montmorillonite

{{Infobox mineral | name = Montmorillonite | category = Phyllosilicate minerals | group = Smectite group | boxwidth = | image = Montmorillonite-Quartz-pala48a.jpg | caption = A sample of montmorillonite | formula = | IMAsymbol = Mnt | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = C2/m | unit cell = a = 5.19 Å, b = 9.02 Å, c = 12.4 Å; β = 94°; Z = 2 | color = White, pale pink, blue, yellow, red, green | habit = Compact masses of lamellar or globular microcrystalline aggregates | twinning = | cleavage = {001} Perfect | fracture = Uneven | tenacity = | mohs = 1–2 | luster = Dull, earthy | streak = | diaphaneity = Translucent | gravity = 2–3 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 1.485–1.535 nβ = 1.504–1.550 nγ = 1.505–1.550 | birefringence = δ = 0.020 | pleochroism = | 2V = Measured: 5° to 30° | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }}

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 include saponite, nontronite, beidellite, and hectorite.