Traditionally,
mineralogy has been an observational science: mineralogists describe new minerals, measure the stability fields of known minerals with respect to intensive thermodynamic variables, solve and refine crystal structures, and attempt to develop empirical schemes of organization of this knowledge, and apply these schemes to problems in the Earth and Environmental Sciences. Most minerals are complex (sensu lato) objects from both structural and chemical perspectives. On the one hand, this makes a quantitative theoretical understanding of the factors controlling structure, chemical composition and occurrence difficult to impossible by established theoretical methods in
physics and
chemistry. On the other hand, the more complicated a mineral, for example, veblenite: KNa(H2O)3[(Fe2+5Fe3+4Mn2+6Ca□)(OH)10(Nb4O4{Si2O7}2(Si8O22)2)O2], the more information it contains about its origin and properties. The principal thrust of Hawthorne's work has been to establish the theoretical underpinnings of more rigorous approach to
Mineralogy. The patterns of linkage of chemical bonds in space contain significant energetic information that may be used for this purpose. Bond Topology combines aspects of
Graph Theory, Bond-Valence Theory, This approach allows all topologically possible local arrangements to be enumerated for specific sets of coordination polyhedra. Infinite arrangements with
translational symmetry may be represented by
finite graphs via
wrapping and extends this method to
crystals. Work by the late
Jeremy Burdett showed that the electronic energy
density of states can be derived using the
method of moments, and that the energy difference between two structures depends primarily on the first few disparate moments of their respective energy
density of states This leads to the following conclusions: (1) zero-order moments define chemical composition; (2) second-order moments define coordination numbers; (3) fourth- and sixth-order moments define local connectivity of coordination polyhedra; and (4) higher moments define medium- and long-range connectivity.
Chemical reactions in minerals Using the moments approach (see above), chemical reactions in minerals may be divided into two types: has shown that short-range order is ubiquitous in
amphiboles and defines the chemical pathways by which these minerals respond to varying temperature and pressure. The theoretical developments that underpin this behaviour indicate that they should apply to all other anisodesmic minerals (2) Minerals in which bond topology is not conserved in chemical reactions form the majority of mineral species, but are less quantitatively abundant; however, they form the majority of the environmentally relevant minerals. The criteria that control the chemical composition and stability of these minerals at the atomic level may be derived from the valence-sum rule and valence-matching principle and much of this complexity can be quantitatively predicted reasonably well, and species in aqueous solution also follow the valence-sum rule, and that their Lewis basicities scale with pH of the solution at maximum concentration of the species in solution Complex species in aqueous solution actually form the building blocks of the crystallizing minerals, and hence the structures retain a record of the pH of the solutions from which they crystallized.
Structure hierarchy A mathematical
hierarchy is an ordered set of elements where the ordering reflects a natural hierarchical relation between the elements. The structure hierarchy hypothesis states that structures may be ordered hierarchically according to the polymerization of coordination polyhedra of higher bond valence. and
Nikolai Belov; (2) if the basis of the classification involves factors that are related to the mechanistic details of the stability and behaviour of minerals, then the physical, chemical and paragenetic characteristics of minerals should arise as natural consequences of their crystal structures and the interaction of those structures with the environment in which they occur. The structure hierarchy hypothesis may be justified by considering a hypothetical structure-building process whereby higher bond-valence polyhedra polymerize to form the structural unit. This hypothetical structure-building process resembles our ideas of crystallization from an aqueous solution, whereby complexes in aqueous and hydrothermal solutions condense to form crystal structures, Structure hierarchies have been developed for several mineral families, e.g. borates, uranyl oxides and oxysalts, phosphate, sulfate, arsenate and oxide-centered Cu, Pb and Hg minerals
Experimental work The role of hydrogen in crystal structures Hydrogen was long considered a fairly unimportant component in minerals, particularly when present as "water of hydration". This view has now changed: the polar nature of hydrogen controls the dimensions of polymerization of strongly bonded oxyanions in crystal structures, giving rise to cluster, chain, sheet and framework structures. Minerals forming in the core, mantle and deep crust do not incorporate so much hydrogen, and hydrogen is also far less polar at high pressures due to symmetrization of donor and acceptor bonds, and minerals generally crystallize as frameworks. Minerals forming in the shallow crust or at the Earth's surface have cluster, chain, sheet and framework structures in response to the constituent hydrogen.
Short-range order-disorder in rock-forming minerals Long-range order (LRO) describes the tendency for atoms to order at a specific location in a structure, averaged over the whole crystal. Short-Range Order (SRO) is the tendency for atoms to locally cluster in arrangements that are discordant with random distribution. A local form of Bond-Valence Theory (i.e., NOT a mean-field approach) can be used to predict patterns of SRO Infrared spectroscopy (IR) in the fundamental OH-stretching region is sensitive to both LRO and SRO of species bonded to OH, and one can combine Rietveld structure refinement and IR spectroscopy to derive patterns of SRO. Thus H can act as a local probe of SRO in many complex rock-forming minerals. Of particular importance are the role of Li, Ti and H in amphiboles, Li and H in staurolite and Li in tourmaline This work has resulted in much improved understanding of the crystal chemistry of these minerals, and the possibility for more realistic activity models for their thermodynamic treatment.
Crystal chemistry of amphibole-supergroup minerals In 1987, Hawthorne began collaboration with Roberta Oberti, Luciano Ungaretti and Giuseppe Rossi in Pavia using large-scale crystal-structure refinement and electron-microprobe analysis of amphiboles to solve many crystal-chemical problems, e.g. This work has had a major impact on the understanding of amphibole structure, chemical composition and occurrence and resulted in a more comprehensive classification and nomenclature for these minerals
Crystal chemistry of tourmaline-supergroup minerals The tourmaline minerals rival the amphiboles in complexity, and were relatively neglected until twenty-five years ago. Hawthorne and his students began crystal-chemical work on these minerals and rapidly identified a new subgroup of tourmaline minerals, showed that tourmaline has more complicated cation-ordering patterns than was hitherto thought, and a new classification scheme for the tourmaline-supergroup minerals was approved by the International Mineralogical Association. There has since been a major increase in tourmaline studies, turning it into a petrogenetically useful mineral.
Description of new minerals Systematic work on the crystal chemistry of rock-forming minerals have led to the discovery many hitherto unrecognized types of chemical substitution, e.g. The main interest with regard to accessory minerals is the opportunity to examine novel crystal structures in relation to the hierarchical organization of structural arrangements in general. Often by serendipity, this work has led to some interesting findings such as the discovery of thiosulphate in sidpietersite and [C4-Hg2+4]4+ groups in mikecoxite Hawthorne has been involved in the discovery of 180 new mineral species. == Honours ==