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Unformatted text preview: y. Some naturally occurring minerals of these two polytype structures are
presented in Table 1. Tab. 1: Composition and symmetry of some natural anionic clays [1, 2]
Name Chemical composition Symmetry Barbertonite Mg6Cr2(OH)16CO3· 4H2O hexagonal Desautelsite Mg6Mn2(OH)16CO3· 4H2O rhombohedral Hydrotalcite Mg6Al2(OH)16CO3· 4H2O rhombohedral Manasseite Mg6Al2(OH)16CO3· 4H2O hexagonal Pyroaurite Mg6Fe2(OH)16CO3· 4.5H2O rhombohedral Reevesite Ni6Fe2(OH)16CO3· 4H2O rhombohedral Sjögrenite Mg6Fe2(OH)16CO3· 4.5H2O hexagonal Stichtite Mg6Cr2(OH)16CO3· 4H2O rhombohedral Takovite Ni6Al2(OH)16CO3· 4H2O rhombohedral 1. Introduction 7 1.2.1. Preparation of hydrotalcite-type compounds (HTs) Many compounds with the same structural elements as the above mentioned
compounds have been synthesised and also found in nature. They will be referred to in
the following as hydrotalcite-type compounds (HTs) as in most publications, though
they are frequently also referred to as Feitknecht’s compounds, mixed hydroxides or
layered double hydroxides. Since not only the metals can be exchanged in a wide
range, but also the anion present, their formula is best described as: [M2+1-xM3+x(OH)2][An-x/n]· mH2O where M are metal cations and A are anions Though a great variety of HT compounds with even more than three different metals
and several anions have been synthesised, three fundamental rules need to be fulfilled
to obtain pure HT phases [1, 25]: The first one concerns the radii of the metals ions. Though a wide range of metal
cations in the oxidation state +2 and +3 can be used for the preparation of HTs, their
radius should not be too different from that of Mg2+. For example Be2+ is too small to
form HTs, Ca2+ and Cd2+ are too large (Tab. 2). They give rise to other types of
compounds [1, 25]. Nevertheless natural and synthetic HTs containing small amounts
of ‘large cations’ like Ca2+ and Cd2+ have been described in the literature [1, 25, 44].
The synthesis of HTs containing ions that have a suitable ion radius but favour a
distorted octahedral structure due to the Jahn-Teller effect, like Cu2+, Ni3+, Mn3+, is
more difficult because these ions are prone to form separate phases [1, 25, 28].
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