Fluorellestadite, Ca10(SiO4)3(SO4)3F2, has been synthesized as single phase. This compound crystallizes in the apatite type structure, s.g. P63/m, with parameters a=9.4417(1) Å, c=6.9396(1) Å and V=535.76(1) Å3. The refinement of its crystal structure converged to RWP=12.33% and RF=4.58%. The atomic parameters have been used to analyze the phase content of mineralized white Portland clinkers. These clinkers contain Ca3SiO5, Ca2SiO4, Ca12Al14O32F2 and Ca10(SiO4)3(SO4)3F2. The agreement between the elemental composition inferred from the Rietveld phase analysis and that measured by XRF is noteworthy. This comparison does not take into account the presence of amorphous phases and unmodeled elemental substitutions in crystalline phases. Similar Rietveld studies on commercial white Portland clinkers are also shown to be feasible.