- © The Mineralogical Society Of America
Epidote-group minerals rich in rare earth elements (REE), in particular allanite, are common accessory phases in igneous, metamorphic, metasomatic, and sedimentary rocks. Small amounts of REE are present in most epidote-group minerals, but in allanite—and the related minerals dissakisite, ferriallanite, dollaseite, khristovite and androsite—the REE are essential structural constituents. An important characteristic of REE-rich epidote-group minerals is that their octahedrally coordinated M sites contain major amounts of divalent cations. This paper summarizes literature data for these minerals and discusses their chemistry, occurrence, phase relations, and petrologic and geologic significance. The chapter emphasizes allanite, because it is the most common and best-studied of the REE-rich epidote-group minerals.
MINERAL CHEMISTRY AND NOMENCLATURE
Epidote-group minerals contain isolated silicon tetrahedra and corner-sharing groups of two tetrahedra, and are thus assigned to the disilicate or sorosilicate structural family (for a detailed description of the structure, see Franz and Liebscher 2004). The epidote-group structural formula is A2M3(SiO4)(Si2O7)(O,F)(OH), or in a simplified form A2M3Si3O11(O,F)(OH), in which A = Ca, Sr, Pb2+, Mn2+, Th, REE3+, and U, and M = Al, Fe3+, Fe2+, Mn3+, Mn2+, Mg, Cr3+, and V3+ (Deer et al. 1986). There are two structurally different A sites, A(1) and A(2), with different coordination numbers, and there are three different M sites, M(1), M(2), and M(3), which are all octahedrally coordinated (Ueda 1955; Dollase 1971).
In epidote-group minerals, trivalent REE are accommodated in the A sites, which in endmember epidote both contain Ca. The incorporation of REE3+ is commonly charge balanced by a divalent cation (Fe2+, Mn2+, Mg) substituted for a trivalent one in the M sites (Table 1 …