mer-

in inorganic nomenclature
https://doi.org/10.1351/goldbook.M03828
An affix used in names to denote meridional; three groups occupying vertices of an octahedral @C01329@ sphere in such a relationship that one is cis to the two others which are themselves @C01092@, not now recommended for precise nomenclature purposes.
Source:
Red Book, 3rd ed., p. 245 [Terms] [Book]