Sn-rich tourmaline from the Land’s End granite, SW England

[摘要] Multiple generations and growth stages of tourmaline from a hydrothermal quartz-tourmaline rock from the Land’s Endgranite, SW England, were investigated by Electron Probe MicroAnalyzer (EPMA) to reveal details of the variation intourmaline composition with emphasis on the distribution of Sn. Tourmaline shows a large range in chemical composition, mostly on the dravite–schorl solid solution and towards more Fe-rich compositions. Several growth zones havevery high Fe levels (>3.5 apfu) with a significant amount of Fe3+ coupled with low Al. The main substitution vectorscontrolling the major element composition are Fe2+Mg–1 and Fe3+Al–1. The Fe–Mg exchange is the main substitution inthe earlier growth stages, whereas the Fe–Al substitution becomes more important towards the end of the crystallizationsequence. Tin is commonly associated with the high-Fe zones, but all Fe-rich zones do not necessarily have elevatedSn content. Octahedral sites in tourmaline, most likely the Y-site, host Sn through the proposed coupled substitution YZSn4+ +2YZFe2+ +5YZFe3+ + WO2–↔2YZMg2+ +6YZAl3+ + WOH–. The thin Sn-rich zones, hosting up to 2.53 wt. % SnO2, areinterpreted to coincide with the onset of cassiterite crystallization, and the lower Sn content in subsequent growth zonesreflects the fluid chemistry and Sn solubility in a cassiterite-buffered hydrothermal system. This study demonstrates thesuitability of quantitative X-ray mapping in identifying and quantifying minor elements in finely-spaced growth zones.

[学科分类] 自动化工程

[关键词] Tourmaline;tin;trace elements;EPMA