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

Chemical composition and cell parameters were ascertained on new data from Ti-Nb-Ta complex oxides from the miarolitic pegmatites of the Baveno pink g ranite (Southern Alps, NW Italy). The crystals are tiny. single or aggregat ed in small sprays, prismatic or tabular, from yellow-orange to brownish in colour. Typical associated minerals include fluorite. zinnwaldite. gadolin ite-group minerals and Sc-minerals. Cavity paragenesis is typical of NYF pe gmatites, and shows two stages of crystallization developing in magmatic-pn eumatolitic and hydrothermal conditions. X-ray data show that some oxides b elong to the aeschynite mineral group; others are polyerase and fersmite. A eschynite and polyerase are chemically heterogeneous and structurally disor dered because of their metamict state. This disorder does not always seem t o be related to radionuclide contents. Two main trends are indicated, consi dering the behaviour of Y. The high Y contents fit with very low Ca and LRE E contents in the A site: the HREE contents follow the Y trend. In the B si te, Ti is the dominant cation. followed by some Nb and very little Ta. Smal l quantities of Y tit with increasing Ca, U, Th and REE contents. In the B site, Nb cations often exceed those of Ti. The Th contents are often greate r than those of U. Besides the already known aeschynite-(Y) and vigezzite, new varieties, 'titano-vigezzite' and 'niobo-aeschynite-(Y)', are identifie d 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 c onsidered 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 o f 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 lit erature. Epitaxial growth of aeschynite on polyerase (sample 3194) allows s ome inferences on the crystallization sequence.