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 H_{1 =} 250 kg/mm², the maximum value H_{2 =} 482 kg/mm²; Mohs’ hardness is 4½. 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. D_{calc =} 3.98 g/cm³; D_{meas =} 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 0.34, F 2.94, H₂O (by Penfield method) 1.17, -O=F₂–1.24, total 99.25. The empirical formula is (Z = 2): Na_2.60K_1.41Ca_1.60Sr_0.09Ba_2.58(Mn_8.17Fe²⁺_6.88Fe³⁺_0.94Mg0.₁₁₅Al_0.01)_∑16.115(Ti_7.17Nb_0.57Zr_0.10)_∑7.84Si_16.06H_4.61F_5.49O_70.51. The simplified formula, taking into account the crystal structure, is (Z = 2): KBa₃Ca₂Na₂(Mn, Fe²⁺, Fe³⁺ )₁₆Ti₈(Si₂O₇)₈O₈(OH)₄(F,O,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(1), 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).