MONOCLINIC MEMBERS OF THE EPIDOTE GROUP - EFFECTS OF THE AL-REVERSIBLE-ARROW-FE3+REVERSIBLE-ARROW-FE2+ SUBSTITUTION AND OF THE ENTRY OF REE(3+)

Minerals of the clinozoisite-epidote series (with Fe3+ ranging from 0.24 to 1.14 a.p.f.u.) and allanites (with REE up to 0.93 a.p.f.u.) were studied from a crystal chemical point of view. Structure refinements and microprobe analyses were performed on twenty-three single crystals; the determined site populations were correlated with structural-geometrical parameters. Important modifications affect the M3 octahedron, which is the preferential site for the entry of Fe3+ and Fe2+ substituting for A1. M1 can accomodate iron too (up to 0.18 a.p.f.u.) and, in allanites, small amounts of divalent cations. Minor geometrical variations occur in the A1 and A2 polyhedra, as well as in the M2 octahedron. Tetrahedra are quite constant in shape and dimensions; however, significant variations of their mutual orientation in the Si2O7 group are observed as a function of the total iron content (p value) and the amount of REE. These two chemical variables also affect the donor-acceptor distance. The relationships between unit cell parameters and chemical composition are underlined.