New Members of the Hydrotalcite-Manasseite Group

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 (Mg_1-xAl_x)(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 × 7.56= 15.12 Å and a calculated density of D_c = 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 D_c = 1.96 g/cm³; (2) an 11-Å phase having the composition [(Mg_3.96Fe_0.06Al_1.98)(OH)₁₂][Na_0.56(SO₄)_1.30·7.3H₂O], hexagonal unit-cell parameters of a = √3 · 3.042 = 5.28 Å, c = 11.16 Å, and D_c = 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 D_c = 2.06 g/cm³; and (4) an 18.5-Å ordered mixed-layer phase having the ideal composition [(Mg₈Al₄)(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 × 18.54 = 55.62 Å and D_c = 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-xR³⁺_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.