Botuobinskite and mirnyite, two new minerals of the crichtonite group included in Cr-pyrope xenocrysts from the Internatsionalnaya kimberlite

Abstract Two new mineral species of the crichtonite group: botuobinskite, ideally SrFe 2+ (Ti 4+ 12 Cr 3+ 6 )Mg 2 [O 36 (OH) 2 ] and mirnyite, ideally SrZr(Ti 4+ 12 Cr 3+ 6 )Mg 2 O 38 , occur as inclusions in mantle-derived Cr-pyrope xenocrysts from the Internatsionalnaya kimberlite pipe, Mirny field, Siberian craton. Botuobinskite forms needle- and blade-like acicular crystals up to 1 mm in length and up to 30 μm in diameter, a large platy inclusion (700 × 700 × 80 μm) and roughly isometric grains (up to 80 μm). Mirnyite occurs as needle-and blade-like elongated inclusions (up to 1 mm). Both minerals are jet-black, opaque and exhibit a metallic lustre. In plane-polarised reflected light, botuobinskite and mirnyite are greyish-white with a weak brownish tint. Between crossed polars, the new species show distinct anisotropy in shades of bluish grey to greenish-brown. Neither bireflectance nor pleochroism is observed. Calculated densities for botuobinskite and mirnyite are 4.3582(5) and 4.3867(3) gm/cm 3 , respectively. The crystal structures of botuobinskite and mirnyite have been refined ( R = 0.0316 and 0.0285, respectively) from single crystal X-ray diffraction data. The minerals are trigonal, crystallise in the space group R $\bar{3}$ (No. 148) and are isostructural with other members of the crichtonite group. The unit cell parameters are a = 10.3644(8) Å, c = 20.6588(11) Å and V = 1921.9(2) Å 3 for botuobinskite and a = 10.3734(8) Å, c = 20.6910 (12) Å and V = 1928.2(2) Å 3 for mirnyite, with Z = 3 for both. The Raman spectra of the minerals show strong peaks at 133, 313 and 711 cm –1 . Infrared spectroscopy data for botuobinskite indicates H–O stretching of the hydroxyl groups. Botuobinskite and mirnyite have been approved by the IMA–CNMNC under the numbers 2018-143a and 2018-144a, respectively. Botuobinskite and mirnyite are named after the Botuobinskaya exploration expedition and Mirny town, respectively. The minerals may be considered as crystal-chemical analogues of other crichtonite-group species occurring in the lithospheric mantle (i.e. loveringite, lindsleyite and mathiasite). Both species commonly occur in intimate association with Cr-pyrope as well as other peridotitic minerals and exert an important control on the partitioning of incompatible elements during mantle metasomatism.