Destinezite ("diadochite"), Fe ₂ (PO ₄ )(OH).6H ₂ O; its crystal structure and role as a soil mineral at Alum Cave Bluff, Tennessee

University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States

A new occurrence of destinezite (diadochite), ideally Fe ₂ (PO ₄ )(SO ₄ )(OH).6H ₂ O, is described from Alum Cave Bluff, Great Smoky Mountains National Park, Tennessee, where it occurs in soil and in a weathered Precambrian phyllite in unusually large crystals associated with other hydrated sulfates such as pickeringite-apjohnite. Destinezite is triclinic, P1, with a = 9.570(1), b = 9.716(1), c = 7.313(1) Aa, alpha = 98.74(1) degrees , beta = 107.90(1) degrees , gamma = 63.86(1) degrees and Z = 2. Its crystal structure consists of infinite chains of Fe(O,OH,H ₂ O) ₆ octahedra, sulfate tetrahedra and phosphate tetrahedra linked by a unique system of vertex sharing. The chains are weakly bonded into slabs by hydrogen bonding between OH and H ₂ O of the Fe(III) octahedra and oxygen ions of the sulfate tetrahedra. Slabs of tetrahedral/octahedral chains alternate with sheets of H ₂ O molecules. The structure thus somewhat resembles hydrated clay minerals, with H ₂ O molecules that act as hydrogen bond donors and acceptors to oxygen atoms of adjacent slabs. Destinezite and diadochite occur at numerous localities worldwide and have been assumed to be identical, but this identity has never been proven. It is proposed that the name "destinezite" be applied to visibly crystalline, triclinic Fe ₂ (PO ₄ )(SO ₄ )(OH).6H ₂ O and "diadochite" to massive to earthy, poorly ordered, X-ray amorphous materials that approximate destinezite in composition. Diadochite/destinezite may be an unrecognized component of soils where weathering of pyrite and apatite has occurred and pH is low. It may thus be a significant sink for phosphorus and sulfur in such soils.

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