Veblenite, K₂☐₂Na(Fe²⁺₅Fe³⁺₄Mn²⁺₇☐)Nb₃Ti(Si₂O₇)₂(Si₈O₂₂)₂O₆(OH)₁₀(H₂O)₃, a new mineral from Seal Lake, Newfoundland and Labrador: mineral description, crystal structure, and a new veblenite Si₈O₂₂ribbon

Abstract Veblenite, ideally K 2 ☐ 2 Na(Fe 2+ 5 Fe 3+ 4 Mn 2+ 7 ☐)Nb 3 Ti(Si 2 O 7 ) 2 (Si 8 O 22 ) 2 O 6 (OH) 10 (H 2 O) 3 , is a new mineral with no natural or synthetic analogues. The mineral occurs at Ten Mile Lake, Seal Lake area, Newfoundland and Labrador (Canada), in a band of paragneiss consisting chiefly of albite and arfvedsonite. Veblenite occurs as red brown single laths and fibres included in feldspar. Associated minerals are niobophyllite, albite, arfvedsonite, aegirine-augite, barylite, eudidymite, neptunite, Mn-rich pectolite, pyrochlore, sphalerite and galena. Veblenite has perfect cleavage on {001} and splintery fracture. Its calculated density is 3.046 g cm –3 . Veblenite is biaxial negative with α 1.676(2), β 1.688(2), γ 1.692(2) (λ 590 nm), 2V meas = 65(1)°, 2V calc = 59.6°, with no discernible dispersion. It is pleochroic in the following pattern: X = black, Y = black, Z = orange-brown. The mineral is red-brown with a vitreous, translucent lustre and very pale brown streak. It does not fluoresce under short and long-wave UV-light. Veblenite is triclicnic, space group P , a 5.3761(3), b 27.5062(11), c 18.6972(9) Å, α 140.301(3), β 93.033(3), γ 95.664(3)°, V = 1720.96(14) Å 3 . The strongest lines in the X-ray powder diffraction pattern [ d (Å) (I)(hkl) ] are: 16.894(100)(010), 18.204(23)(0 1), 4.271(9)( , 040, 120), 11.661(8)(001), 2.721(3)( ), 4.404(3)( , ), 4.056(3)(031, 1 2; , ), 3.891(2)(003). The chemical composition of veblenite from a combination of electron microprobe analysis and structural determination for H 2 O and the Fe 2+ /Fe 3+ ratio is Nb 2 O 5 11.69, TiO 2 2.26, SiO 2 35.71, Al 2 O 3 0.60, Fe 2 O 3 10.40, FeO 11.58, MnO 12.84, ZnO 0.36, MgO 0.08, BaO 1.31, SrO 0.09, CaO 1.49, Cs 2 O 0.30, K 2 O 1.78, Na 2 O 0.68, H 2 O 4.39, F 0.22, O = F –0.09, sum 95.69 wt.%. The empirical formula [based on 20 (Al+Si) p. f. u. is (K 0.53 Ba 0.28 Sr 0.03 ☐ 0.16 ) Σ1 (K 0.72 Cs 0.07 ☐ 1.21 ) Σ2 (Na 0.72 Ca 0.17 ☐ 1.11 ) Σ2 (Fe 2+ 5.32 Fe 3+ 4.13 Mn 2+ 5.97 Ca 0.70 Zn 0.15 Mg 0.07 ☐ 0.66 ) Σ17 (Nb 2.90 Ti 0.93 Fe 3+ 0.17 ) Σ4 (Si 19.61 Al 0.39 ) Σ20 O 77.01 H 16.08 F 0.38 . The simplified formula is (K, Ba, ☐) 3 (☐, Na) 2 (Fe 2+ , Fe 3+ , Mn 2+ ) 17 (Nb,Ti) 4 (Si 2 O 7 ) 2 (Si 8 O 22 ) 2 O 6 (OH) 10 (H 2 O) 3 . The infrared spectrum of the mineral contains the following bands (cm –1 ): 453, 531, 550, 654 and 958, with shoulders at 1070, 1031 and 908. A broad absorption was observed between ~3610 and 3300 with a maximum at ~3525. The crystal structure was solved by direct methods and refined to an R 1 index of 9.1%. In veblenite, the main structural unit is an HOH layer, which consists of the octahedral (O) and two heteropolyhedral (H) sheets. The H sheet is composed of Si2O 7 groups, veblenite Si 8 O 22 ribbons and Nb-dominant D octahedra. This is the first occurrence of an eight-membered Si 8 O 22 ribbon in a mineral crystal structure. In the O sheet, (Fe 2+ , Fe 3+ , Mn 2+ ) octahedra share common edges to form a modulated O sheet parallel to (001). HOH layers connect via common vertices of D octahedra and cations at the interstitial A (1,2) and B sites. In the intermediate space between two adjacent HOH layers, the A (1) site is occupied mainly by K; the A (2) site is partly occupied by K and H 2 O groups, the B site is partly occupied by Na. The crystal structure of veblenite is related to several HOH structures: jinshanjiangite, niobophyllite (astrophyllite group) and nafertisite. The mineral is named in honour of David R. Veblen in recognition of his outstanding contributions to the fields of mineralogy and crystallography.

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