Arsenoflorencite-(La), ideally LaAl3(AsO4)2(OH)6, was studied at high pressure by single-crystal X-ray diffractometry. The unit cell was determined at nine pressures up to 7.471(8) GPa; no evidence of a phase transformation was found in this range. The pressure–volume data (refined simultaneously) were fitted to a third-order Birch–Murnaghan equation of state which gave V0 = 710.71(8) Å3, KT0 = 106(2) GPa and K′ = 9.2(9). These values were confirmed independently from an FEfE plot. The crystal structure was refined at 1.596, 3.622, 5.749 and 7.471 GPa, the first time this has been done for a member the alunite supergroup. The compressibility of arsenoflorencite-(La) is strongly anisotropic, with βc > βa. The main compression mechanism was found to be governed by the internal angle O3–La–O2 of the La polyhedron, where the O2 and O3 atoms move toward the c axis during compression.

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