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

Aaron Lussier*, Neil A. Ball, Frank C. Hawthorne, Darrell J. Henry, Rentaro Shimizu, Yoshihide Ogasawara, Tsutomu Ota

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    25 Citations (Scopus)


    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) Å3, 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 (K0.53Na0.19Ca0.260.02)ΣX=1.00(Mg1.19Fe0.552+Fe0.053+ Ti0.14Al1.07)□Y=3.00(Al5.00Mg1.00)(Si5.97Al0.03O18)(BO3)3(OH)3(O0.602 F0.16OH0.24). 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.

    Original languageEnglish
    Pages (from-to)355-361
    Number of pages7
    JournalAmerican Mineralogist
    Issue number2
    Publication statusPublished - 2016 Jan 1


    • crystal-structure refinement
    • electron-microprobe analysis
    • Kokchetav Massif
    • Maruyamaite
    • microdiamond inclusions
    • new mineral
    • northern Kazakhstan
    • optical properties
    • tourmaline
    • ultrahigh-pressure

    ASJC Scopus subject areas

    • Geochemistry and Petrology
    • Geophysics


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