Abstract The basic serpentine structure is extremely simple. In spite of the simple crystal-chemical features involving the nearest neighbours (namely, the coordination polyhe-dra), complexity arises because of the different ways in which the basic polyhedra assemble together, forming flat layers in lizardite, curled layers in chrysotile, alternating layers in antigorite, flat kinked layers in polygonal serpentine, and flat geodesically kinked layers in polyhedral serpentine. Further complexity is derived from not-nearest-neighbour relations, such as polytyp-ism and polysomatism, that may occur as ordered and disordered distributions. A peculiar feature of chrysotile and polygonal serpentine is the presence of local fivefold symmetry. Chrysotile shares many nanoscale properties with synthetic nanotubes and nanowires. Serpentine minerals may be mutually associated, or interleaved with other layer silicates. Serpentinization and deserpentinization reactions have important implications for many extremely important large-scale processes occurring in the Earth’s crust and outer mantle. Due to their occurrence as tiny disordered crystals, meaningful structural study of serpentine minerals deals mostly with the nanoscale and may require alternative, unconventional methods. For this reason, electron microscopy techniques have long been used widely in the study of serpentine minerals, revealing a fascinating microstructural world.

Abstract This chapter describes the crystal habit of mineral fibres. This topic is so complex and multifaceted that it deserves a detailed and systematic description in order to guide the reader through the jungle of the intricate, overlapping, sometimes contradictory definitions of ‘mineral fibre’ and the complex relationship between crystal structure and growth and overall crystal habit. The definition of ‘mineral fibre’ or ‘fibre’ itself is intrinsically chameleon-like because it assumes different meanings and relevance in different areas and scientific fields. As a matter of fact, if you ask ten different people to define what ‘fibre’ means to them, you may receive ten different answers. Even among specialists in the field of ‘mineral fibres’, there is no clear agreement on what we call a ‘fibre’. It is important to be able to rely upon indisputable and unanimous definitions and share a common language because the crystal habit of ‘mineral fibres’, especially asbestos, is considered one of the major characteristics affecting their toxicity, pathogenicity and carcinogenicity and hence it has a paramount importance for the social and legal issues.

Abstract This chapter deals with the crystal structure of regulated and unregulated mineral fibres. The aim is to provide readers, both specialists and researchers broadly interested in environmental problems, with up-to-date information on a topic that is expanding daily. The chapter describes specifically the structure of the fibrous modification whenever available and outlines possible differences from the corresponding prismatic variety. Details of the experimental techniques used for structure determination/refinement are reported also, if appropriate, to outline the experimental difficulties faced due to the small dimensions, sensitivity and chemical complexity of mineral fibres.