Goyazite and florencite from two African carbonatites

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Abstract
Summary: Florencite with ω 1·653, εd 1·661, G 3·457, a 6·971 ± 0·004 Å, c 16·42±0·13 Å, and rhombohedral unit-cell contents Sr0·42Ce0·52Ca0·14Al2·83(PO4)1·77(SO4)0·10 F0·41(OH)5·24(H2O)0·85 occurs in the Kangankunde carbonatite in Nyasaland, and goyazite with G 3·386, a 6·982±0·001 Å, c 16·54±0·02 Å, and rhombohedral unit-cell contents Ba0·05Sr0·50Ce0·41Ca0·08A2·78(PO4)1·62(SO4)0·19F0·30(OH)5·30(H2O)1·18 on the Wigu carbonatite in Tanganyika. Hydrothermal decomposition experiments in the Wigu material indicate stability below 535 ± 10 °C at pH2O 200 bars and 565 ± 10°C at pH2O 2500 bars ; both minerals exhibit endothermic peaks at 630°C in differential thermal analyses. The crystal chemistry of the goyazite series is discussed in the light of the two new analyses and of newly determined unit-cell dimensions for four other members of the goyazite series. Goyazite, florencite, and gorceixite appear to have crystallized during late-stage replacement processes or under supergene conditions in carbonatites.

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