The crystal structure of stilpnomelane. Part 1. The subcell: Clays and Clay Minerals 13

SUMMARY, The structural formula of stilpnomelane averaged from 37 representative literature analyses and based on the determined structure is (Ca,Na,K)4(Ti0.1A12.~Fe35.sMn0.sMg~.a)[Si~Al,] (O,OH)2~.nH20. Ferrostilpnomelanes have up to I2 R 3+ cations in the octahedral sheet, ferristilp-nomelanes commonly have between z4 and 30 R a+ cations, rarely up to 37- The two groups can be distinguished by accurate X-ray powder diffraction, ferrostilpnomelanes having d00 ~ less and a greater than have ferristilpnomelanes. Theunit cell is triclinic, a-- b ~ 21 "8 A, 7 = I zo ~ d001 ~ r z.z Ik, PL Hexagonal groups or 'islands ' of z4 silicon-oxygen tetrahedra co-ordinate othe octahedral sheet, the centres of the 'islands ' are displaced by 5a/I2, 8b/iz on either side of the octahedral sheet as a result of warping of this sheet o permit articulation with the smaller tetrahedral sheet. The z4-tetrahedra 'islands ' are linked laterally by 6-member rings of inverted tetrahedra, which in turn link at their apices to like rings connecting the next sheet of 'islands'. Thus a single tetrahedral layer is 4 tetrahedra thick and has symmetry 6/m. The 6-member rings of tetrahedra act as hinges permitting up to o'5 ~ extension or contraction of the tetrahedral sheet o accommodate variations in the octahedral sheet dimensions with varying R2+/R ~+ ratio. IN Part I of this study (The Subcell, Eggleton and Bailey, I965) it was shown that st i lpnomelane was triclinic with a = b = zP72 A, c = 17"74 A, ~ = I24 ~ = 96~ 7 = ~ 2o ~ A subcell having a ~ and b ~ one quarter of the true a and b dimensions was defined, and the crystal lography, composit ion, and average structure of the subcell was described. The composit ion of the subcell as revealed by the present study is ~i i ~ (O,OH)I~.5.nH20. A<I,R s T~.~ Since Part I was published, addit ional analysed samples of st i lpnomelane have generously been made available to the author, and other workers (e.g. Blake, I965