Crystal chemistry of tourmalines and spinels

Summary

Tourmalines

Tourmaline group minerals are complex borocyclosilicates which occur in a wide variety of igneous, metamorphic and sedimentary rocks, and are known as useful indicator minerals of the composi-tional evolution of their host rocks. However, the chemical compo-sition of tourmaline is significantly controlled also by crystal-structural constraints.

In this collaboration project, we are studying the effects of OH-groups on structural stability and long- and short-range ordering properties, by means of spectroscopic (FTIR, Mössbauer spec.) and X-ray diffraction methods (SC-XRD). The main aim of the project is to understand how stable local atomic arrangements regulate the compositional limits of the tourmaline structure.

Spinels

Spinel group minerals are important ore metal sources, and are the most important carriers of natural remanent magnetism in the Earth’s crust. They have found numerous industrial and technological applications, and are also used to retrieve petrogenetic information from various geological settings.

In this project, we investigate natural spinel minerals as well as synthetic single crystals grown under temperatures and oxygen fugacities corresponding to the conditions during natural magmatic crystallization. The overall aim of the project is to investigate how composition and cation ordering affects the spinel crystal structure, as well as how this modify optical and physical properties. To reach comprehensive crystal chemical descriptions, we analyse spinel single crystals by a range of methods, for example electron microprobe technique, X-ray single crystal structure refinements and spectroscopic methods (Mössbauer, OAS, FTIR).

Project participants at the museum

• Henrik Skogby
• Ulf Hålenius (Professor Emeritus)

External project participants

• Ferdinando Bosi, University of Rome, Italy
• Giovanni Andreozzi, University of Rome, Italy
• Davide Lenaz, University of Trieste, Italy

Selected publications

• Andreozzi, G.B., D’Ippolito, V., Skogby, H. and Hålenius, U. (2019): Color mechanisms in spinel: a multi-analytical investigation of natural crystals with a wide range of coloration. Physics and Chemistry of Minerals 46, 343-360.
• Bosi, F., Skogby, H., Hålenius, U. and Ciriotti, M.E. (2018): Experimental cation redistribution in the tourmaline lucchesiite, CaFe2+ 3Al6(Si6O18)(BO3)3(OH)3O. Physics and Chemistry of Minerals 45, 621-632.
  
• Bosi, F., Skogby, H. and Hålenius, U. (2019): Thermally induced cation redistribution in fluor-elbaite and Fe-bearing tourmalines. Physics and Chemistry of Minerals 46, 371-383.