NEUTRON DIFFRACTION AS A TOOL FOR MINERALOGY

Neutrons are commonly used as a tool in the study of functional materials, where the greater sensitivity to lighter elements is frequently used to locate hydrogen, lithium, etc. within crystal structures. The magnetic moment of neutrons also makes them ideal for probing the magnetic structures of materials under different conditions. Many of the same questions arise in mineral structures but neutrons are rarely applied to these problems.

The diffraction instruments available at the Spallation Neutron Source (Oak Ridge National Laboratory) will also be introduced together with the capabilities that they can bring to the mineralogical community.

An example will be used to illustrate a case where neutron diffraction data were key in understanding the detailed nuclear and magnetic structure, in ambient and non-ambient conditions. A study was recently undertaken to examine the changes that occur on transforming blue dioptase (Cu₆Si₆O₁₈·6H₂O) to black dioptase (Cu₆Si₆O₁₈). The electronic environment of copper in dioptase is intimately tied to the coordination of water, and its increased mobility in the blue phase eliminates the Jahn-Teller distortion present in the more common green phase.  Given the changes in the electronic structure it is not surprising that the magnetic susceptibility and Neel transition temperatures differ between the phases. Green dioptase displays antiferromagnetic ordering at low temperature but the nuclear and magnetic structures of the blue phase are currently unknown.