TITLE:
A Pragmatic Approach for Using Conventional Electron Microprobe Analyses to Obtain Reliable Formulae for Differentiated Alkali Igneous Rock Amphiboles by Assessing the Possible Presence of Oxy-Amphibole Components
AUTHORS:
C. Michael B. Henderson, Roger H. Mitchell
KEYWORDS:
Amphibole Stoichiometry, Oxo-Amphiboles, Vacant Cation Sites, Ti-Rich Amphiboles, 24 Anion Calculation, Fe3+/?Fe Modelling, Alkaline Rocks
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.14 No.6,
June
26,
2026
ABSTRACT: Conventional electron probe microanalysis (EPMA) provides major and minor element data that lack water and Fe valence information, requiring calculation of Fe3+/Fetotal ratios based on amphibole stoichiometry. Amphibole compositions, particularly in alkaline rocks, show the presence of extra oxygen (denoted WO2? here) replacing (OH) (i.e., the oxyamphibole molecule). Micro-analytical and spectroscopic published work provides ‘full’ analyses which allow refinement of protocols for modelling amphibole formulae and calculation of Fe3+/Fetotal ratios for a wide-range of amphiboles in alkaline igneous rocks. In this paper formula calculation protocols are checked against the compositions of hypothetical stoichiometric amphiboles with chosen proportions of vacant sites and fixed values for Fe3+ and Fe2+. A spreadsheet for EMP analyses of Ca- and Na-rich amphiboles in alkaline igneous rocks with any content of vacancies in A, is provided to calculate formulae on a stoichiometric (23 O + WO2?/2) formula basis, with Fe3+ estimated for a 13 cation calculation. Ca-Na amphiboles generally have higher WO2? than Na-amphiboles requiring their WO2? contents to be assessed if reliable Fe-oxidation states are to be estimated. The preferred Fe3+/ΣFe calculation protocol is assessed for published amphibole compositions with Fe-valence values determined by M?ssbauer spectroscopy confirming the earlier reports that many amphiboles, especially Ti-rich samples, have high extra-O and may possess excess cations in the C cation group. A scheme is developed to quantify this excess (denoted ?C) giving the relationship Σ13cations = Σ16cations ? Σ(Ca + Na + K) ? ?C. It is found that many amphiboles have relatively low Fe3+/ΣFe values (0.1 - 0.3) consistent with equilibration near the QFM buffer.