|Crystal symmetry||(3R polytype) Trigonal hexagonal scalenohedral
H-M symbol: 32/m
Space group: R3m
|Unit cell||a = 3.065 Å, c = 23.07 Å; Z = 3/8;|
|Color||White with possible brownish tint|
|Crystal habit||Subhedral platey crystals, lamellar-fibrous, rarely euhedral prismatic; commonly foliated, massive|
|Tenacity||Flexible, not elastic|
|Mohs scale hardness||2|
|Luster||Satiny to greasy or waxy|
|Specific gravity||2.03 - 2.09|
|Optical properties||Uniaxial (-)|
|Refractive index||nω = 1.511 - 1.531 nε = 1.495 - 1.529|
|Birefringence||δ = 0.016|
|Other characteristics||Greasy feel|
Hydrotalcite is a layered double hydroxide of general formula Mg6Al2CO3(OH)16·4(H2O), whose name is derived from its resemblance with talc and its high water content. The layers of the structure may stack in different ways, to produce a 3-layer rhombohedral structure (3R Polytype), or a 2-layer hexagonal structure (2H polytype) which was formerly known under the name manasseite. The two polytypes are often intergrown. The carbonate anions that lie between the structural layers are weakly bound, so hydrotalcite has anion exchange capabilities.
It was first described in 1842 for an occurrence in a serpentine - magnesite deposit in Snarum, Modum, Buskerud, Norway. It occurs as an alteration mineral in serpentinite in association with serpentine, dolomite and hematite.
Hydrotalcite has been studied as potential getter for iodide in order to scavenge the long-lived 129I (T1/2 = 15.7 million years) and also other fission products such as 79Se (T1/2 = 295 000 years) and 99Tc, (T1/2 = 211 000 years) present in spent nuclear fuel to be disposed under oxidising conditions in volcanic tuff at the Yucca Mountain nuclear waste repository. Unfortunately carbonate easily replaces iodide in its interlayer. Another difficulty arising in the quest of an iodide getter for radioactive waste is the long-term stability of the sequestrant that must survive over geological time scales. Layered double hydroxides are well known for their anion exchange properties.
Hydrotalcite is also used as an antacid.
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