New Members of the Hydrotalcite-Manasseite Group

V. A. Drits, T. N. Sokolova, G. V. Sokolova and V. I. Cherkashin
Geological Institute of the U.S.S.R. Academy of Sciences Pyzhevsky 7, Moscow, U.S.S.R.

Abstract: Several new minerals, structurally and chemically similar to the hydrotalcite-manasseite group, have been found in the saline deposits of the central pre-Caspian depression and in those of Middle Asia. Their structures consist of layers of (Mg1-xAlx)(OH)2 plus interlayers of anions and molecular water. In addition to manasseite itself, [(Mg4Al2)(OH)12][(CO3) · 3H2O], having unit-cell parameters of a = 3.042, c = 2 x 7.56 = 15.12 Å and a calculated density of Dc = 2.15 g/cm3, other varieties were identified whose interlayers contain alternately SO42- or SO42- and CO32- anions. The following new phases were recognized: (1) an 8.85-Å phase having the composition [(Mg4Al2)(OH)12][(SO4) · 3H2O], subcell dimensions of a′ = 3.04, c′ = 8.85 Å, and Dc = 1.96 g/cm3; (2) an 11-Å phase having the composition [(Mg3.96 Fe0.06 Al1.98)(OH)12] [Na0.56(SO4)1.30 · 7.3H2O], hexagonal unit-cell parameters of a = 3 x 3.042 = 5.28 Å, c = 11.16 Å, and Dc = 1.90 g/cm3; (3) a 16.5-Å mixed-layer phase having the composition [(Mg4Al2)(OH)12][(SO4)0.5(CO3)0.5 · 3H2O] and an ordered ABAB … alternation of 7.56-Å layers (i.e., a brucite-like layer + a CO32--containing interlayer) and 8.94-Å layers containing SO42- anions in interlayers; this phase has unit-cell dimensions of a′ = 3.05, c′ = 16.5 Å and Dc = 2.06 g/cm3; and (4) an 18.5-Å ordered mixed-layer phase having the ideal composition [(M8Al4)(OH)24][M+0.5(SO4)1.25(CO3)1.0 · 9H2O] and an alternation of 7.56-Å, CO3-containing layers and 11-Å layers having SO42- anions and Na and Mg cations in interlayers (M+0.5 corresponds to interlayer cations). This last phase has unit-cell dimensions of a = 3.046, c = 3 x 18.54 = 55.62 Å and Dc = 1.99 g/cm3. Some of the varieties containing SO42- in the interlayers swelled with ethylene glycol or glycerol.

For this diverse family of minerals whose structures are based on brucite-like layers of the composition (R2+1-x R3+x)(OH)2, a unifying system of nomenclature is offered for consideration. In describing these brucite-like structures intercalated with anion- and cation-containing interlayers, the composition of the hydroxide sheet and the interlayers, the periodicity of the layers, and the mixed-layer nature of the material should be described. By means of symbols, a single term might be used to describe the crystal chemistry of any member of the group having a given cation composition for the brucite-like layers, for example, manasseite, a 2H polytype of hydrotalcite that has a repeat distance of 7.56 Å and CO3-containing interlayers, might be designated as 7.56-Å CO3-hydrotalcite-2H.

Key Words: Carbonate • Hydrotalcite • Interlayer • Nomenclature • Sulfate • X-ray powder diffraction

Clays and Clay Minerals; December 1987 v. 35; no. 6; p. 401-417; DOI: 10.1346/CCMN.1987.0350601
© 1987, The Clay Minerals Society
Clay Minerals Society (www.clays.org)