They are in the broader category of mixed anion compounds.
They are not to be confused with tetrafluoroborates (BF4) or the fluorooxoborates which have fluorine bonded to boron.
Uniaxial (-) nω = 1.580 nε = 1.485 Max birefringence δ = 0.095 [1] transparent Biaxial (+) nα = 1.609 nβ = 1.620 nγ = 1.642
Max birefringence: δ = 0.033 Uniaxial (-) nω = 1.570 nε = 1.534 Max birefringence δ = 0.036
Max birefringence δ = 0.024 Jeremejevite hexagonal P63/m a = 8.5591, c = 8.1814 519 density 3.28 Uniaxial (-) nω = 1.653 nε = 1.640 Max birefringence δ = 0.013 dark brown Uniaxial (-) nω = 1.645(2) nε = 1.621(2) Max birefringence δ = 0.024 Uniaxial (-) Uniaxial (-) nω = 1.637 nε = 1.621 Max birefringence δ = 0.016 Uniaxial nω = 1.637 - 1.668 nε = 1.619 - 1.639 Max birefringence δ = 0.018 - 0.029 Uniaxial (-) Uniaxial (-) nω = 1.660 - 1.661 nε = 1.636 - 1.637 Max birefringence δ = 0.024 brown Uniaxial (-) nω = 1.735 nε = 1.655 Birefringence 0.080 brownish Uniaxial (-) Uniaxial (-) nω = 1.753 nε = 1.740 Max birefringence δ = 0.013 n = 1.627 - 1.685 Uniaxial (-) nω = 1.666 nε = 1.653 Max birefringence δ = 0.013 dark brown Uniaxial (-) nω = 1.741(3) nε = 1.720(3) Max birefringence δ = 0.021 Uniaxial (-) nω = 1.757 nε = 1.722 Max birefringence δ = 0.035