Ercit reported in 2002 that "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.
On the basis of the above procedure to calculate a formula, the results of a number of published studies were re-examined that lists unusual compositions of allanite-subgroup minerals based on dominant constituents of the various sites, and on interpretation of these compositions. Many of these would seem to represent new species if interpreted on the basis of dominant-site constituent rather than dominant end-member components. For example, sample 8 of Carcangiu, et al. (1997) would seem to represent a new species because of its unique composition, with Al dominant at M3 and Ln(Ce) dominant at A2. However, it is impossible to construct an end-member for such a composition without invoking a significant proportion of vacancies at A1, contrary to the observed full occupancy of A1 for sample 8. Instead, it is more correct to interpret sample 8 as an intermediate composition consisting mostly of a solid solution between clinozoisite and allanite-(Ce), with other less significant components.
The same logic applies to the following three compositions, which are more correctly interpreted as the products of solid solution between clinozoisite and allanite-(Ce), then epidote and allanite-(Ce), and epidote and allanite-(Y), respectively. However, the five remaining compositions feature significant amounts of components of as-of-yet
unrecognized end-members. The proportions of these new components are sufficient to warrant descriptions of new species; however, to do so requires further work
on these samples.
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