Hydrotalcite-like and manasseite-like minerals occur as polytypes in rhombohedral and hexagonal forms, respectively. The general formula is [Mg_1-xAl_x(OH)₂]^x+ [(CO₃)_x/2 ^. nH₂O]^x- , where x = 0.25 to 0.33 (Drits et al., 1987). In general, the structure is a brucite-like positively charged layer separated by CO₃ anions and H₂O as interlayer material, but substitutions are common. The International Mineralogical Association recognizes different species (below), based primarily on chemical differences, although there are many other un-named forms. These minerals occur in saline deposits, pegmatites, and serpentinites. Hydrothermal synthesis is relatively easy, as is anion exchange. 
Cf., anionic clay (Part A), double metal hydroxides (Part A)

barbertonite Mg₆Cr₂(OH)₁₆CO₃ ^. 4(H₂O)

carrboydite (Ni,Cu)_5.90Al_4.48(OH)_21.69(SO₄,CO₃)_2.78 ^. 3.67(H₂O)

chlormagaluminite (Mg_3.55Fe²⁺_0.27Na_0.05)(Al_1.93Fe³⁺_0.07Ti_0.01)(OH)₁₂ ^. Cl₂CO₃ ^. 2(H₂O)

coalingite Mg₁₀Fe³⁺₂(OH)₂₄CO₃ ^. 2(H₂O); Mg₁₆Fe³⁺₂(OH)₃₆CO₃ ^. 2(H₂O)

desautelsite Mg₆Mn₂(OH)₁₆CO₃ ^. 4(H₂O)

honessite [Ni_5.55Mg_0.10Fe³⁺_2.35(OH)₁₆](SO₄)_1.18 ^. nH₂O

hydrohonessite [Ni_5.43Fe³⁺_2.57(OH)₁₆](SO₄)_1.286.95H₂O ^. 0.98NiSO₄

hydrotalcite Mg₆Al₂(OH)₁₆CO₃ ^. 4(H₂O); Mg₄Al₂(OH)₁₂SO₄ ^. 3(H₂O)

iowaite Mg_4.63Fe³⁺_1.32(OH)₁₂Cl_1.33 ^. 1.95(H₂O)

manasseite Mg₆Al₂(OH)₁₆CO₃ ^. 4(H₂O); Mg₄Al₂(OH)₁₂CO₃ ^. 3(H₂O)

meixnerite Mg₆Al₂(OH)₁₆(OH)₂ ^. 4(H₂O)

motukoreaite [Mg_1.82Mn_0.03Zn_0.02Al_1.12(OH)_5.15] ^. [Na_0.07K_0.07 (CO₃)_0.40(SO₃)_0.41 ^. 2.7(H₂O)]

mountkeithite [(Mg_8.15Ni_0.85)(Fe³⁺_1.31Cr_1.02Al_0.65)(OH)₂₄](CO₃)_1.11(SO4)_0.38(Mg_1.76Ni_0.18)(SO₄)_1.94(H₂O)_9.39

pyroaurite Mg₆Fe³⁺₂(OH)₁₆CO₃ ^. 4.5(H₂O); Mg₄Ni²⁺₂Fe³⁺₂(OH)₁₆CO₃ ^. 4(H₂O)

reevesite Ni₆Fe²⁺₂(OH)₁₆CO₃ ^. 4(H₂O)

sjögrenite Mg₆Fe³⁺₂(OH)₁₆CO₃ ^. 4.5(H₂O)

stichtite Mg₆Cr₃(OH)₁₆CO₃ ^. 4(H₂O); [Mg_5.94(Cr_1.29Al_0.51Fe³⁺_0.25)(OH)_15.1][(CO₃)_1.473.7(H₂O)]

takovite Ni₆Al₂(OH)₁₆CO₃OH ^. 4(H₂O); [Ni₅Mg_0.10Fe³⁺_0.13Al_2.81(OH)_14.42](CO₃)_2.27 ^. 5.4(H₂O);

             Ni₆Al₂(OH)₁₆SO₄OH ^. nH₂O

wermlandite [Mg_3.55(Al_0.57Fe³⁺_0.41)₂(OH)₁₈](Ca_0.6Mg_0.4)(SO₄)₂ ^. 12(H₂O)

woodwardite Cu₄Al₂(OH)₁₂SO₄ ^. 2-4H₂O