The quantitative chemical composition of a sample of erionite-Na from Rome, Oregon (USA) was retrieved by SEM-EDX standardless analysis. The analysed fibres have a diameter commonly smaller than 3 μm, i.e. those more easily inhalable, therefore representing a typical case encountered during environmental investigations. The claimed dependence of the toxicity and biological activity of the fibres to their crystal chemical features renders the reliable chemical characterization of paramount importance. To reduce the errors induced by particle size/morphology effects, an empirical procedure was adopted. In particular, correction factors were calculated for each oxide as a function of the particle diameter using shards of zeolites and feldspathoids of known bulk composition. Those correction factors have to be considered as sample specific. Experimental conditions were optimised in order to minimize the effect of alkali metal migration. Results indicate that particle effects cause on the oxide concentrations a relative underestimation of Al2O3 and K2O by up to ca. 10 %, whereas Na2O is overestimated by up to ca. 15 %. Moreover, the SiO2 concentration is independent from dimension even in the case of very small particles. Finally, comparison between electron-microprobe and SEM-EDX bulk analyses of crystals pointed out a relative underestimation of Na, K, (of ca. 10–15 %) and Ca (of ca. 5 %) for the SEM data, in good agreement with reference structural investigations.