Hydrotalcite
Hydrotalcite with serpentine, Snarum, Modum, Buskerud, Norway. Size: 8.4 × 5.2 × 4.1 cm
General
Category	Carbonate mineral
Formula (repeating unit)	Mg6Al2CO3(OH)16·4H2O
IMA symbol	Htc[1]
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	[2][3][4][5]

Hydrotalcite, or formerly also völknerite,^[6] is a layered double hydroxide (LDH) of general formula Mg
₆Al
₂CO
₃(OH)
₁₆·4H
₂O, whose name is derived from its resemblance with talc and its high water content. Multiple structures containing loosely bound carbonate ions exist. The easily exchanged carbonates allow for applications of the mineral in wastewater treatment and nuclear fuel reprocessing.

Structure and discovery

It was first described in 1842 for an occurrence in a serpentine–magnesite deposit in Snarum, Modum, Buskerud, Norway.^[2] It occurs as an alteration mineral in serpentinite in association with serpentine, dolomite and hematite.^[3] 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.^[2][3][5]

Applications

Nuclear fuel reprocessing

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 = 327,000 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. However, carbonate anions easily replace iodide anions in its interlayer and therefore the selectivity coefficient for the anion exchange is not favorable. 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 (LDH) are well known for their anion exchange properties.^{[citation needed]}

Medical

Hydrotalcite is also used as an antacid, such as Maalox (magnesium-aluminium oxide).^[7]

Wastewater treatment

Treating mining and other wastewater by creating hydrotalcites often produces substantially 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 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 settling, centrifuge, or other mechanical means.^[8]