Montmorillonite [K⁺_0.58(Al,Mg)₂(Si,Al)₄O₁₀(OH)₂]

Structure Monoclinic
Space Group C2/m (No. 12)
a=5.200 Å b=9.200 Å c=10.130 Å
a=90.00, b=99.00, g=90.00

Atomic Positional Parameters

Al 0.000 0.333 0.000
K  0.500 0.000 0.500
Mg 0.000 0.000 0.000
O1 0.481 0.500 0.320
O2 0.172 0.728 0.335

O3 0.348 0.691 0.110
OH 0.419 0.000 0.105
Si 0.417 0.329 0.270

References
SI Tsipursky and VA Drits (1984) Clay Minerals, 19, 177-193

SW Bailey (Ed.) (1988) Hydrous Phyllosilicates, Reviews in Mineralogy, 19, Mineralogical Society of America.

Mineral Chemistry
The montmorillonite group of minerals are clay minerals formed from very small and imperfect crystals. This group is basically of the pyrophyllite-type but substitutions in either or both octahedral and tetrahedral layers allows weak bonding of exchangeable cations in interlayer positions. Interlayer water or cation exchange occurs readily, with accompanying large changes in the c-dimension, hence the term 'swelling clays'.

The simplest end-member formula (which is that of pyrophyllite) is Al₄Si₈O₂₀(OH)₄.nH₂O; the term 'smectite' encompasses the group of clay minerals that are derived from this formula.

The structure displayed here was analysed following K-saturation of interlayer cation sites.

The CHIME figure shows magnesiums and aluminiums in octahedral coordination. Hydroxyl groups (OH) are shown as blue spheres. Interlayer potassiums are shown as crimson spheres. The Si-O sheets are shown with ochre and red spheres for silicon and oxygen respectively. The unit cell is outlined.