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Pyromorphite is one of the important end member lead apatites that has potential applications in environment remediation. The thermal behavior of natural pyromorphite, Pb₁₀(PO₄)₆Cl₂, has been investigated up to 1373 K at room-pressure using a powder X-ray diffraction device equipped with a heating system. Pyromorphite experiences melting and decomposing at 1373 K into lead phosphate (Pb₃(PO₄)₂), and lead dioxide (PbO₂) with reaction with air. The fit of the temperature–volume data yields a linear volume expansion coefficient <i>α_V</i> = 4.5 (±0.02) × 10⁻⁵ K⁻¹. The linear expansion coefficients for lattice parameters present the anisotropic thermal expansibility, i.e., <i>α_a</i> = (±0.06) × 10⁻⁵ K⁻¹ and <i>α_c</i> = 2.2 (±0.06) × 10⁻⁵ K⁻¹. We looked into the crystal chemistry and proposed an expression to quantitatively evaluate the structural evolution of pyromorphite upon high temperature by calculating the twist angle of the Pb(1)O₆ metaprism, which decreases at elevated temperatures. A distinct drop of the twist angle was observed at ~1100–1200 K, which might be a sign for the phase transition to a low symmetric subgroup. The variation of the twist angle is more sensitive than that of the unit cell; therefore, it can be applied to monitor the structural and phase changes of apatite group materials in general.