In this work, 99.999% pure magnesium (Mg) single microcrystals have been deformed in [0001] compression (c-axis compression) and [10 (1) over bar0] tension (c-axis contraction) conditions at room temperature and under loading rates ranging from 5 x 10(-4) up to similar 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 degrees far from a classical simple shear twin on {10 (1) over bar5} planes. In c-axis compression, at the highest applied strain rate, no twin could be detected in the 5 mu m sized pillars of 2:1 (height to width) aspect ratio. Plasticity is thus purely mediated by slip. However, the appearance of newly oriented grains was observed by lowering the sample size or by reducing the aspect ratio. Their crystallographic features suggest a mechanism of unit cell reconstruction through the transformation from pyramidal I to basal plane. The results presented in this study impose to consider twinning as a reorientation mechanism not necessarily limited to a simple shear.