The mess that is allanite

Allanite is a poorly defined collection of species belonging to the epidote group. In the past, allanite was defined as merely being lanthanon-bearing, but more recently, it is defined as being Ln-dominant at the A2 site. Species of the allanite subgroup include allanite-(Ce), allanite-(La), allanite-(Y), androsite-(La), dissakisite-(Ce), dollaseite-(Ce) and khristovite-(Ce). Lack of recognition of the recommendations of Nickel & Mandarino (1987), and of the relation between members of the subgroup with allanite in their species name and those without, has led to a number of errors in naming species and in use of the term allanite. Ten methods have recently been published describing how the relevant formulae may be calculated. Each is reviewed in conjunction with new observations on the behavior of Si, Cr, V, Mn2+ and A-site vacancies. This evaluation results in a recommended procedure for the calculation of the formula involving a basis of 6 (M + T) cations and 12(O,F,Cl) + 1(OH). Recalculation of the formulae of published compositions of allanite-subgroup minerals shows that some apparently new species are better interpreted as intermediate solid-solutions between conventional end-members; there are, however, at least five potentially new species awaiting description.