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Crystal chemistry and synthesis of selected borosilicate minerals. Scott, Graeme

Abstract

Borosilicates are oxygen-bearing boron minerals in which SiO₄ tetrahedra form an important structural component. They represent widespread constituents of rocks originating in the Earth’s crust and commonly provide insight into the rock-forming processes. Important borosilicates include tourmaline, axinite, werdingite, boralsilite, dumortierite, holtite, howlite, and grandidierite. Tourmaline is the most widespread borosilicate mineral and it can be used to study a wide variety of geological processes based on its high compositional variability. Dumortierite [(Al,☐)Al₆(BO₃)Si₃O₁₃(O,OH)₂] is second only to tourmaline as the most abundant borosilicate mineral, but remains relatively understudied in comparison. Isostructural holtite [(Al,Ta,Nb,☐)Al₆(BO₃)(Si,Sb,As)₃O₁₂(O,OH,☐)₃] is poorly constrained chemically. The ability to incorporate elements such as As, Sb, Bi, Nb and Ta make dumortierite and holtite unusual for silicate minerals. Synthesis experiments designed to better determine the relationship between holtite and dumortierite by synthesizing dumortierite and gradually replacing Si with As and Sb were carried out at the GeoForschungZentrum (GFZ) in Potsdam, Germany. Synthesis conditions were designed based on the previous work by Werding and Schreyer (1990). Experiments ranged from 3-5 kbar at 550-650 °C and 15-20 kbar at 600-700 °C. The less common borosilicate boralsilite [Al₁₆B₆Si₂O₃O₃₇] was the dominant phase produced in the attempts at synthesizing dumortierite. The results showed an increased stability range for boralsilite than had been previously studied and gave insight to simpler methods of synthesis. A detailed case study of the mineralogy of the Uvil’dy Lake pegmatite revealed a potentially unique locality for dumortierite and tourmaline evolution. The tourmaline shows a high Mn/(Fe + Mg) ratio within primary phases and increased Li, Fe and Mn-enrichment in later phases. Apart from being unusually yellow, the dumortierite from Uvil’dy was also anomalously high in Bi (~0.03 apfu). Eight additional samples of dumortierite from different global localities were analysed and were compared to approximately 1100 dumortierite and holtite compositions assembled from both published and unpublished data from worldwide localities. Including the analysed Uvil’dy samples, this extensive dataset gives a detailed look at the chemical relationship between dumortierite and holtite and allows them to be better constrained in terms of their chemical constituents.

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