Tungusite: New Data, Relationship with Gyrolite and Structural Model

Giovanni Ferraris, Alessandro Pavese and Svetlana V. Soboleva
Dip. Scienze Mineralogiche e Petrologiche, Univ. Torino, Via Valperga Caluso 37, 10125 Torino, Italy
Dip. Scienze della Terra, Sez. Mineralogia, Univ. Milano, Via Botticelli 23, 20133 Milano, Italy
Inst. Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Acad. Sci., Staromonetny per. 35, Moscow, Russia

Abstract: New chemical analyses, electron and X-ray powder diffraction data, and comparison with gyrolite and reyerite show that tungusite has the ideal formula [Ca14(OH)8](Si8O20)(Si8O20)2[Fe92+(OH)14], symmetry P1¯ and a = 9.714(9), b = 9.721(9), c = 22.09(3), α = 90.13(1)°, β = 98.3(2)°, γ = 120.0(1)°, Z = 1. A structural model for tungusite is derived by splitting the double tetrahedral layer of reyerite and inserting a trioctahedral X sheet which is ideally occupied by Fe2+. Polytypism phenomena due to different relative positions between tetrahedral and X sheets are discussed. A substitutional solid solution represented by the formula [Ca14(OH)8]Si24−yAlyO60[NaxM9−(x+z)z(OH)14−(x+y+2z)·(x+y+2z)H2O] includes tungusite (x = y = z = 0, M = Fe2+ and gyrolite (x = 1, y = 1, z = 6, M = Ca).

Keywords: tungusite • reyerite • gyrolite • structural model

Mineralogical Magazine; September 1995 v. 59; no. 396; p. 535-543; DOI: 10.1180/minmag.1995.059.396.13
© 1995, The Mineralogical Society
Mineralogical Society (www.minersoc.org)