A study of the late-stage mineralogy of agpaitic environments: borosilicates, niobate hydrates and development of laser ablation ICP-MS as a tool to analyze for high concentrations of light elements (Li, Be, and B)

Abstract

To date, over 400 mineral species have been discovered at Mont Saint-Hilaire (MSH) many of which are new to science. These new minerals tend to occur as late-stage phases with an enrichment high field strength elements (HFSE) (i.e. Nb, Ti, Zr) as well as light elements (i.e. Li, Be and B). Characterization of these minerals has been hindered by their small crystal sizes (i.e. μm scale) as well as enrichment in light elements. This thesis discusses the chracterization and overall implications of franconite group minerals as well as the new boro silicate minerals steedeite and nolzite. Finally the effectiveness of laser ablation ICP-MS as a method of accurately measuring major concentrations of light elements is discussed. Franconite group minerals (FGM) are hydrous-alkali niobates, inferred to have formed from Nbrich precursor minerals at low temperatures (~150 °C). The structures of these minerals are held together by hydrogen bonds and are highly flexible with Na, Ca, Mg and possilbly Fe2+ and K substituting for one another. Although FGM are Nb-rich, they are notably devoid of Ta despite the two elements being geochemical twins. The FGM are crystallo-chemically similar to scandia octahedral molecular sieves, a group of synthetic compounds known for their cation exchange properties. Twelve borosilicate phases, including the new minerals steedeite and nolzeite are known from MSH. These are crystallo-chemically similar, both minerals containing of single loop-branched dreier chains in their structures. Such chains occur in synthetic compounds (e.g. Li2Mg2[Si4O11]) but are not known to occur in other naturally occurring minerals. Steedeite is highly ordered with respect to Si and B whereas nolzeite contains a mixed Si/B site despite the two minerals being associated with one another. Accurate chemical analysis of late-stage minerals, rich in light elements, is hindered by the fact that such elements have low atomic masses. Laser ablation ICP-MS analysis of a variety of mienrals with major concentrations of light elements gave measured concentrations of Li, Be and B that were within ± 10% of expected ideal values for most minerals. Beam widths varying from 6 to 30 μm were employed however; they had no effect on the measured light element concentrations. Four standards including the NIST610 & 612 glasses, natural beryl, and lithium metaborate were used. Of these NIST610 is considered the most suitable as it has the smallest associated errors (1σ error: ± 0.05 to ± 1.68 ppm). Based on this LA-ICP-MS is considered to be a suitable alternative to secondary ion mass spectrometry for analysis of light elements.