Bruce Godfrey Hyde

25: Inorganic and Mineral Structures Reconsidered (1986)

Introduction

25: inorganic and mineral structures reconsidered (1986). Reconsidering the "Ionic Model" in solid-state chemistry and mineralogy. This lecture proposes an alternative, successful in silicates & complex structures, emphasizing cation dominance.

Abstract

Liversidge Research Lecture delivered before the Royal Society of New South Wales, September 24th, 1986, at The University of Sydney. Reproduced by permission of the Royal Society of New South Wales from J. Proc. Roy. Soc. N.S.W., 1986, 119, 153-164."For 60 years or so the "Ionic Model" has been fundamental to solid state chemistry and mineralogy. It has been useful, but the ideas involved have become sacrosanct, even when they do not work! Quantum mechanical methods are becoming increasingly important and useful, but they lack the simple "physical" approach and, in any case, so far can only be applied to the simpler structures.""An alternative approach, as simple, naive and "physical" as the ionic model, is successful where the latter succeeds and where it fails (e.g. in silicates). It can often be useful for simple and complicated structures; and it avoids the ionic/covalent dichotomy. Like the successful quantum methods, it sees no difference in principle between non-molecular structures and those of small molecules (another unhappy dichotomy). It emphasises that, as in organic chemistry, one "size" for an atom is insufficient for understanding structure; at the crudest level one needs a bonding size (for first nearest neighbour interactions) and a non-bonding size (for second and further neighbours).""It transpires that cations, far from being small in size and influence, often dominate crystal structure and behaviour."

Review

This paper, based on the Liversidge Research Lecture delivered in 1986, offers a provocative and timely re-evaluation of fundamental principles in solid-state chemistry and mineralogy. For decades, the "Ionic Model" has served as the bedrock for understanding inorganic and mineral structures, despite its acknowledged limitations and occasional failures. The abstract highlights a critical juncture where established models, though useful, had become "sacrosanct," while emerging quantum mechanical methods, though powerful, lacked a simple physical intuition and were often computationally intensive for complex systems. This sets the stage for a much-needed critical introspection into how we conceptualize and teach crystal structures. The core of the lecture presents an alternative approach, designed to be as conceptually straightforward and "physical" as the ionic model, yet demonstrably more versatile and accurate. This new perspective purports to succeed where the conventional model falters, notably in complex silicate structures, and is applicable across a spectrum of structural complexities. A significant strength lies in its ability to transcend the oftenproblematic ionic/covalent dichotomy, fostering a unified understanding that bridges the gap between non-molecular solids and discrete molecules. Central to this proposed framework is the novel concept that an atom requires not just one "size," but distinct "bonding" and "non-bonding" sizes to adequately describe its interactions within a crystal lattice, akin to the nuanced understanding prevalent in organic chemistry. Perhaps the most impactful insight from this lecture is the radical re-positioning of cations within crystal structures. Far from being mere "small in size and influence," the abstract emphatically states that cations "often dominate crystal structure and behaviour." This challenges a long-held perception and, if substantiated by further research, could fundamentally alter our understanding of crystallographic principles and material properties. The lecture, even in abstract form, promises a powerful conceptual tool for researchers and educators alike, urging a departure from entrenched models towards a more nuanced and accurate representation of the forces governing inorganic and mineral architectures.

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