AbstractThe petrographical, crystal-chemical and petrogenetical aspects of rutile rich in antimony (up to 33.75 wt.% Sb₂O₅; equal to 0.2 Sb⁵⁺ per O = 2) from St. Marcel-Praborna in the Aosta Valley, Italy, were re-examined. These compositions occur in two different petrographical environments (within the rock matrix or as microinclusions within Sb-rich titanite) in the manganese concentrations at this locality. The new data confirm our earlier hypothesis that two distinct petrogenetical/crystal-chemical processes both occurred: 1. Sb-metasomatism of pre-existing Sb-free rutile inclusions; and 2. creation of neoblastic Sb-rich rutile by the expulsion of Ti from pre-existing Sb-free titanite being metasomatized by Sb to form Sb-rich titanite. In the literature, Sb in minerals is variably considered as being trivalent and/or pentavalent. This work demonstrates that within rutile it is entirely Sb⁵⁺, substituting for Ti⁴⁺ by the following heterovalent cation exchange mechanism which is also the dominant one in the host Sb-rich titanite: 2 ^viR⁴⁺ = ^viR³⁺ + ^viR⁵⁺, where ^viR³⁺ = (Al,Cr,Mn,Fe)³⁺. A near-perfect correlation of ΣR⁵⁺vs. ΣR³⁺ (r > 0.98) is perturbed only by the presence of trace amounts of Ca²⁺, Sr²⁺ and Ba²⁺. Traces of Mn⁴⁺, Si⁴⁺ and/or (OH)⁻ might also be present. These alkaline earth cations are the largest cations ever recorded in the rutile structure and are seemingly too large to occupy normal octahedral sites. The cation exchange mechanism involved might be that found in the ‘trirutile’ mineral group: 3 ^viR⁴⁺ = ^viR²⁺ + 2 ^viR⁵⁺. Alternatively these large divalent cations may be situated in the lozenge-shaped tunnels of the rutile structure, by analogy with other large cations occupying the wider subrectangular tunnels in the analogous cryptomelane/hollandite/priderite, romanéchite and todorokite mineral groups. This leads to a possible new cation exchange mechanism for the rutile structure: 2 ^viR⁴⁺ + ^tunelvacant = 2 ^viR³⁺ + ^tunnelR²⁺.