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

• Main Authors: S.I. Lahti, V. Suominen
• Format: Article
• Language: English
• Published: Geological Society of Finland 1988-06-01
• Series: Bulletin of the Geological Society of Finland
• Subjects: granites; fluocerite; bastnaesite; cerianite; intergrowths; unit cell; chemical composition; rare earths; Fjälskär; Houtskär; Finland
• Online Access: http://www.geologinenseura.fi/bulletin/Volume60/sgs_bt_060_1_pages_045_053.pdf

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 P63mcm) forms in the mineral syntaxial intergrowth with bastnaesite- (Ce) (a = 7.11 Å, c = 9.74 Å, space group P62c). The cerianite phase (a = 5.45 Å, space group Fm3m) is intergrown with fluocerite-bastnaesite in a more complex way. The {0011} plane of fluocerite-bastnaesite is parallel to the {111} plane of cerianite and the {1000} plane of fluocerite-bastnaesite is parallel to the (110) 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% RE203). The Fjälskär granite is exceptionally enriched in REEs mainly incorporated in fluocerite and associated REE minerals. The CO2-bearing fluids altered fluocerite to bastnaesite as crystallization drew to an end in the rock. Separation of the cerianite phase was due to later oxidation of Ce3+. Because of structural and chemical similarities, the crystallographic axes of the various phases are preferentially oriented in the intergrowth.