Synchrotron X-ray analysis of norbergite, Mg_2.98 Fe_0.01Ti_0.02Si_0.99O₄ (OH_0.31F_1.69) structure at high pressure up to 8.2 GPa

The natural norbergite, Mg_2.98Fe_0.01 Ti_0.02Si_0.99O₄ (OH_0.31 F_1.69) is examined by synchrotron X-ray diffraction analysis at pressures up to 8.2 GPa. The measured linear compressibilities of the crystallographic axes are β_a = 2.18(4) × 10^-3, β_b = 2.93(7) × 10^-3, and β_c = 2.77(7) × 10^-3 (GPa^-1), respectively and the calculated isothermal bulk modulus of the norbergite is K_T = 113(2) GPa based on the Birch-Murnaghan equation of state assuming a pressure derivative of K′ = 4. The crystal structures of norbergite are refined at room temperature and pressures of 4.7, 6.3, and 8.2 GPa, yielding R values for the structure refinements of 4.6, 5.3, and 5.3%, respectively. The bulk moduli of the polyhedral sites are 293(15) GPa for the tetrahedron, 106(5) GPa for the M2 octahedron, 113(2) GPa for the M3 octahedron, and 113(3) GPa for the total void space. The bulk modulus exhibits a good linear correlation with the filling factor for polyhedral sites in structures of the humite minerals and forsterite, reflecting the Si⁴⁺ + 4O_2- ⇔ □ + 4(OH, F)^- substitution in the humite minerals. Moreover, two simply linear trends were observed in the relationship between bulk modulus and packing index for natural minerals and dense hydrous magnesium silicate minerals. This relationship would reflect that the differences in compression mechanism were involved with hydrogen bonding in these minerals.