Maruyamaite, K(MgAl₂)(Al₅Mg)Si₆O₁₈(BO₃)₃(OH)₃O, a potassium-dominant tourmaline from the ultrahigh-pressure Kokchetav massif, northern Kazakhstan: Description and crystal structure

Maruyamaite, ideally K(MgAl₂)(Al₅Mg)Si₆O₁₈(BO₃)₃(OH)₃O, was recently approved as the first K-dominant mineral-species of the tourmaline supergroup. It occurs in ultrahigh-pressure quartzofeldspathic gneisses of the Kumdy-Kol area of the Kokchetav Massif, northern Kazakhstan. Maruyamaite contains inclusions of microdiamonds, and probably crystallized near the peak pressure conditions of UHP metamorphism in the stability field of diamond. Crystals occur as anhedral to euhedral grains up to 2 mm across, embedded in a matrix of anhedral quartz and K-feldspar. Maruyamaite is pale brown to brown with a white to very pale-brown streak and has a vitreous luster. It is brittle and has a Mohs hardness of ∼7; it is non-fluorescent, has no observable cleavage or parting, and has a calculated density of 3.081 g/cm³. In plane-polarized transmitted light, it is pleochroic, O = darkish brown, E = pale brown. Maruyamaite is uniaxial negative, ω = 1.634, ε = 1.652, both ±0.002. It is rhombohedral, space group R3m, a = 15.955(1), c = 7.227(1) Å, V = 1593(3) ų, Z = 3. The strongest 10 X-ray dif- fraction lines in the powder pattern are [d in Å(I)(hkl)]: 2.581(100)(051), 2.974(85)(132), 3.995 (69)(240), 4.237(59)(231), 2.046(54)(162), 3.498(42)(012), 1.923(36)(372), 6.415(23)(111), 1.595(22)(5.10.0), 5.002(21)(021), and 4.610(20)(030). The crystal structure of maruyamaite was refined to an R₁ index of 1.58% using 1149 unique reflections measured with MoKα X-radiation. Analysis by a combination of electron microprobe and crystal-structure refinement gave SiO₂ 36.37, Al₂O₃ 31.50, TiO₂ 1.09, Cr₂O₃ 0.04, Fe₂O₃ 0.33, FeO 4.01, MgO 9.00, CaO 1.47, Na₂O 0.60, K₂O 2.54, F 0.30, B₂O₃(calc) 10.58, H₂O(calc) 2.96, sum 100.67 wt%. The formula unit, calculated on the basis of 31 anions pfu with B = 3, OH = 3.24 apfu (derived from the crystal structure) and the site populations assigned to reflect the mean interatomic distances, is (K_0.53Na_0.19Ca_0.26□_0.02)_ΣX=1.00(Mg_1.19Fe0.552+Fe0.053+ Ti_0.14Al_1.07)_□Y=3.00(Al_5.00Mg_1.00)(Si_5.97Al_0.03O₁₈)(BO₃)₃(OH)₃(O0.602 F_0.16OH_0.24). Maruyamaite, ideally K(MgAl₂) (Al₅Mg)(BO₃)₃(Si₆O₁₈)(OH)₃O, is related to oxy-dravite: ideally Na(MgAl₂)(Al₅Mg)(BO₃)₃(Si₆O₁₈)(OH)₃O, by the substitution ^XK → ^XNa.