THE PROFILE OF TRACE ELEMENTS, INCLUDING THE REE, IN GEM-QUALITY GREEN ANDRADITE FROM CLASSIC LOCALITIES

摘要

Crystals of gem-quality andradite (variety "demantoid") from Val Malenco, Italy, have been analyzed for major, rare-earth and other trace elements (Sc, Ti, V, Cr, Co, Ni, Zn, Sr, Y, Zr), together with samples from Aosta Valley, Italy, and from the classic deposits of the world (Russia, Iran, Pakistan, Namibia, Madagascar). They are all of gem quality and vary from bright green to yellow-green in color. The samples are homogeneous within the limit of the analytical error and cover a restricted range of composition from almost pure andradite (Adr > 98 mol.%) to members of andradite-uvarovite (Adr_(81-96)Uv_(3-18)) or andradite-grossular (Adr_(92-94)Grs_(6-8)) solid solution, with the sole exception of the sample from Namibia, showing a composition varying from pure andradite to almost pure grossular (Adr_(11)Grs_(89)). All the samples have a low abundance of most trace elements, except for Cr, which ranges from a few ppm to more than 5 wt% Cr_2O_3. The substitution of Fe~(3+) and Cr~(3+) for Al at the [Y] site significantly controls the geometry of the structural sites and the incorporation of the REE. In particular, (i) the samples showing a composition close to pure andradite exhibit LREE-enriched and HREE-depleted patterns with a strong positive Eu anomaly, whereas (ii) the uvarovite-enriched samples show flatter patterns with a small positive Eu anomaly, and (iii) grossular-rich samples are LREE-depleted with no Eu anomaly or a negative one. However, such a compositional variation may also arise from differences in the bulk composition of the host rocks and from changes in the physicochemical conditions during growth.