Homology and typism in heterophyllosilicates: An alternative approach
Christiansen, C. C.; Makovicky, E.; Johnsen, O.
published: Dec 1, 1999
ArtNo. ESP154017502003, Price: 29.00 €
The crystal structures of the astrophyllite homologous series are composed of an octahedral sheet (O-sheet) sandwiched between two heterogeneous silicate sheets (H-sheet) in a mica-like way. The heterogeneous sheet is basically a tetrahedral sheet in which rows of octahedra replace rows of diorthosilicate groups. By describing the structures as stackings of HHO-packets, these structures are divided into rod shaped C and T-modules. The T-modules represent the mica-like parts of the structures, and the C-modules represent the octahedral parts of the H-sheets together with the backing octahedra of the O-sheet. The order number n of T-modules between two C-modules varies in the homologous series, where astrophyllite has the order n = 2 and nafertisite the n = 3. A proposed model of the crystal structure of perraultite indicates that this mineral constitutes the n = 1 homologue. The heterophyllosilicates form a family of minerals plesiotypic to this series, where the interconnections of the adjacent H-sheets differ from the astrophyllite series. Different kinds of polytypism occur within this family, with different crystal-chemical implications, and a classification of the polytypes is presented dividing the structures into 00 and non-OD polytypes and "improper" polytypes. The OD and non-OD polytypes both result from different stackings of the HHO-packets; they differ from each other also by having different order of preserved coordination spheres. There exists an extensive merotypic family of heterophyllosilicates. These structures all contain HOH-layers similar to that of the heterophyllosilicates but have intercalated additional layers of H20, nacaphite, barite or of a carbonate structure. In these structures non-OD polytypism also occurs. The interrelation of sobolevite and quadruphite is recognised as an example of OD polytypism. All the observed relationships have been worked into a classification tree for the heterophyllosilicate structures.