| Mineral | MONTMORILLONITE |
| Chemistry | (NaCa) 0.33 (AlMg) 2 Si 4 O 10 (OH) 2 ·nH 2 O |
| Class | Silicates |
| Uses | as a drilling mud and as a water clogging agent in soil. |
| Transparency | crystals are translucent and masses are opaque. |
| CrystalSystem | Monoclinic |
| CrystalHabits | : never in large individual crystals, usually found in compact or lamellar masses.Also seen as inclusions in quartz as fibers and powder-like masses. |
| Fracture | perfect in one direction, basal; not seen in massive specimens. |
| Hardness | 1½ - 2 |
| Characteristics | crystals expand to many times their original volume when added to water. |
| Indicators | softness, color, soapy feel, luster and expandability when added to water. |
| DensityNote | average |
| IMAStatus | Approved |
| Discovered | 1847 |
| CellParameters | a = 5.17b = 8.94c = 9.95 β=99.54° |
| Ratio | a:b:c = 0.578 : 1 : 1.113 |
| OpticalType | Biaxial (-) |
| RI | n α =1.485 - 1.535 n β =1.504 - 1.550 n |
| Birefringence | δ=0.020 |
| SurfaceRelief | Low |
| TypeLocality | Montmorillon Vienne Poitou- Charentes France |
| Strunz | 8/H.19-20 8 : Silicates H : Phyllosilicates (layered)Tetragonal or pseudotetragonal layered structures [Si4O10]4- and others 19 : Brammallite ? - Yakhontovite series |
