidal slip systems were evaluated. In c-axis contraction, at strain rates of 45 s− 1, the formation of a new grain whose crystallographic characteristics do not correspond to those of well-known twin systems could be observed. An explanation was found but it requires the breakdown of the invariant plane strain condition, and a unit cell reconstruction via pyramidal II to basal plane transformation. This unconventional twin is at 2.1° far from a classical simple shear twin on {10 1¯ 5} planes. In c-axis compression, at the highest applied strain rate, no twin could be …