Magnesio-lucchesiite, CaMg3Al6(Si6O18)(BO3)3(OH)3O, a new species of the tourmaline supergroup

• Published in: American Mineralogist
• Main Authors: Scribner E. D., Cempirek J., Groat L. A., James Evans R., Biagioni C., Bosi F., Dini A., Halenius U., Orlandi P., Pasero M.
• Other Authors: Scribner, E. D., Cempirek, J., Groat, L. A., James Evans, R., Biagioni, C., Bosi, F., Dini, A., Halenius, U., Orlandi, P., Pasero, M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Mineralogical Society of America 2021
• Subjects: beryllium and boron: Quintessentially crustal; Elba Island; lamprophyre dike; Lithium; Magnesio-lucchesiite; new mineral specie; O'Grady Batholith; San Piero in Campo; Northwest Territories
• Online Access: http://hdl.handle.net/11573/1571893; https://doi.org/10.2138/am-2021-7496

Magnesio-lucchesiite, ideally CaMg3Al6(Si6O18)(BO3)3(OH)3O, is a new mineral species of the tourmaline supergroup. The holotype material was discovered within a lamprophyre dike that cross-cuts tourmaline-rich metapelites within the exocontact of the O'Grady Batholith, Northwest Territories (Canada). Two additional samples were found at San Piero in Campo, Elba Island, Tuscany (Italy) in hydrothermal veins embedded in meta- serpentinites within the contact aureole of the Monte Capanne intrusion. The studied crystals of magnesio-lucchesiite are black in a hand sample with vitreous luster, conchoidal fracture, an estimated hardness of 7-8, and a calculated density of 3.168 (Canada) and 3.175 g/cm3 (Italy). In plane-polarized light, magnesio-lucchesiite is pleochroic (O = dark brown, E = colorless) and uniaxial (-); its refractive index values are nω = 1.668(3) and nϵ = 1.644(3) (Canada), and nω = 1.665(5) and nϵ = 1.645(5) (Italy). Magnesio-lucchesiite is trigonal, space group R3m, Z = 3, with a = 15.9910(3) Å, c = 7.2224(2) Å, V = 1599.42(7) Å3 (Canada) and with a = 15.9270(10) Å, c = 7.1270(5) Å, V = 1565.7(2) Å3 (Italy, sample 1). The crystal structure of magnesio-lucchesiite was refined to R1 = 3.06% using 2953 reflections with Fo > 4σ(Fo) (Canadian sample; 1.96% / 1225 for the Italian sample) The Canadian (holotype) sample has the ordered empirical formula X(Ca0.60Na0.39K0.01)ς1.00YMg2.02Fe0.622+Fe0.093+Ti0.25 V0.01Cr0.01ς3.00zAl5.31Fe0.693+ς6.00TSi5.98Al0.02ς6.00O18BO33v(OH)2.59O0.41ς3.00 ${ }^{mathrm{Y}}left(mathrm{Mg}_{2.02} mathrm{Fe}_{0.62}^{2+} mathrm{Fe}_{0.09}^{3+} mathrm{Ti}_{0.25} mathrm{~V}_{0.01} mathrm{Cr}_{0.01} ight)_{Sigma 3.00} mathrm{z}left(mathrm{Al}_{5.31} mathrm{Fe}_{0.69}^{3+} ight)_{Sigma 6.00}left[{ }^{mathrm{T}}left(mathrm{S} mathrm{i}_{5.98} mathrm{Al}_{0.02} ight)_{Sigma 6.00} mathrm{O}_{18} ight]left(mathrm{BO}_{3} ight)_{3}{ }^{mathrm{v}}left[(mathrm{OH})_{2.59} mathrm{O}_{0.41} ight]_{Sigma 3.00}$W(O0.78F0.22)ς1.00. The Italian (co-type) material shows a wider chemical ...