Smectite group

Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite group, is a 2:1 clay, meaning that it has two tetrahedral sheets of silica sandwiching a central octahedral sheet of alumina. The particles are plate-shaped with an average diameter around 1 μm and a thickness of 0.96 nm; magnification of about 25,000 times, using an electron microscope, is required to resolve individual clay particles. Members of this group

Saponite is a trioctahedral mineral of the smectite group. Its chemical formula is . It is soluble in sulfuric acid. It was first described in 1840 by Svanberg. Varieties of saponite are griffithite, bowlingite and sobotkite.

Nontronite is the iron(III) rich member of the smectite group of clay minerals. Nontronites typically have a chemical composition consisting of more than ~30% Fe2O3 and less than ~12% Al2O3 (ignited basis). Nontronite has very few economic deposits like montmorillonite. Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchangeable cations.

Hectorite is a rare soft, greasy, white, lithium-rich clay mineral with a chemical formula of .

Saliotite is a rare colorless to pearl white phyllosilicate mineral in the smectite group with formula . It is an ordered 1:1 interstratification of cookeite and paragonite. It has perfect cleavage, a pearly luster and leaves a white streak. Its crystal structure is monoclinic, and it is a soft mineral with a hardness rated 2-3 on the Mohs scale.

Sauconite is a complex phyllosilicate mineral of the smectite clay group, formula Na0.3Zn3(SiAl)4O10(OH)2·4H2O. It forms soft earthy bluish white to red-brown monoclinic crystals typically massive to micaceous in habit. It has a Mohs hardness of 1 to 2 and a specific gravity of 2.45. Optically it is biaxial positive with refractive index values of nα = 1.550 – 1.580, nβ = 1.590 – 1.620 and nγ = 1.590 – 1.620. It is found in vugs and seams in the oxidized zones of zinc and copper deposits. It occurs in association with hemimorphite, smithsonite, chrysocolla, coronadite and various iron oxides.

Aliettite is a complex phyllosilicate mineral of the smectite group with a formula of (Ca0.2Mg6(Si,Al)8O20(OH)4·4H2O) or .

Stevensite is a white clay mineral composed of hydrated calcium, sodium and magnesium silicate, with the chemical formula (Ca,Na)xMg3-x(Si4O10)(OH)2. The mineral is a member of smectite. The mineral is named after Edwin Augustus Stevens.