Fluid-induced chemical and phase heterogeneity in pegmatitic apatite: implications for alteration conditions and anomalous optical biaxiality

This study documents the formation of optically anomalous biaxial apatite resulting from partial fluid-mediated metasomatic alteration of pegmatitic apatite containing a britholite component. The optical anomaly was identified using a refractometer. SEM and EDX data constrain alteration conditions to interaction with acidic, oxidizing, highly saline, Si-bearing fluids at temperatures below 400–500 °C, accompanied by elevated fluorine activity. Pervasive partial metasomatic modification produced patchy compositional zoning and increased porosity within the apatite crystals. Microscale variations in F, OH, and Cl define compositional domains with differing proportions of fluorapatite, hydroxylapatite, and chlorapatite end members. These chemical differences result in variable light propagation velocities and refractive indices, giving rise to anomalous biaxial optical behavior. Single-crystal X-ray diffraction shows that all crystals retain the hexagonal apatite structure (P6₃/m). At the same time, fluid-induced apatite alteration generated non-uniform channel-site occupancies, accompanied by substitution of apatite by britholite along with the formation of vacancies in the calcium positions, and local structural distortions without symmetry lowering. The integration of optical, structural, and chemical data demonstrates that microscale compositional heterogeneity, produced by fluid–apatite interaction, is responsible for the observed optical anomaly.