fac-

https://doi.org/10.1351/goldbook.F02313
An affix used in names to designate three groups occupying the corners of the same face of an octahedral @C01329@ sphere, not now generally recommended for precise nomenclature purposes.
Sources:
Blue Book, p. 464 [Terms] [Book]
Red Book, 3rd ed., p. 245 [Terms] [Book]