New SIMS procedures for the characterization of a complex silicate matrix, Na3(REE,Th,Ca,U)Si6O15·2.5H 2O (sazhinite), and comparison with EMPA and SREF results

Abstract

Analytical methods based on secondary ion mass spectrometry were developed for the characterisation of a complex layered silicate REE-mineral, named sazhinite, for which a number of issues are still open regarding its chemistry and structure. Such procedures involved the analysis and quantification of light, volatile, alkaline, medium-Z, rare earth and actinide elements. The accuracy of the SIMS data is within the assigned precision of the concentration values assumed as reference in the calibration standards employed. REE and actinide data yield a good agreement in terms of calculated site scattering at the M site: 58.42 electrons per formula unit (epfu) vs. 60.39 epfu obtained by Single Crystal Structure-Refinement (SREF). Accuracy is estimated on the order of 5% rel. for H, Li, Be and B, and 10% rel. for F. Na analysis was crucial to solve the open questions about the structure, and excellent agreement was obtained by comparing data from SIMS (REE, Y, actinides, Na) + EMPA (SiO 2, CaO, SO3 and K2O) with information derived from SREF: site scattering of the M site + Na sites = 92.56 epfu calculated from chemical data, against 91.95 epfu from SREF. Such procedures can be easily extended to the analysis of variously complex, silicate REE-minerals.