THERMAL EXPANSION IN THE TOURMALINE MINERAL SYSTEM

Tourmaline is an acentric, rhombohedral boron silicate mineral formed in igneous and metamorphic rocks that can be useful as an indicator of rock petrogenesis. As such, it is important to understand its chemical, structural, and thermodynamic properties.

Our principal goal is to determine how thermal expansion along different crystallographic axes is affected by chemical composition. Additionally, this study reveals decomposition temperatures under oxidizing conditions for the various compositions studied. Here we report (1) X-ray powder diffraction data for zone-free, near-end-member tourmaline samples and (2) preliminary conclusions on the manner in which tourmaline chemistry affects thermal expansion for individual crystallographic axes as well as volume.

The end-members studied to date are schorl, dravite, elbaite, rossmanite, liddicoatite, uvite, buergerite, olenite and feruvite. X-ray powder diffraction data were collected at Lafayette College between 15° and 80° 2Θ on a PANalytical Empyrean X-ray powder diffractometer equipped with an Anton Paar furnace. Temperature calibrations between 22° to 900 °C were made via reversible polymorphic transformations for several different compounds; these showed a consistent 25°- 30° temperature difference between set and actual temperatures. NIST silicon was used as an internal standard. Peaks were indexed manually based on data in the literature. The software of Holland and Redfern (1997) was utilized for the calculation of unit-cell dimensions.

Data analyses reveal that end-members with high iron (Fe²⁺) content initiated breakdown between 628°C - 728°C. End-members with hydroxide (OH^-) as the dominant anion initiated breakdown between 778°C - 828°C. Schorl with high iron (Fe²⁺) has a flatter V-T slope and thus expands less than the other specimens analyzed. Currently, we are analyzing the location of Fe²⁺ in the structure relative to the observed variations of the unit-cell dimensions with temperature. We also shall be making measurements on additional specimens between abstract submission and the NE GSA meeting.

We thank the National Museum of Natural History, George Rossman and Frank Hawthorne for supplying the samples for this work, which was funded by the Earth Sciences Division of the National Science Foundation.