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	05.DA.50
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;
Identification
Color	White with possible brownish tint
Crystal habit	Subhedral platey crystals, lamellar-fibrous, rarely euhedral prismatic; commonly foliated, massive
Crystal system	Trigonal
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

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 (T_1/2 = 15.7 million years) and also other fission products such as ⁷⁹Se (T_1/2 = 295 000 years) and ⁹⁹Tc, (T_1/2 = 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.

See also

• Brucite, Mg(OH)₂
• Fougerite
• Layered double hydroxide
• Magnesium hydroxide