See hydrotalcite group.

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

The hydroxy interlayer is comprised of Al–OH^–,H₂O complexes (also possibly with Fe³⁺ or Mg2+) and is located in the space (or interlayer) between the 2:1 layers of phyllosilicates. The cations are octahedrally coordinated by the H₂O and OH^–. The hydroxy-interlayer does not form continuous sheets and additional H₂O may occur above and below the Al–OH^–, H₂O complexes. See hydroxy-interlayered mineral.
Cf., hydroxy interlayering, hydroxy-interlayered smectite, hydroxy-interlayered vermiculite

Hydroxy interlayering is a process that commonly occurs in upper soil horizons where 2:1 layer phyllosilicates (e.g., smectite, vermiculite, mica) are transformed to their hydroxy-interlayered equivalents by incorporation and polymerization of Al³⁺ and OH^– and/or H₂O. These Al–(OH)^–, H₂O complexes (also possibly with Fe³⁺ or Mg²⁺) are located in the interlayer between the 2:1 layers, and do not form continuous sheets. In nature, the origin of the Al³⁺ is from the dissolution of silicates, such as feldspar and chlorite. See hydroxy-interlayered mineral.
Cf., hydroxy interlayer, hydroxy-interlayered smectite, hydroxy-interlayered vermiculite, mica, smectite, vermiculite

Hydroxy-interlayered minerals are 2:1 layer phyllosilicates with Al³⁺ and possibly Mg²⁺ and Fe³⁺ complexes in the interlayer. These complexes are formed by polymerization of usually Al3+ cations octahedrally coordinated by (OH)^– and/or H₂O. Complexes are limited to one to five octahedra without forming a continuous sheet or Keggin ions (Al₁₃), although it is possible that additional H₂O may occur adjacent to the 2:1 layer. The permanent layer charge of HIMs per formula unit is believed to vary from 0.2 (equivalent to low-charge smectite) to 1.0 (equivalent to true mica). HIMs commonly form in upper soil horizons from smectite, vermiculite, or mica during the acidic weathering of soils. The fixed interlayer complexes reduce the ability of swelling and shrinking and thus soil fertility. These phases are commonly identified by X-ray diffraction and cation exchange capacity measurements.
Cf., chemical weathering, hydroxy-interlayer, hydroxy-interlayering, hydroxy- interlayered smectite, hydroxy-interlayered vermiculite, Keggin ion, pillared clay

Hydroxy-interlayered smectites are 2:1 phyllosilicates with a permanent layer charge of approx. 0.2 – 0.6 per formula unit and with fixed interlayer complexes. These complexes are composed of cations, mainly Al³⁺, but possibly Mg²⁺ and Fe³⁺, octahedrally coordinated by H₂O and OH^–. Presumably, the high-charge Al³⁺ fixes the complex, which thus limits swelling or shrinking capabilities. Depending on the degree of hydroxy-interlayering (proportion of exchangeable cations replaced by fixed complexes), the transition between smectite and hydroxy-interlayered smectite is poorly defined. Smectite and HIS phases are commonly interstratified.
See hydroxy-interlayered mineral.
Cf., hydroxyl interlayer, hydroxyl interlayering, hydroxy-interlayered vermiculite, smectite, vermiculite

Hydroxy-interlayered vermiculites are 2:1 layer phyllosilicates with a permanent layer charge of 0.6 – 0.9 per formula unit and with fixed complexes in the interlayer. These complexes are composed of cations, mainly Al³⁺, but possibly Mg²⁺ and Fe³⁺, octahedrally coordinated by H₂O and OH^–. Presumably, the high-charge Al³⁺ fixes the complex, which thus limits swelling or shrinking capabilities. Depending on the degree of hydroxy-interlayering (proportion of exchangeable cations replaced by fixed complexes), the transition between vermiculite and hydroxy-interlayered vermiculite is poorly defined. Vermiculite and HIV phases are often interstratified.
See hydroxy-interlayered mineral.
Cf., hydroxy interlayer, hydroxy interlayering, hydroxy-interlayered smectite, smectite, vermiculite

An obsolete term for annite.

An obsolete term for biotite.

In micromorphology of soils, hypo-coating is a pedofeature that consists of a layer of material that is related and immediately adjacent to the surface of, for example, voids, grains, or aggregates.
Syn., neo-cutan;
Cf., coating, cutan, quasi-coating