Polysomatic approach in the crystal chemical study of minerals
Published:January 01, 1997
The concept of polysomatism developed in the ’70s with the fundamental work by Thompson (1970, 1978) on biopyriboles. Since then, this concept proved to be of invaluable help in the description – sometimes in the prediction – of relevant crystal chemical features in several mineralogical families.
In what follows we shall illustrate, as examples, two polysomatic families of silicate minerals, the former displaying in addition extensive polytypic features. These families are the polysomatic series existing between pyroxene and spinel, and the polysomatic series of lintisite. Our choice depends on two main reasons:
the former is related to the didactics: the aim is to present polysomatic families having different characteristics. Therefore we shall first discuss the pyroxene–spinel family, in which there exist two building blocks only and the various polysomes arise from different ratio between them, and from different sequences; then we shall discuss the lintisite family, which are based on three distinct building blocks, which can link between each other in pairs or individually.
the second – more practical – reason is related to the fact that the crystallographic team at the University of Pisa has been concerned with the polytypic and polysomatic features of both the pyroxene-spinel and the lintisite families since a long time.
Finally, we shall briefly describe the relationship between shattuckite and planchéite, two copper silicates. These minerals can be considered as two members of a possible polysomatic family which should have striking resemblance with the famed family of biopyriboles.
Figures & Tables
Modular Aspects of Minerals
Since the first beginning of the crystal chemical study of the inorganic compounds, a simple modular approach was developed, by looking at the crystal structures as built up through the assembling of simple polyhedral units. This approach was no only useful for a vivid and insightful description of the complex atomic arrangements of natural and synthetic compounds, but, through the use of simple and powerful rules for assembling polyhedral units, it displayed an extraordinary heuristic power, suggesting reliable models for many complex structural assemblages. The polyhedral approach also laid the basis for meaningful classifications which were applied to all the classes of inorganic compounds.