Veblenite, K_2□_2Na(Fe_5~(2+)Fe_4~(3+)Mn_7~(2+)□)Nb_3Ti(Si_2O_7)_2(Si_8O_22)_2O_6(OH)_10(H_20)_3, a new mineral from Seal Lake,Newfoundland and Labrador: mineral description, crystalstructure, and a new veblenite Si_8O_22 ribbon

摘要

Veblenite, ideally K_2□_2Na(Fe_5~(2+)Fe_4~(3+)Mn_7~(2+)□)Nb_3Ti(Si_2O_7)_2(Si_8O_22)_2O_6(OH)_10(H_20)_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 a 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 PI, a 5.3761(3), b 27.5062(11), c 18.6972(9) A, α 140.301(3), β 93.033(3), γ 95.664(3)°, V= 1720.96(14) A3. The strongest lines in the X-ray powder diffraction pattern [d(A)(l)(hkl)] are: 16.894(100)(010), 18.204(23)(0ll), 4.271(9)(141, 040, 120), 11.661(8)(001), 2.721(3)(195), 4.404(3)(132, 142), 4.056(3)(031, 112; 152, 143), 3.891(2)(003).The chemical composition of veblenite from a combination of electron microprobe analysis and structural determination for H_2O and the Fe~(2+)/Fe~(3+) ratio is Nb_2O_5 11.69, Ti02 2.26, Si02 35.71, A1_2O_3 0.60, Fe_2O_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_2O 0.30, K_2O 1.78, Na_2O 0.68, H_2O 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_(O.53)Ba_(0.28)Sr_(0.03)□_(0.16))_(Σ1)(K_(0.72)Cs_(0.07)□_(1.21))_(Σ2)(Na_(0.72)Ca(017)□_(1.11))_(Σ2)(Fe_(5.32)~(2+)Fe_(4.13)~(3+)Mn_(5.97)~(2+)Ca_0.70Zn_(0.15)Mg_(0.07)□_(0.66))_(Σ17)(Nb_(2.90)Ti_(0.93)Fe_(0.17)~(3+))_(Σ4)(Si_(19.61)Al_(0.39))_7)_(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_2O_7)_2 (Si_8O_(22))_2O_6(OH)_(10)(H_2O)_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 Ri 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 Si_2O_7 groups, veblenite Si_8O_(22) ribbons and Nb-dominant D octahedra. This is the first occurrence of an eight-membered Si_8O_(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 .4(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_2O 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.