The chemical and crystallographic data presented in Part I suggest that fluorapophyllite and natroapo-phyllite are isostructural. Crystal structures of tetragonal potassium-rich fluorapophyllite have been determined by Taylor and Náray-Szabó (1931), and refined by Colville et al. (1971), Chao (1971), Prince (1971), and Bartl and Pfeifer (1976); and that of tetragonal potassium-rich hydroxyapophyllite by Rouse et al. (1978). Crystal structures of ortho-rhombic potassium-rich apophyllite (Sahama, 1965; Belsare, 1969), however, have not been determined because they are complexly twinned. It is well known that minor amounts of Na replace K in apophyllite, but no reports on the crystal structure of natural nat-roapophyllite with Na/K ≫ 1 have been published.

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