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

Abstract

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/mm 2 , the maximum value H 2 = 482 kg/mm 2 ; Mohs’ hardness is 4½. Biaxial, negative, β= 1.858(10), γ= 1.888(10); 2 V = 65(5)°; α= 1.790 (calculated from 2 V ). Optical orientation: X = b , Z ∧ a = 34°. Dispersion is strong, r . Pleochroism: Y (orange) > Z (bright-yellow) ≥ X (yellow). Microtwinning on (001) is observed. D calc = 3.98 g/cm 3 ; D meas = 3.84(10) g/cm 3 . IR and Mossbauer spectra are given. Chemical composition is (electron microprobe combined with Mossbauer data, wt.%): Na 2 O 2.27, K 2 O 1.87, CaO 2.53, SrO 0.26, BaO 11.16, MgO 0.13, MnO 16.32, FeO 13.92, Fe 2 O 3 2.11, Al 2 O 3 0.02, SiO 2 27.17, TiO 2 16.14, Nb 2 O 5 2.14, ZrO 2 0.34, F 2.94, H 2 O (by Penfield method) 1.17, -O=F 2 –1.24, total 99.25. The empirical formula is ( Z = 2): Na 2.60 K 1.41 Ca 1.60 Sr 0.09 Ba 2.58 (Mn 8.17 Fe 2+ 6.88 Fe 3+ 0.94 Mg0. 115 Al 0.01 ) ∑16.115 (Ti 7.17 Nb 0.57 Zr 0.10 ) ∑7.84 Si 16.06 H 4.61 F 5.49 O 70.51 . The simplified formula, taking into account the crystal structure, is ( Z = 2): KBa 3 Ca 2 Na 2 (Mn, Fe 2+ , Fe 3+ ) 16 Ti 8 (Si 2 O 7 ) 8 O 8 (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, C 2, a = 10.723(1), b = 13.826(2), c = 20.791(4) A, 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 , A ( 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).