Motivation

Aims

Modern Mineralogy is focussed on the fine-scale characterization of the structure and properties of minerals and related compounds, thus being essentially a Geo-material Science. Although the number of newly characterised mineral species is low when compared to that of small molecules, the capability of their characterization is rapidly improving, and presently allows the understanding of very fine-scale details with are crucial to model physical and chemical properties. In particular, crystallographic methods are a fundamental tool for mineral characterization, and can provide constraints for interpreting the results of other techniques and a basis for a reliable modelling of mineral behaviour.

Target materials

These models are particularly important as they provide extrapolation to intensive conditions (P, T, fo2, Xh2o, bulk composition) which cannot be experimentally verified by characterization of natural material and/or the products of syntheses under controlled conditions. Rock samples coming from the deep interior of the Earth are sporadically available, and cannot represent the possible geological situations. For instance, a sample blasted out from 250 km in a kimberlite pipe in Lesoto (South Africa) represents the deepest available rock, whereas the deepest drilling in Kola Peninsula (Russia) could achieve "only" the level of 12.2 km. Uplifting geological processes (and erosion) brought to surface few metamorphic rocks from (perhaps) 100 km; however, they most probably re-equilibrated during uplift. Within the last few decades extraterrestrial material (lunar, Martian and interstellar) became also available, and will be increasingly accessible for mineralogical studies.

Technologies

In this respect, mineralogists are particularly interested in the development of high-pressure/high-temperature (crystallographic) techniques, which allow simulation of the matter in the interior of the Earth and in extraterrestrial conditions. The development and availability of intense- and micro-beam radiation sources (synchrotron radiation) also is opening a new era for characterizing micrometric samples and for the physics and chemistry of minerals, in that it allows in situ determination of (very rapid) cation-ordering processes and phase transitions even on very small crystals or on powdered material. The combination of structural data with the new information from spectroscopies based on synchrotron radiation and (pulsed) neutron sources is also giving extremely promising results.

Summary

To sum up, the experimental and theoretical problems to be faced by mineralogists, physicists and chemists of minerals span from synthesis and crystal growth to the characterization of the structure and properties, through modelling and development of new methods.

For the study of minerals, all crystallographic methods are fundamental tools, from diffraction (X-rays, neutrons, electrons) to microscopy and spectroscopy, from mathematical to physical and crystal-chemical aspects of the crystalline solid state. In particular, powerful methods for the solution of very complex structures from poorly crystalline materials, and for the improvement of the accuracy and the precision of the refinement's results would be important. That follows from the fact that most of the challenging aspects of modern crystallography are present in minerals, such as:

• Complicated crystal-chemistry due to the presence of isomorphous substitutions and the absence of discrete units (molecules); minerals are actually infinite arrays of atoms.
• Atomic order/disorder at the long- and short-range level which reflect the conditions experienced by the host rock and may be used to accurately model thermodynamics and kinetics of the geological processes.
• Real structures with defects and modulation, resulting, from deviations from thermodynamic equilibrium.
• Structural modularity (polytypism, polysomatism).
• Phase transitions.
• Low crystallinity and disordered materials.