Raman Spectroscopy of Near-Endmember Tourmalines Dravite and Magnesio-Foitite to 24 GPa

Abstract

The tourmaline supergroup comprises the borosilicates most abundant on Earth, making these minerals a very significant boron repository in the crust. Additionally, the tourmaline structure is extremely chemically flexible and has low intra-volume diffusion, allowing tourmalines to exhibit well-defined compositional zoning. As such, tourmalines have great potential for use as petrogenetic indicators for fluid composition, pressure and temperature conditions, and boron isotope signatures. We use diamond anvil cells and Raman spectroscopy to examine the pressure response of two synthetic tourmaline crystals, dravite [XNa YMg3 ZAl6 TSi6 O18 (BO3)3 V(OH)3 W(OH)] and magnesio-foitite [X Y(Mg2Al) ZAl6 TSi6 O18 (BO3)3 V(OH)3 W(OH)], of near-endmember compositions, at pressure up to 24 GPa. We identified and tracked through pressure 14 Raman bands in dravite and 17 in magnesio-foitite. Our results are as follows: (1) for both dravite and magnesio-foitite, the Raman bands in the T-site spectral region (600-800 cm-1) split into two independently coherent band clusters around 9 GPa; (2) a new band with a Raman shift of ~560 cm-1 appears at ~15 GPa for dravite and ~13 GPa for magnesio-foitite; (3) a prominent band at ~370 cm-1 is observed in both samples to develop into two prominent bands by 15 GPa in magnesio-foitite and after 24 GPa in dravite. Our results provide insight into the atomistic controls on the high-pressure mechanical behavior of the tourmaline structure, as well as evidence supporting the presence of a subtle phase transition at ~15 GPa. We also present experimentally-determined values for the Grüneisen parameter of both samples calculated from vibrational modes: dravite = 0.63 +/- 0.36, Mg-foitite = 0.39 +/- 0.37. These values are broadly in agreement with values for the Grüneisen parameter obtained from thermodynamic calculations.