Potentially new Ba–Sr phase with (Ca, Ba, Sr)10-x□x[(SO4)3(PO4)3](F−, O2−, Cl−)2 (1 < x < 2) composition belonging to the apatite supergroup was discovered in a recrystallized low-grade combustion metamorphic rocks of the Belqa Group, Central Jordan. The phase occurs as a fibrous subparallel aggregate and was originated by pseudomorphical replacement of precursor fish bone tissues embedded in biomicritic bituminous calcareous sediments. The mineralized fish bone was primarily composed of biogenic carbonate-fluorapatite. The Ba–Sr phase is monoclinic with space group P21/b, the unit cell parameters a = 18.881(19), b = 7.091(12), c = 8.951(12) Å, β = 119.68(8)o, V = 1041.1(14) Å3, and Z = 4. The empirical formula of the Ba–Sr phase is (Ca5.19Ba2.35Sr1.07Na0.06)Σ8.67[(S3.31P2.63Al0.03Si0.02V0.01)Σ6.00O24](F− 1.33O2− 0.58Cl− 0.01)Σ2.00. The primary textural heterogeneity of the bone tissues has controlled sharp compositional zonation of the Ba–Sr phase expressed in patchy distribution of Sr, Ba, Ca, P, and S. The newly-formed Ba–Sr phase represents the extreme case of interaction between precursor fossil biogenic carbonate-fluorapatite and associated Ba and Sr depot minerals (barite and celestine). The reaction took place during long-term and low-grade (T = 450–500 °C) combustion metamorphism of the calcareous sediments under dry conditions. The fossil bones were affected by drastic physical and chemical changes that were completed by the formation of a new mineral phase.