  The intermetallic compound H (Mn$_7$Si$_2$V) phase in the Mn-Si-V alloy
system can be regarded as an approximant phase of the dodecagonal quasicrystal
as one of the two-dimensional quasicrystals. To understand the features of the
approximant H phase, in this study, the crystallographic features of both the H
phase and the (\sigma $\, \rightarrow$ H) reaction in Mn-Si-V alloy samples
were investigated, mainly by transmission electron microscopy. It was found
that, in the H phase, there were characteristic structural disorders with
respect to an array of a dodecagonal structural unit consisting of 19
dodecagonal atomic columns. Concretely, penetrated structural units consisting
of two dodecagonal structural units were presumed to be typical of such
disorders. An interesting feature of the (\sigma $\, \rightarrow$ H) reaction
was that regions with a rectangular arrangement of penetrated structural units
(RAPU) first appeared in the \sigma $\,$ matrix as the initial state, and H
regions were then nucleated in contact with RAPU regions. The subsequent
conversion of RAPU regions into H regions eventually resulted in the formation
of the approximant H state as the final state. Furthermore, atomic positions in
both the H structure and the dodecagonal quasicrystal were examined using a
simple plane-wave model with twelve plane waves.
