The Mess That Is "Allanite"

Abstract

"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, Mn 2+ 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.

Spectroscopic study of inclusions in gem corundum from Mercaderes, Cauca, Colombia

“…Energy-dispersive X-ray analysis in the SEM yielded an approximate chemical composition of 43 wt% SiO 2 , 29 wt% Al 2 O 3 , 13 wt% CaO, 6 wt% FeO, 3.5 wt% Ce 2 O 3 , 2.5 wt% La 2 O 3 , 1.4 wt% MgO, <1 wt% Nd 2 O 3 , and <1 wt% Y 2 O 3 . We hence assign tentatively the mineral under discussion to a REE-bearing member of the epidote group, but in most cases presumably allanite-(Ce) (for a discussion of chemical compositions of REE-bearing epidote-group minerals see Ercit 2002). This assignment is supported by our PL spectra (Fig.…”

Recommended nomenclature of epidote-group minerals

“…Ercit (2002), in his article "The mess that is allanite", asks the provocative question: What is "allanite"? He states that the name "allanite" is often incorrectly used to describe any REE-bearing epidote.…”

“…Until 1988, the mineralogy of REE-rich epidote-group minerals was very simple because there was only the one official root name allanite with the end-member formula CaREEAl 2 Fe 2+ [Si 2 O 7 ] [SiO 4 ]O(OH). An additional source of confusion (Ercit, 2002) is that cations in the formula of REE-rich epidote-group minerals are commonly grouped rather than assigned to specific sites. However, specific site assignments determine the species.…”

“…However, specific site assignments determine the species. In the absence of appropriate guidelines, Ercit (2002) discusses several approaches for using electron-microprobe analytical data to calculate formulae of REE-bearing epidote-group minerals. Then he uses analytical data from the literature to obtain a simplified end-member formula corresponding to a mineral name.…”

Micro-structural and compositional variations of hydrothermal epidote-group minerals from a peralkaline granite, Corupá Pluton, Graciosa Province, South Brazil, and their petrological implications

“…In general, allanite compositions close to the ferriallanite end-member represent allanite from zone I, while others are mainly from zone II. The allanite zone IV is relatively more Al-rich, and (REE+Y)-poor and their compositions are somewhat displaced The best estimation methods, according to Ercit (2002), are based on 6 (M+T) or 8 (A+M+T) cations and 25 negative charges, the first being superior as it does not depend on A-site vacancies. However, the second method was indicated by the IMA, taking into account the data obtained by Bonazzi et al (1996) clinozoisite-epidote-allanite-ferriallanite (cf.…”