Ba and Sr mineralization of fossil fish bones from metamorphosed Belqa group sediments, Central Jordan: an integrated methodology

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) ų, and Z = 4. The empirical formula of the Ba–Sr phase is (Ca_5.19Ba_2.35Sr_1.07Na_0.06)_Σ8.67[(S_3.31P_2.63Al_0.03Si_0.02V_0.01)_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.