Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
Review Articles
Mode of Occurrence, Crystal Chemistry, and Origins of Li Minerals of Metasomatic Albitite: Murakamiite from Iwagi Islet, SW Japan
Teruyoshi IMAOKAMariko NAGASHIMA
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2022 Volume 131 Issue 2 Pages 235-256

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Abstract

 The mineralogical characteristics of metasomatic Li-minerals in Iwagi Islet, sugilite, katayamalite, murakamiite, Li-rich pectolite, zektzerite, sogdianite and taeniolite are summarized. These Li-minerals occur in metasomatic albitite, which contains 500 ppm Li. Such a high-Li albitite content is unusual amongst metasomatic rocks in Setouchi Province of SW Japan. Albitite forms small bodies that are several tens of centimeters to tens of meters in size, and are disseminated in a host granite of Late Cretaceous age. It shows conspicuous strain-induced textures. Murakamiite and Li-rich pectolite form a solid solution with Li × 100/(Li + Na) atomic ratios ranging from 44.2 to 60.1, and Na line profiles show a zoning structure in which Na decreases from core to rim. Albitite-normalized element concentrations vary systematically with the ionic radius of the element; normalized concentrations of cations with the same valence roughly form a simple convex parabolic curve when plotted against the ionic radius. This indicates that the element partitioning of murakamiite and pectolite during metasomatism to form albitite took place under the strong control of a crystal structure, quasi-equilibrated with metasomatic fluids and coexisting minerals. The δ7Li values of murakamiite and Li-rich pectolite show a wide range from −9.1 to +0.4‰ (average −2.9‰), and no obvious correlation with Li content is observed. These δ7Li values should have resulted from hydrothermal fluid–rock interactions at temperatures of 300-600°C (hydrothermal stages). The very low δ7Li values down to −9.1‰ may have originated from intra-crystalline Li isotope diffusion, or involvement of deep-seated, Li–Na-enriched subduction-zone fluids with low δ7Li values. The occurrence of porous zircon, dalyite, and mantles of zektzerite and/or sogdianite on resorbed zircon in albitite suggests that those zirconium silicate minerals are the products of metasomatic mineral replacement reactions by dissolution–reprecipitation processes associated with Na-, K-, and Li-rich hydrothermal fluids.

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