The chemical and crystallographic data presented in Part I of this study suggest that the crystal structures of fluor- and hydroxyapophyllite are identical except for the OH ⇋ F substitution. However, since cases exist in which this substitution produces a structural change, e.g. hexagonal fluorapatite vs. monoclinic hydroxyapatite (Elliot et al., 1973), a structure determination of the pure hydroxyl end-member has been undertaken, the results of which are presented here. In addition, Prince (1971) has proposed that the fluorine in fluorapophyllite from Centreville, Virginia, is present as HF “molecules” rather than F ions. We shall examine this hypothesis in the light of new chemical data on Centreville apophyllite.

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