AMFORM, a new mass-based model for the calculation of the unit formula of amphiboles from electron microprobe analyses

The precision of estimating the amphibole formula [e.g., Si-T +/- 0.02, Al-C +/- 0.02, (A)(Ca+Na+K) +/- 0.04 apfu] is 2-4 times higher than when using methods published following the last IMA recommended scheme (2012). It is worth noting that most methods using IMA1997 recommendations (e.g., PROBE-AMPH) give errors that are about twice those of IMA2012-based methods. A linear relation between O-W(2-) and the sum of (C)(Ti, Fe3+) and (A)(Na+K) contents, useful to estimate the iron oxidation state of highly oxidized amphiboles typical of post-magmatic processes, is also proposed. A step by step procedure (Appendix(1) 1) and a user-friendly spreadsheet (AMFORM. xlsx, provided as supplementary material(1)) allowing one to calculate amphibole unit-formulas from EMP analyses are presented. This work opens new perspectives on the unit-formula calculation of other minerals containing OH and structural vacancies (e.g., micas).