Veblenite: a new ti-silicate related to HOH structures

• Main Authors: CAMARA ARTIGAS, Fernando, Sokolova E, Hawthorne FC, Rowe R, Grice J, Tait K.
• Other Authors: Cámara F, Tait K
• Format: Conference Object
• Language: English
• Published: 2011
• Subjects: structure; layered; silicate; Newfoundland
• Online Access: http://hdl.handle.net/2318/91950; http://journals.iucr.org/a/issues/2011/a1/00/a48236/a48236.pdf

Veblenite, ideally KNa(Fe2+5Fe3+4Mn7)Nb4(Si2O7)2(Si8O22)2 O6(OH)10(H2O)3, triclicnic, P¯1, 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) Å3, has no natural or synthetic analogues. It is a new mineral occurring at Ten Mile Lake, Seal Lake area, Labrador, Newfoundland, Canada, as red brown single laths and fibres included in feldspar. In the crystal structure of veblenite, M octahedra share common edges to form a wavy octahedral sheet parallel to (001). SiO4 tetrahedra share corners to form Si2O7 groups and Si8O22 ribbons. This is the first occurrence of an eightmembered Si-O ribbon in a mineral crystal structure. The Nb-dominant D octahedra, Si2O7 groups and Si8O22 ribbons share common vertices to form a heteropolyhedral (H) sheet. Two H sheets and an O sheet constitute an HOH block. Along [001], HOH blocks connect via common vertices of D octahedra and cations at the A(1,2) and B interstitial sites. In the intermediate space between two adjacent HOH blocks, two [100] channels of different size can be identified. A narrow channel of the width of an Si2O7 group contains the A(1) site mainly occupied by K. A large channel has the width of four Si2O7 groups (Si8O22 ribbon) and contains A(2) sites partly occupied by K and H2O groups. The crystal structure of veblenite is related to several HOH structures where the O sheet is ideally composed of Fe2+ octahedra, such as jinshanjiangite [1], niobophyllite [2] (astrophyllite group) and nafertisite [3]. Veblenite differs from those structures in the topology and chemistry of the H sheet. In jinshajiangite, the H sheet consists of (Si2O7) groups and Ti-dominant octahedra. In niobophyllite, the H sheet is composed of (Si4O12) astrophyllite ribbons [a double ribbon of (Si2O7) groups] and Nb-dominant octahedra. In nafertisite, the H sheet is composed of (Si6O17) ribbons [a triple ribbon of (Si2O7) groups] and Ti-dominant octahedra. In veblenite, the H sheet is composed of (Si2O7) groups (as in jinshajiangite), (Si8O22) ...