What is "vicinal planes"

Vicinal \Vic"i*nal\ (?; 277), a. [L. vicinalis: cf. F. vicinal.]

1. Near; vicine.
   --T. Warton.

2. (Organic Chem.) Having the substituted groups on the same carbon atom.

   Vicinal planes (Min.), subordinate planes on a crystal, which are very near to the fundamental planes in angles, and sometimes take their place. They have in general very complex symbols.