Effect Of Thermal Treatment On The Cation Exchange And Disordering In Tourmaline

摘要

Tourmaline is an aluminoborocyclosilicate mineral with a complex arrangement of atoms. With highly variable chemistry and multiple cation sites, tourmaline is one of the last complex minerals whose structure was unraveled, and its response to changes in Pressure-Temperature-Time (P-T-X) are not well understood. Due to its stability at high temperature and pressure, tourmaline has the potential to be an informative mineral in terms of petrogenetic indicators and could be used in assessing provenance, thermobarometry and geochronology. 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. Using single crystal X-ray diffraction and stepwise heating, the extent and effect of the exchange between the Y- and Z-sites in response to changes in temperature was described.In response to increased temperature, equivalent amounts of Fe2+, Fe3+, Mg2+ of the Y-sites exchange with Al of the Z-sites. This leads to decreases in Y-site average bond length, increases in Z-site average bond length, shortening of a lattice parameters, lengthening of c lattice parameters and decreases in quadratic elongation. Additionally, the T-site experienced an increased in tetrahedral rotation and ditrigonality and changes to the crimping of the tetrahedral ring upon heating. The cation exchange and disordering in these samples relates to the stability of tourmaline at elevated temperatures in that tourmaline will undergo cation exchange and disordering to maintain the stability of the mineral. This has implications on the conditions in which tourmaline is formed as well as stability of tourmaline and other minerals and materials in different P-T-X conditions.