Doppelschichtstrukturen mit brucitähnlichen Schichtionen [Me(II)1-xMe(III)x(OH)2)x+

Abstract

By introducing trivalent ions Me³⁺ into brucite-like layers Me(II)(OH)₂, layer ions [Me(II)_1-xMe(III)_x(OH)₂)^x+ can be obtained. In these layer ions the charge compensation of an O^2--ion by the metal ions is in the average 1+x/2 , leaving only a bond strength of 1-x/2 for the O–H bond, i. e. the charge of H⁺ must partly be compensated by additional anions X^-. This is possible by forming hydrogen bonds O—H • • • X or, with interfering H₂O, O—H • • • OH—H • • • X. The additional anions and water molecules are forming interlayers [x X • n H₂O]^x- interleaved between the brucite-like mainlayers. The water molecules can be removed reversibly without destroying the double layer structure. 

Three main types of double layers structures have been found.

1) The pyroaurite-sjögrenite type with random distribution of Me²⁺ and Me³⁺ (range about 0.2 < x < 0.4). The a-lattice constant is about 3 Å. The approximate formula of both minerals is [Mg₆Fe₂(OH)₁₆] • [CO₃ • ~4 H₂O]. They differ only in the stacking mode of the layers.

2) The type of the laminar calcium aluminates with ordered main layers [Ca₂Me(III)(OH)₆]⁺, a₀~5,8 Å = √3 • 3.35 Å. The Ca-ions in these layers are ~0.6 Å off the center of the Ca (OH)₆-octahedron in direction to an H₂O molecule of the interlayer, such obtaining the coordination number 7.

3) Double layer structures with two kinds of layer ions: chlorites, lithiophorite, koenenite, valerite.

Finally a structure is proposed for the green basic cobalt bromide [4Co(OH)₂ < > CoBrOH • n H₂O].