Nb-Ta oxide minerals from miarolitic pegmatites of the Baveno pink granite, NW Italy

摘要: Chemical composition and cell parameters were ascertained on new data from Ti-Nb-Ta complex oxides from the miarolitic pegmatites of the Baveno pink granite (Southern Alps, NW Italy). The crystals are tiny, single or aggregated in small sprays, prismatic or tabular, from yellow-orange to brownish in colour. Typical associated minerals include fluorite, zinnwaldite, gadolinite-group minerals and Sc-minerals. Cavity paragenesis is typical of NYF pegmatites, and shows two stages of crystallization developing in magmatic-pneumatolitic and hydrothermal conditions. X-ray data show that some oxides belong to the aeschynite mineral group; others are polycrase and fersmite. Aeschynite and polycrase are chemically heterogeneous and structurally disordered because of their metamict state. This disorder does not always seem to be related to radionuclide contents. Two main trends are indicated, considering the behaviour of Y. The high Y contents fit with very low Ca and LREE contents in the A site; the HREE contents follow the Y trend. In the B site, Ti is the dominant cation, followed by some Nb and very little Ta. Small quantities of Y fit with increasing Ca, U, Th and REE contents. In the B site, Nb cations often exceed those of Ti. The Th contents are often greater than those of U. Besides the already known aeschynite-(Y) and vigezzite, new varieties, ‘titano-vigezzite’ and ‘niobo-aeschynite-(Y)’, are identified here in the Baveno miarolitic cavities. Samples 14 (analysis a) and 2502 (analysis b) have Ca as the main occupant of the A site, followed by Y, Th and REE; in the B site, Ti prevails over Nb. These compositions cannot be considered as pure vigezzite, but as a new variety called ‘titano-vigezzite’. In the same way, analysis a of sample 3 may be considered a new variety of aeschynite-(Y), with Nb prevailing over Ti in the B site, and here called ‘niobo-aeschynite-(Y)’. Neither variety has ever been mentioned in the literature. Epitaxial growth of aeschynite on polycrase (sample 3194) allows some inferences on the crystallization sequence.