EFFECT OF THERMAL TREATMENT ON THE CATION EXCHANGE AND DISORDERING IN TOURMALINE

Tourmaline has a complex atomic arrangement that has been well studied at room temperature. However, the response of cation ordering in the tourmaline structure to elevated temperatures is not as well understood. In this study single crystal X-ray diffraction and stepwise heating were used to describe the extent and effect of cation exchange between the Y- and Z-sites in response to changes in temperature. Three reactions were proposed to understand the cation exchange and disordering between the Y- and Z-sites in the tourmaline structure. These reactions include: 1. ^YFe²⁺ + ^ZAl + OH ↔ ^ZFe³⁺ + ^YAl + O + H↑ in two samples with varying Fe²⁺ content. 2. ^YMg + ^ZAl ↔ ^ZMg + ^YAl. 3. ^YFe³⁺ + ^ZAl ↔ ^ZFe³⁺ + ^YAl.

In response to increased temperature, equivalent amounts of Fe²⁺, Fe³⁺, Mg²⁺ in the Y-sites exchange with Al of the Z-sites. This leads to a decrease of the Y-site average bond length, increases of the Z-site average bond length, shortening of lattice parameter a, lengthening of lattice parameter c and decreases in quadratic elongation. Bond valence sums for the cations in the Y- and Z-sites were calculated at each temperature step to assess cation stability. Cation exchange and disordering in these samples allows tourmaline to maintain stability at elevated temperatures. This research allows for observation of the tourmaline structure when it crystallizes at high temperatures and how disordering and distortion allow the structure to remain crystalline at high temperatures.