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

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Marco E. Ciriotti
Messaggi: 24606
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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AMFORM, a new mass-based model for the calculation of the unit formula of amphiboles from electron microprobe analyses

Messaggio da Marco E. Ciriotti » gio 05 lug, 2018 15:07

Referenza:
▪ Ridolfi, F., Zanetti, A., Renzulli, A., Perugini, D., Holtz, F., Oberti, R. (2018): AMFORM, a new mass-based model for the calculation of the unit formula of amphiboles from electron microprobe analyses. American Mineralogist, 103, 1112–1125.

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 WO2– 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 (R2 = 0.998).
The precision of estimating the amphibole formula [e.g., TSi ± 0.02, CAl ± 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 WO2– 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 (Appendix1 1) and a user-friendly spreadsheet (AMFORM.xlsx, provided as supplementary material1) 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).
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

VINCENZO MILETTO
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Re: AMFORM, a new mass-based model for the calculation of the unit formula of amphiboles from electron microprobe analys

Messaggio da VINCENZO MILETTO » lun 09 lug, 2018 22:32

Ciao Marco,

un lavoro molto interessante che apre nuove prospettive di studio su tanti minerali!
Grosso modo la formula base degli anfiboli è composta da 40 atomi, dove la sostituzione di un elemento non influisce sulla sua natura, ma comporta poi un enorme cambiamento a livello macroscopico, quindi visivo, dove gli atomi sono poi a milioni, e non è detto che siano sostituiti tutti con un unico elemento.
Ho vicino a me una ex orneblenda di Traversella grossa come una mela, attraversata da una vena di magnetite e ti assicuro che già a 40 ingrandimenti c'è un mondo "infinito" di forme e colori.
Ma voler chiudere in un'unica formula un mondo così complesso non è come cercare una legge unica che governi tutto l'universo?
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Marco E. Ciriotti
Messaggi: 24606
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

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

Messaggio da Marco E. Ciriotti » mar 10 lug, 2018 10:14

Ma voler chiudere in un'unica formula un mondo così complesso non è come cercare una legge unica che governi tutto l'universo?
Se i materiali sono molto omogenei tutto regge.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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