Montmorillonite
>
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. Wikipedia
Color: White, pale pink, blue, yellow, red, green
2V angle: Measured: 5° to 30°
Cleavage: Perfect
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Compact masses of lamellar or globular microcrystalline aggregates
Crystal system: Monoclinic
Formula (repeating unit): (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O