SHOULD THE DEFINITION OF THE TERM “MINERAL” BE CHANGED?: THE CASE BEING MADE BY ENVIRONMENTAL MINERALOGY AND NANOMINERALOGY

Although one would reasonably expect that the definition of a mineral, by the 21^st century, would be clear and straightforward, we would argue that this is simply not the case. Many of the pillars of the classical definition of a mineral have been called into question, particularly by the burgeoning fields of environmental mineralogy and nanomineralogy. In this talk, and as a tribute to Frank Hawthorne, arguably the greatest mineralogical crystallographer who ever lived, we discuss the lack of crystallinity within certain materials that are classified as minerals, and argue that it is time to rethink mineral’s venerable definitions.

Knowledge of atomic structure has proven crucial time and again in understanding mineral properties including, especially in the elevated temperature and pressure regimes of the Earth, compressibility, elasticity, thermal behavior, and transformation characteristics (this has been one of the greatest contributions of Professor Hawthorne), and in the lower T and P regimes, aqueous reactivity and even specific surface properties such as adsorption and desorption tendencies. Keeping all this in mind, we now argue that adherence to the classic definition of a mineral has undoubtedly left large numbers of natural non-crystalline (amorphous) and poorly crystalline materials on the periphery, or worse, understudied and even unnoticed. Environmental mineralogy and nanomineralogy recognize that such materials are of exceptional importance in near-surface aqueous, soil, and atmospheric environments. These are some of the most important factors in environmental science as a whole. Indeed, relatively recently, there are environmentally important substances that have been formally recognized as minerals, such as ferrihydrite and schwertmannite, that are poorly ordered, to very poorly ordered using the traditional terminology. Perhaps they have been recognized as official minerals because of their abundance and/or geochemical importance. Certainly, our understanding of the structures of these materials, including the nature of structural order and disorder, has markedly improved due to the advancement and development of a suite of atomic structure analysis tools.