#66
Cccm
Example – Cordierite, Mg2Al4Si5O18
From the French mining engineer and geologist P. L. A. Cordier (1777-1861).
The polymorphism of cordierite I: The crystal structure of low cordierite
Gibbs G V
American Mineralogist, 51 (1966) p.1068-1087
http://webmineral.com/data/Cordierite.shtml
CIF
http://rruff.geo.arizona.edu/AMS/download.php?id=00218.cif&down=cif
SiO4 tetrahedra (orange)
AlO4 tetrahedra (blue)
Mg/Fe green/brown
H2O/Li/Na/K in the centres of the channels running along the c axis
The Fascination of Crystals and Symmetry 