hydrotalcite

{{Infobox mineral | name = Hydrotalcite | category = Carbonate mineral | image = Hydrotalcite-200667.jpg | imagesize = 260px | caption = Hydrotalcite with serpentine, Snarum, Modum, Buskerud, Norway. Size: 8.4 × 5.2 × 4.1 cm | formula = | IMAsymbol = Htc | molweight = | strunz = 5.DA.50 | dana = | system = 3R polytype: Trigonal 2H polytype: Hexagonal | class = 3R polytype: Hexagonal scalenohedral (m) H-M symbol: ( 2/m) 2H polytype: Dihexagonal dipyramidal (6/mmm) | symmetry = Rm | unit cell = a = 3.065 Å, c = 23.07 Å; Z = 3 | color = White with possible brownish tint | colour = | habit = Subhedral platey crystals, lamellar-fibrous, rarely euhedral prismatic; commonly foliated, massive | twinning = | cleavage = {0001}, perfect | fracture = | tenacity = Flexible, not elastic | mohs = 2 | luster = Satiny to greasy or waxy | streak = White | diaphaneity = Transparent | gravity = 2.03–2.09 | density = | polish = | opticalprop = Uniaxial (−) | refractive = nω = 1.511 – 1.531 nε = 1.495 – 1.529 | birefringence = δ = 0.016 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = Greasy feel | alteration = | references = }} Hydrotalcite, or formerly also völknerite, is a layered double hydroxide (LDH) of general formula , whose name is derived from its resemblance with talc and its high water content. Multiple structures containing loosely bound carbonate () ions exist. The easily exchangeable carbonates enable applications of the mineral in wastewater treatment and the immobilisation of radioactive waste after 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. It occurs as an alteration mineral in serpentinite in association with serpentine, dolomite and hematite. 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.