Apatite crystals co-existing with liquid or with vapour in the system CaO-CaF 2 -P 2 O 6 -H 2 O-CO 2 are equant whereas those precipitated from a liquid during a quench are greatly elongated parallel to the c -axis and they exhibit skeletal forms. Basic rocks in the Karroo volcanic succession of Southern Rhodesia contain apatite crystals with forms very similar to the synthetic quench apatites. Acicular apatite crystals may prove to be of value as a criterion for the former existence of a liquid phase in rocks whose origin is debatable.
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Wyllie et al. (1962) studied this question.