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

Abstract

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.