Hydrotalcite

Hydrotalcite

Hydrotalcite with serpentine, Snarum, Modum, Buskerud, Norway. Size: 8.4 x 5.2 x 4.1 cm
General
Category Carbonate mineral
Formula
(repeating unit)		 Mg6Al2CO3(OH)16·4(H2O)
Strunz classification 5.DA.50
Crystal system 3R polytype: Trigonal
2H polytype: Hexagonal
Crystal class 3R polytype: Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
2H polytype: Dihexagonal dipyramidal (6/mmm)
Space group R3m
Unit cell a = 3.065 Å,
c = 23.07 Å; Z = 3
Identification
Color White with possible brownish tint
Crystal habit Subhedral platey crystals, lamellar-fibrous, rarely euhedral prismatic; commonly foliated, massive
Cleavage {0001}, perfect
Tenacity Flexible, not elastic
Mohs scale hardness 2
Luster Satiny to greasy or waxy
Streak White
Diaphaneity Transparent
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
References

[1][2][3]

[4]

Hydrotalcite is a layered double hydroxide of general formula Mg
6Al
2CO
3(OH)
16·4(H
2O), whose name is derived from its resemblance with talc and its high water content. The layers of the structure stack in multiple ways, to produce a 3-layer rhombohedral structure (3R Polytype), or a 2-layer hexagonal structure (2H polytype) formerly known as manasseite. The two polytypes are often intergrown.[1][2][4] 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.[1] It occurs as an alteration mineral in serpentinite in association with serpentine, dolomite and hematite.[2]

Applications

Nuclear fuel reprocessing

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. 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.

Anion exchange

Layered double hydroxides are well known for their anion exchange properties.

Medical

Hydrotalcite is also used as an antacid.

Wastewater treatment

Treating wastewater by creating hydrotalcites produces less sludge than lime. In one test, final sludge reductions reached up to 90 percent. This alters the concentration of magnesium and aluminum and raises the pH of the water. As the crystals form, they trap other waste substances including radium, rare earths, anions and transition metals. The resulting mixture can be removed via centrifuge.[5]

See also

References

Further reading

This article is issued from Wikipedia - version of the 11/24/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.