Apophyllite, KCa4Si8O20(F,OH)·8H2O, has been redefined in terms of a solid-solution series between the two phases hydroxyapophyllite, KCa4Si8O20(OH)·8H2O, and fluorapophyllite, KCa4SI8O20F·8H2O. The pure hydroxyl end-member is tetragonal, P4/mnc, with a = 8.978(3), с = 15.83(1) A, and two formula weights per unit cell. The observed and calculated densities are 2.37 and 2.36 g/cm3, respectively. Optically it is uniaxial positive with ϵ = 1.543 and ω = 1.542. The type locality is the Ore Knob mine, Jefferson, Ashe County, North Carolina. Due to the similarity of their properties, hydroxy- and fluorapophyllite can only be distinguished by chemical means. Nomenclature revisions are presented.

A refinement of the crystal structure of hydroxyapophyllite has been carried out using 757 X-ray reflections and least-squares, the final residual being 0.038. The structure is not significantly different from that reported for fluorapophyllite, except for the OH for F substitution. The unconventional formula KCa4(Si4O10)2F1−x(HF)x·[(H2O)8−x(OH)x] with x≃1, previously proposed for fluorapophyllite from Centreville, Virginia, is shown on the basis of new chemical data to be invalid.

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