Journal
AMERICAN MINERALOGIST
Volume 101, Issue 1-2, Pages 355-361Publisher
MINERALOGICAL SOC AMER
DOI: 10.2138/am-2016-5359
Keywords
Maruyamaite; tourmaline; new mineral; electron-microprobe analysis; optical properties; crystal-structure refinement; Kokchetav Massif; northern Kazakhstan; ultrahigh-pressure; microdiamond inclusions
Categories
Funding
- University of Manitoba Graduate Fellowship
- Canada Research Chair in Crystallography and Mineralogy
- Natural Sciences and Engineering Research Council of Canada
- Canada Foundation for Innovation
- NSF [EAR-9405747]
- Japan Society for the Promotion of Science [15204050]
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Maruyamaite, ideally K(MgAl2)(Al5Mg)Si6O18(BO3)(3)(OH)(3)O, 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 similar to 7; it is non-fluorescent, has no observable cleavage or parting, and has a calculated density of 3.081 g/cm(3). In plane-polarized transmitted light, it is pleochroic, 0 = darkish brown, E = pale brown. Maruyamaite is uniaxial negative, w = 1.634, e = 1.652, both 0.002. It is rhombohedral, space group R3rii, a = 15.955(1), c = 7.227(1) angstrom, V= 1593(3) angstrom(3), Z= 3. The strongest 10 X-ray diffraction lines in the powder pattern are [d in A(1)(hkl)]: 2.581(100)(051), 2.974(85)(T32), 3.995 (69) (240), 4.237(59)(231), 2.046(54)(762), 3.498(42)(012), 1.923(36)(372), 6.415(23)(T11), 1.595(22) (5.10.0), 5.002(21)(021), and 4.610(20)(030). The crystal structure of maruyamaite was refined to an R, index of 1.58% using 1149 unique reflections measured with MoKa X-radiation. Analysis by a combination of electron microprobe and crystal-structure refinement gave Si02 36.37, A1203 31.50, TiO2 1.09, Cr2O3 0.04, Fe2O3 0.33, FeO 4.01, MgO 9.00, CaO 1.47, Na2O 0.60, K20 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 (K3.53Na0.19Ca0.26H0.02)Dc=1.00(Mg(1.19)Fer(55)FerosTi(0.14) Al(1.0)7)2Y-3.00(Al5.00MgO)(S15.97A10.03018)(BO3)(3)(OH)(3)(OhoFo.160110,24). Maruyamaite, ideally K(MgAl2) (A15Mg)(BO3)3(Si6018)(OH)(3)O, is related to oxy-dravite: ideally Na(MgAl2)(Al5Mg)(BO3)3(S16018) (OH)(3)O, by the substitution xK-: xNa.
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