Goyazite and florencite from two African carbonatites

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 Sr_0·42Ce_0·52Ca_0·14Al_2·83(PO₄)_1·77(SO₄)_0·10 F_0·41(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 Ba_0·05Sr_0·50Ce_0·41Ca_0·08A_2·78(PO₄)_1·62(SO₄)_0·19F_0·30(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.