Occurrence, crystallography and chemistry of the fluocerite-bastnaesite-cerianite intergrowth from the Fjälskär granite, southwestern Finland

Abstract

LAHTI, SEPPO I. and SUOMINEN, VELI, 1988: Occurrence, crystallography and chemistry of the fluocerite-bastnaesite-cerianite intergrowth from the Fjälskär granite, southwestern Finland. Bull. Geol. Soc. Finland 60, Part 1, 45-53. White crystals composed mainly of fluocerite were identified during separation of the accessory minerals of the Middle Proterozoic Fjälskär granite, southwestern Finland. Detailed X-ray diffraction and microprobe studies showed that the mineral is intergrown with bastnaesite and cerianite. X-ray single crystal studies reveal that fluocerite-(Ce) (a = 7.11 Å, c = 7.28 Å, space group P6 3 mcm) forms in the mineral syntaxial intergrowth with bastnaesite-(Ce) (a = 7.11 A, c = 9.74 Å, space group P6 2 c). The cerianite phase (a = 5.45 Å, space group Fm3m) is intergrown with fluocerite-bastnaesite in a more complex way. The [00011 plane of fluocerite-bastnaesite is parallel to the (111J plane of cerianite and the [1000] plane of fluocerite-bastnaesite is parallel to the (llO) or corresponding plane of cerianite. The fluocerite-bastnaesite-cerianite intergrowth is dominated by LREEs, especially Ce, La and Nd. Its composition varies slightly; pure separate phases cannot be distinguished with the microprobe. Zircon and fluorite associated with fluocerite have minor REEs (below 0.3 wt% RE 2 0 3). The Fjälskär granite is exceptionally enriched in REEs mainly incorporated in fluocerite and associated REE minerals. The C0 2-bearing fluids altered fluocerite to bastnaesite as crystallization drew to an end in the rock. Separation of the ceri-anite phase was due to later oxidation of Ce 3 +. Because of structural and chemical similarities, the crystallographic axes of the various phases are preferentially oriented in the intergrowth.