Surkhobite:: revalidation and redefinition with the new formula, (Ba, K)2CaNa(Mn, Fe2+, Fe3+)8Ti4(Si2O7)4O4(F, OH, O)6

Surkhobite, a new mineral related to the members of the jinshajiangite-perraultite series, was approved in 2002 (IMA No. 2002-037) and later discredited (IMA decision 06-E). It is redefined here with a new formula and revalidated with the original name (IMA 07-A). It occurs as platy crystals up to 1 mm and grains up to 2 × 1 × 0.4 cm in the association with aegirine, microcline, albite, quartz, amphibole, annite, bafertisite, astrophyllite, zircon, fluorite, polylithionite, stillwelite, sogdianite, tadjikite in alkaline pegmatite at the massif Dara-i-Pioz, Tajikistan. Surkhobite is translucent, brownish-red, lustre vitreous, streak white, cleavage perfect on {001}; hardness is anisotropic: the minimum value H1 = 250 kg/mm2, the maximum value H2 = 482 kg/mm2; Mohs' hardness is 41/2. Biaxial, negative, β = 1.858(10), γ = 1.888(10); 2V = 65(5)°; α = 1.790 (calculated from 2V). Optical orientation: X = b,Z ∧ a = 34°. Dispersion is strong, r < v. Pleochroism: Y (orange) > Z (bright-yellow) ≥ X (yellow). Microtwinning on (001) is observed. Dcalc = 3.98 g/ cm3; Dmeas. = 3.84(10) g/cm3. IR and Mössbauer spectra are given. Chemical composition is (electron microprobe combined with Mössbauer data, wt.%): Na2O 2.27, K2O 1.87, CaO 2.53, SrO 0.26, BaO 11.16, MgO 0.13, MnO 16.32, FeO 13.92, Fe2O3 2.11, Al2O3 0.02, SiO2 27.17, TiO2 16.14, Nb2O5 2.14, ZrO2 O.34, F 2.94, H2O (by Penfield method) 1.17, -O=F2 -1.24, total 99.25. The empirical formula is (Z = 2): Na2.60 K1.41Ca1.60 Sr0.09 Ba2.58 (Mn8.17 Fe6.882+ Fe0.943+ Mg0.115 Al0.01) ∑16.115 (Ti7.17 Nb0.57 Zr0.10) ∑7.84 Si16.06 H4.61 F5.49O70.51. The simplified formula, taking into account the crystal structure, is (Z = 2): KBa3Ca2Na2(Mn, Fe2+, Fe3+)16 Ti8(Si2O7)8 O8(OH)4(FO,OH)8. The crystal structure was refined on a single crystal to R= 0.043 with 3686 independent reflections (F > 2σ). Surkhobite is monoclinic, C2, a = 10.723(l), b = 13.826(2), c = 20.791(4) Å, β = 95.00(1)°. Surkhobite is the Mn-dominant analogue of jinshajiangite and differs from perraultite in that Ca is ordered onto and is dominant in the site A(6). The strongest lines of the powder difraction pattern [d, Å (I, %) (hkl)] are: 10.39 (20) (002), 3.454 (100) (006), 3.186 (15) (321), 2.862 (15) (225), 2.592 (70) (008), 2.074 (40) (048). © 2008 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart.