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).
In this work, we have studied the relationships between mass concentration and unit formula of amphibole using 114 carefully selected high-quality experimental data, obtained by electron microprobe (EMP) + single-crystal X-ray structure refinement (SREF) +/- secondary-ion mass spectrometry (SIMS) analyses, of natural and synthetic Li-free monoclinic species belonging to the Ca and Na-Ca subgroups, and 75 Li-free and Mn-free C2/m end-members including oxo analogs of Ca amphiboles. Theoretical considerations and crystal-chemical driven regression analysis allowed us to obtain several equations that can be used to: (1) calculate from EMP analyses amphibole unit-formulas consistent with SREF +/- SIMS data, (2) discard unreliable EMP analyses, and (3) estimate O-W(2-) and Fe3+ contents in Li-free C2/m amphiboles with relatively low Cl contents (<= 1 wt%). The AMFORM approach mostly relies on the fact that while the cation mass in Cl-poor amphiboles increases with the content of heavy elements, its anion mass maintains a nearly constant value, i.e., 22O + 2(OH,F,O), resulting in a very well-defined polynomial correlation between the molecular mass and the cation mass per gram (R-2 = 0.998).
AMFORM, a new mass-based model for the calculation of the unit formula of amphiboles from electron microprobe analyses
Zanetti Alberto;Oberti Roberta
2018-01-01
Abstract
In this work, we have studied the relationships between mass concentration and unit formula of amphibole using 114 carefully selected high-quality experimental data, obtained by electron microprobe (EMP) + single-crystal X-ray structure refinement (SREF) +/- secondary-ion mass spectrometry (SIMS) analyses, of natural and synthetic Li-free monoclinic species belonging to the Ca and Na-Ca subgroups, and 75 Li-free and Mn-free C2/m end-members including oxo analogs of Ca amphiboles. Theoretical considerations and crystal-chemical driven regression analysis allowed us to obtain several equations that can be used to: (1) calculate from EMP analyses amphibole unit-formulas consistent with SREF +/- SIMS data, (2) discard unreliable EMP analyses, and (3) estimate O-W(2-) and Fe3+ contents in Li-free C2/m amphiboles with relatively low Cl contents (<= 1 wt%). The AMFORM approach mostly relies on the fact that while the cation mass in Cl-poor amphiboles increases with the content of heavy elements, its anion mass maintains a nearly constant value, i.e., 22O + 2(OH,F,O), resulting in a very well-defined polynomial correlation between the molecular mass and the cation mass per gram (R-2 = 0.998).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
