The First (and Second) Known Occurrences of Bazzite in Canada – The Quadeville Rose Quartz Quarry, Ontario, and the Bugaboo Castles Aquamarine Deposit, British Columbia – Description and Crystal Structure

Abstract

Abstract Bazzite, ideally Be3Sc2Si6O18, has been discovered for the first time at two granitic pegmatite localities in Canada: the Quadeville Rose Quartz quarry, Ontario (Bz-ON/CMNMC 90604), and the Bugaboo Castles aquamarine deposit, Purcell mountains, British Columbia (Bz-BC/CMNMC 90725). Bazzite from both Canadian localities occurs as colorless, hexagonal prismatic to acicular crystals up to 0.6 mm in length. The crystal chemistry and structure of the two samples, Bz-ON and Bz-BC, was determined and compared with bazzite from other known occurrences. The average composition of Bz-ON is Be2.99(Sc1.39Mg0.37Fe3+0.10Fe2+0.09Al0.06Mn0.01)Σ2.02Si5.99O18·[Na0.47(H2O)], and that of Bz-BC is Be3.00(Sc1.43Fe2+0.28Mg0.20Al0.05Fe3+0.03Mn0.01)Σ2.01Si6.00O18[(Na0.46Cs0.01)Σ0.47(H2O)]. Bazzite from the Quadeville Rose Quartz pegmatite contains Mg = 0.33–0.41 (avg. 0.37) apfu, Fe3+ = 0.00–0.18 (avg. 0.10) apfu, and Fe2+ = 0.05–0.13 (avg. 0.09) apfu, whereas bazzite from Bugaboo Castles is more Fe2+-rich, with Fe2+ = 0.19–0.38 (avg. 0.28) apfu, Mg = 0.16–0.22 (avg. 0.20) apfu, and Fe3+ = 0.00–0.08 (avg. 0.03) apfu. Both samples have low Al, Mn, Ca, and Cs contents and, compositionally, are more similar to bazzite from alpine fissures and orogenic pegmatites than that from more common anorogenic NYF pegmatites. Bazzite from Canada is hexagonal, P6/mcc, with unit cell parameters a = 9.52741(14), c = 9.19326(15) Å for Bz-ON and a = 9.54396, c = 9.16495(20) Å for Bz-BC, respectively. The structure is composed of SiO4 tetrahedra that share corners to form Si6O18 rings perpendicular to the c axis. Each Si6O18 ring is linked via corners to a 12-membered ring of edge-sharing BeO4 tetrahedra and AO6 octahedra (A), resulting in channels parallel to the c axis (2a and 2b sites). Voids in the channels contain H2O (2a) and large alkali cations (Na, 2b). Both Bz-ON and Bz-BC have fully occupied 2a sites and 2b sites which are 50% occupied by Na. Increased substitution of the smaller cations Mg ([6]r = 0.72 Å) and Fe3+ ([6]r = 0.65 Å) for Sc ([6]r = 0.75 Å) results in significant changes in the bazzite structure, including a decrease in the <A–O> distance and decreased distortions (compression) in the AO6 and SiO4 tetrahedra. Bazzite from Quadeville and Bugaboo Castles are late-stage minerals, the product of breakdown of beryl and/or bertrandite and a Sc-bearing mineral. At Bugaboo Castles, bazzite is the result of alteration of Sc-rich spessartine (up to 3000 ppm) and primary beryl by late-stage OH-bearing fluids. At Quadeville, the source of Sc for bazzite is less evident and suggested to be Sc-bearing columbite-group minerals (up to 0.23 apfu Sc), or primary ferromagnesian minerals.