The alunite supergroup under high pressure: the case of natrojarosite, NaFe3(SO4)2(OH)6

A single crystal of natrojarosite, NaFe3(SO4)2(OH)6, was investigated by single-crystal X-ray diffraction at high-pressure conditions (up to 8.8 GPa) using a diamond-anvil cell. The unit-cell parameters were determined at 11 different pressures and no indications of a phase transition were found up to the maximum pressure reached. The volume and axial moduli were fitted to a third-order Birch–Murnaghan equation-of-state which gave the following values: V = 769.6(2) A3, K T0 = 50.6(9) GPa, K ′ = 9.9(4); a = 7.3172(6) A, K T0 = 104(2), K ′ = 7.6(9); c = 16.5965(20) A, K T0 = 24.6(4) and K ′ = 7.1(2). The crystal structure of natrojarosite was refined at seven different pressures up to 8.779(11) GPa [ a = 7.3170(4), c = 16.5955(5) A and V = 769.46(9) A3 in R 3I m at 0.00010(1) GPa and a = 7.1594(8), c = 15.6003(17) A and V = 692.49(8) A3 at 8.779(11) GPa]. The structural analysis shows that the 12-fold Na polyhedron accommodates most of the deformation by a large volume decrease (14%) and strong polyhedral distortion (63%). Our results indicate that natrojarosite has the most compressible structure of the supergroup studied so far, and has a very strong axial anisotropy.