The role of SIMS in the investigation of the complex crystal-chemistry of mica minerals

Mica is one of the most common minerals present on the Earth Crust (~ 4 vol%) in all kinds of rocks: metamorphic, sedimentary and igneous ones. In addition, mica can be found in the lower crust as well as in the upper and deeper mantle, in association with other minerals. It is of paramount importance in the fields of mineralogy, crystal chemistry, petrography, petrology, geochemistry and volcanology. Indeed, due to the peculiarity of its atomic organisation, mica is able to record in its structure information about the geological history the host rock underwent. Actually, its complex chemical composition and the frequently occurring structural disorder often prevent or limit the knowledge of the structural details suitable for comparative crystal-chemical studies. This is particularly true for trioctahedral micas of volcanic origin, essentially owing to the different structural role played by some cations (e.g., Al, Fe, Ti), their isomorphic substitutions, and their different oxidation states (Fe, Ti) combined with frequent grain-to-grain and/or inter-grain chemical heterogeneity. In mica literature the importance of Fe3+-oxy mechanisms in determining the explosive nature of volcanism is often highlighted. Since the first applications of secondary ion mass spectrometry (SIMS) to geologic samples, it has been evident that this technique can play a key role in the characterisation of minerals and rocks, owing to its high spatial resolution and sensitivity from H to U. However, SIMS suffers from a major intrinsic drawback, i.e., the so-called «matrix-effects» affecting sputtering-ionisation phenomena, which cannot be eliminated but only calibrated. To date, the available standards for micas are very rare and insufficient to face the variety of their chemical compositions. Moreover, the «matrix effects» are further complicated by the existence of crystallographic orientation effects relative to the ion beam, which are related to its strongly anisotropic structure. By means of an analytical procedures developed by us, it is possible to use the ion microprobe to obtain accurate concentrations of the light elements, i.e., H, Li, (Be, B), F (Cl) in mica by means of a small number of silicate standards and mathematical corrections on the ion yields. Examples of SIMS applications to the solution of the complex crystal chemistry in micas from volcanic areas of Southern Italy are presented. In particular, the possibility to measure hydrogen by SIMS has resulted to be fundamental in assessing the correct structural formula and substitution mechanisms occurring in micas, which in turn constitutes the basis of a sound derivation of petrologic/thermodynamic parameters in mica-bearing systems; while the knowledge of Li- and F-contents have contributed to understanding the processes the host rock underwent. For instance the finding of fluorophlogopite in the volcanic rocks of Biancavilla (Sicily) provided evidence that metasomatic processes affected the pre-existing benmoreitic rocks of the Etnean volcanic area.