Structural accommodation of K at dravitic tourmaline’s X-site: insight from Raman spectroscopy and single crystal X-ray diffraction

English Abstract:

Potassium incorporation in dravite increases with pressure, temperature, and its relative concentration in the crystallizing fluid, leading to K-dominant tourmaline (maruyamaite [K(Mg₂Al)Al₆Si₆O₁₈(BO₃)₃(OH)₃O]) forming at high-grade conditions given a K-rich fluid. The question remains of how the large K⁺ ion influences tourmaline’s crystal structure, particularly in comparison to the more common end-member, dravite [NaMg₃Al₆Si₆O₁₈(BO₃)₃(OH)₃(OH)]. In addition, most Raman spectroscopic studies have been conducted on natural tourmaline, which are solid solutions of multiple end-members. The presence of many site substitutions in these tourmaline leads to complicated Raman spectra, hindering unambiguous band assignments.

To understand how tourmaline accommodates the K⁺ ion, we characterized synthetic maruyamaite by electron microprobe, single crystal and powder XRD, Raman and IR absorption spectroscopy. Our synthetic tourmalines have a constrained composition and correspondingly simpler Raman spectra allowing us to unambiguously assign the O-H stretching bands to specific site compositions. The incorporation of K in dravitic tourmaline expands the unit cell volume from a value of 1570 ų for our dravite to 1588 ų for our maruyamaite. In detail, the X-site polyhedra of the latter expand, with average <X-O> distances of 2.737(12) Å (cf. ~2.69 Å for dravite). The other coordination polyhedra are similar in size to dravite. Comparison of the Raman spectra of dravite and maruyamaite reveals K’s effect on the local bonding environment. The Raman band assigned to the occupied X-site shifts from 3750 (dravite; ^XNa) to 3769 cm^-1 (maruyamaite; ^XK), reflecting the shortening of the O-H bond in the X-site-neighbouring W(O1) site. In addition, the presence of vacancies at the X-site is common to all synthesized tourmaline, as indicated by a band at 3618 – 3622 cm^-1. The lower frequency of this band indicates that vacancies are associated with a lengthening of the O-H bond at the W(O1) site.

The investigation of Raman spectra of well-characterized synthetic tourmaline allows unambiguous band assignment. Moreover, the occurrence and relative positions of these bands combined with single crystal XRD provides insightful information about the effect of composition on tourmaline’s crystal structure.