A novel pilot program for homogeneity testing of natural analogs of synthetic mineral reference materials

A novel pilot program for homogeneity testing of natural analogs of synthetic mineral reference materials

Michalak, P. P.; Renno, A. D.; Wiedenbeck, M.; Merchel, S.; Munnik, F.

The growing demand for elements (PGE, REE and refractory metals) desired by the high-tech industry stresses the need for searching for them in non-conventional raw materials. Natural variability of distribution of such elements within the matrices of mineral carriers, frequently, at the micro- and submicrometer level requires employing high-resolution spatial microanalytical techniques as an essential step in quantitative measurements as well as quality assurance procedures (Renno et al. 2010). The necessity for providing reliable, traceable and comparable results for such measurements at the picogram sampling scale makes the use of certified reference materials to be of critical importance. The database of certified reference materials in the form of solutions, powders, pellets, glass beads etc. has been developed by several research groups as well as governmental agencies with certifying capabilities (Jochum 2010). Unfortunately, among such materials natural and synthetic minerals are scarce (Jochum 2010, Wiedenbeck 2010).
Such a lack of mineral reference materials was an incentive for creating a consortium of several German scientific and federal institutions with a common goal of providing a sufficient quantity of synthetic mineral reference materials with concentrations of economically important elements certified at the submicrometer level useful for in-situ analyses with a number of microanalytical methods.
Due to various geological processes natural minerals may develop heterogeneous elemental distribution as well as many structural features that seriously limit their suitability as reference materials. A logical assumption on superiority of synthetic minerals over natural minerals can be made as far as one is able to control chemical and structural homogeneity of the grown crystal. This can be assured by inventing robust homogeneity testing procedure employing both absolute and matrix-corrected methods.
In this study we introduce a systematic approach to homogeneity testing of reference materials using their natural analogs. Microanalytical techniques were selected and introduced on a step-by-step basis in a sequence, depending on their sensitivity and spatial resolution: light and electron microscopy, EPMA, SIMS, PIXE-PIGE, LA-ICPMS, μ-SXRF. The pilot program was launched using three minerals, each representing a different group of minerals of a specific interest to the industry: pyrite, columbite-tantalite and sanidine. All specimens were tested with reflected-light microscopy and scanning electron microscopy (BSE images) and showed no inhomogeneities apart from cracks and some minor inclusions. The results of the chemical homogeneity test will be presented in detail.
The research was supported by the Free State of Saxony, European Union and Helmholtz Association.
[1] A.D. Renno et al., 2010: A development strategy for creating a suite of reference materials for the in-situ microanalysis of non-conventional raw materials. Abstract V51C-2210 presented at 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 Dec.
[2] K.P. Jochum, 2010: Reference materials for in situ microanalysis: Successes and needs, Goldschmidt Conference Abstracts 2010, A470.
[3] M. Wiedenbeck, 2010: Challenges facing the production of RMs for geochemical microanalyses, Goldschmidt Conference Abstracts 2010, A1130.

Keywords: reference materials; spatially resolved analysis

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