AbstractTitanites rich in antimony (up to 12.59 wt.% Sb₂O₅; equal to 0.165 Sb⁵⁺ per Si⁴⁺ = 1.000) arc described for the first time. They coexist with various greenschist-facies minerals in three different petrographical environments in the manganese concentrations at St. Marcel-Praborna in the Aosta Valley, Italy. These titanites are chemically zoned, the Sb-richest parts generally occurring alongside grain boundaries, microfractures, pores or inclusions. Moderately-good positive correlations exist between the Sb, Al and Fe contents, with negative correlations of each of these elements with Ti. Cr has perturbed the Al and Fe contents since a better negative correlation with Ti is obtained with [Al + Cr + Mn + Fe]. The Sr, Ba, Mn and F contents are rather small and vary irregularly. Several crystal-chemical hypotheses are presented concerning the method of structural formula calculation and the valencies and site distributions of Sb and the companion additional minor elements. The favoured interpretation is to place: Sr²⁺, Ba²⁺, Mn²⁺, Fe²⁺ and possibly also Sb³⁺ in the ^viiCa²⁺ site; Sb⁵⁺, Cr³⁺, Al³⁺, Sb³⁺, Fe³⁺ and possibly also Mn³⁺ in the ^viTi⁴⁺ site; and Al³⁺ in the ^ivSi⁴⁺ site; this indicates the existence of a natural silicate containing not only both trivalent and pentavalent Sb, but also both valencies in the same site. The principal ionic substitution is believed to be: 2 ^viTi⁴⁺ = ^vi[Al³⁺ + Cr³⁺ + Mn³⁺ + Fe³⁺] + ^viSb⁵⁺ accompanied by a substantial minor proportion of: 2 ^viTi⁴⁺ = ^viSb³⁺ + ^viSb⁵, or possibly = 2 ^viSb⁴⁺ The data indicate a very late stage penetration of pre-existing titanites by mobilized Sb, Al, Cr, Mn, Fe, F and presumably also (OH) at this locality, whereas Sr and Ba were mobile at an earlier stage. Manganese, which existed in the sedimentary protoliths, did not enter the titanite structure in very significant quantities.