>Can anybody help me out here by either supplying me with a
>montmorillonite structure or giving me a reference which contains one?
In the Wyckoff Crystal Structures Vol. 4, p. 372 (1968) :
"The more hydrated clay minerals, such as halloysite and
montmorillonite, have been extensively studied although satisfactory
determinations of atomic arrangements have not been made.
Both of these have been given monoclinic units very similar to
those of dickite and kaolinite."
Seems that 32 years later, the same comments may apply to
montmorillonite because of some pathology disordering the
layer orientations and variable interlayer content.
You cannot even find it in the mineralogy database :
http://www.geo.arizona.edu/xtal-cgi/test
nor kaolinite, nor halloysite, but you can find dickite.
Armel Le Bail
http://sdpd.univ-lemans.fr/course/
