Liandratite from Karkonosze pegmatites, Sudetes, Southwestern Poland

The chemical composition of liandratite, U6+(Nb,Ta)2O8, was determined from material collected in Niobium, Yttrium, Fluorine type (NYF) pegmatites of the Karkonosze intrusion (Sudetes, SW Poland). Liandratite occurs mainly as rims, up to 40 µm thick, and fracture infillings in fergusonite-(Y) and other Nb-Ta-Ti minerals. Its formation was related to the fluid-driven alteration of primary minerals by three potential mechanisms: (i) direct replacement of a primary mineral by liandratite; (ii) breakdown of the primary mineral to liandratite and a product with the composition of minerals of the pyrochlore group; (iii) multistage alteration, which involved: removal of A-site cations (mostly Y + REE with the exception of U4+) and formation of phases with the composition of pyrochlore group minerals; then crystallization of U-, Bi-, Pb-rich pyrochlores and their replacement by liandratite. The chemical compositions of liandratite formed by the breakdown of different primary minerals are also different, mainly in their U, Ti and Nb contents. Excess Ti, relative to the U6+Nb2O8 end-member, is incorporated into the structure together with additional U. The Ti content of liandratite, and partially through this the U content, are dependent on the nature of the precursor mineral.