Secondary phosphates in montebrasite and amblygonite from Nagatare, Fukuoka Prefecture, Japan

摘要

Montebrasite and amblygonite in an Li-Cs-Ta enriched (LCT) pegmatite from Nagatare, Fukuoka Prefecture, Japan, contain various alteration minerals: fluorapatite, crandallite, goyazite, waylandite, wardite, viitaniemiite, morinite, muscovite, lepidolite, and cookeite. They are associated with lacroixite, quartz, and topaz. Among these minerals, wardite, viitaniemiite, morinite, and lacroixite are newly discovered in Japan. The secondary phosphates and fine-grained mica form fine veins along cleavages and composition planes of polysynthetic twins in the montebrasite and amblygonite. Lacroixite has a different texture from other phosphates, which suggests a possibility of exsolution within montebrasite-amblygonite series. Various secondary phosphates show Ca-, Na- and Sr-metasomatism with leaching of Li, and the formation of low-F montebrasite from montebrasite-amblygonite series indicates an F-OH exchange. However, fluorapatite, morinite, and viitaniemiite crystallized in an F-rich environment. Montebrasite-amblygonite series minerals undergo an acidic alteration to muscovite in the last stage, which is the same process that other Li minerals undergo, such as tourmaline and spodumene.