In the crystallographical literature the asterix is replaced by a horizontal score above the previous number.
c is a glide plane with the translation component parallel to the c-axis.
2 / c means a two-fold rotation axis perpendicular to the (glide) plane.
R means that the shape of the unit cell is that of a rhombohedron.
3 means a three-fold rotation axis.
3* means a three-fold roto-inversion axis, which in turn means that the crystal will regain its original orientation when it is first subjected to a rotation about that axis of 120 degrees (or a multiple thereof) and then subjected to an inversion, i.e. reflected in a point, namely (in) its geometrical center.
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