New Members of the Hydrotalcite-Manasseite Group

Abstract

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-x Al x )(OH) 2 plus interlayers of anions and molecular water. In addition to manasseite itself, [(Mg4Al 2 )(OH) 12 ][(CO 3 )-3H 2 O], 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/cm 3 , other varieties were identified whose interlayers contain alternately SO 4 2− or SO 4 2− and CO 3 2− anions. The following new phases were recognized: (1) an 8.85-Å phase having the composition [(Mg 4 Al 2 )(OH) 12 ][(SO 4 )·3H 2 O], subcell dimensions of a ' = 3.04, c ' = 8.85 Å, and D c = 1.96 g/cm 3 ; (2) an 11-Å phase having the composition [(Mg 3.96 Fe 0.06 Al 1.98 )(OH) 12 ][Na 0.56 (SO 4 ) 1.30 ·7.3H 2 O], hexagonal unit-cell parameters of a = √3 · 3.042 = 5.28 Å, c = 11.16 Å, and D c = 1.90 g/cm 3 ; (3) a 16.5-Å mixed-layer phase having the composition [(Mg 4 Al 2 )(OH) 12 ][(SO 4 ) 0.5 (CO 3 ) 0.5 ·3H 2 O] and an ordered ABAB... alternation of 7.56-Å layers (i.e., a brucite-like layer + a CO 3 2− -containing interlayer) and 8.94-Å layers containing SO 4 2− anions in interlayers; this phase has unit-cell dimensions of a ' = 3.05, c ' = 16.5 Å and D c = 2.06 g/cm 3 ; and (4) an 18.5-Å ordered mixed-layer phase having the ideal composition [(Mg 8 Al 4 )(OH) 24 ][M + 0.5 (SO 4 ) 1.25 (CO 3 ) 1.0 ·9H 2 O] and an alternation of 7.56-Å, CO 3 -containing layers and 11-Å layers having SO 4 2− 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/cm 3 . Some of the varieties containing SO 4 2− 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 (R 2+ 1-x R 3+ 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 2 H polytype of hydrotalcite that has a repeat distance of 7.56 Å and CO 3 -containing interlayers, might be designated as 7.56-Å CO 3 -hydrotalcite-2 H .