High-Temperature and High-Pressure Raman Spectroscopy Study on Heat-Treated Natural Zircon: A Pressure Calibrator for DAC Experiments

Radiation-induced structural damage in natural zircon, known as metamictization, alters its crystallinity and affects its high-pressure behavior. Previous studies report a wide range of phase transition pressures (19.7–28.6 GPa) for natural zircon, likely due to various metamictization. To address this, we first heated natural zircon at 1673 K and found its structure restored and then conducted pressure-induced phase transition of heat-treated natural zircon and synthesized zircon by Raman spectroscopy combined with diamond anvil cells (DACs). Our experiments reveal that the phase transition pressure of heat-treated natural zircon aligns with synthesized zircon at approximately 26.8 GPa. Furthermore, we established the relationship between SiO₄ vibrational frequency (v₃) and temperature–pressure (300–800 K and 0–18 GPa), yielding Δv_T (cm^–1) = −4.97 × 10^–6(T – T₀)² – 0.03(T – T₀), and P (GPa) = [Δv₃(P,T) – Δv_T] × 5.13^–1. Calibrated by heat-treated natural zircon, we successfully determined the crystallization and dissolution conditions of ammonium fluorosilicate in a hydrothermal DAC.