Small a tendency with many exceptions tourmaline group

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Unformatted text preview: 配位多面體共用組成與結構穩定性之關係)。 The existence of edges, and particularly of faces, common to two coordination polyhedra decreases the stability of ionic structures. ◎ Particularly prominant when the following conditions occur: (1) The cations are highly charged. charged. → Highly charged cations tend to: (a) maintain as large a separation as possible within a structure. (b) have anions intervening between them so as to screen one from the other. → Lower repulsive forces. → Reduce crystal potential energy. (2) The cations have small coordination numbers. numbers. (3) The RA/RB ratio is near the lowest limit tolerated by the anion polyhedron. 國立成功大學地球科學系 礦物學(一) 礦物學(一)_江威德_Lecture 12 江威德 11 2009/11/24 Pauling’s Rules 國立成功大學地球科學系 Edge-sharing Face-sharing Distance relative to the corner-sharing case a Corner-sharing 國立成功大學地球科學系 礦物學(一) 江威德 Distance relative to the polyhedral edge length t d 國立成功大學地球科學系 礦物學(一) 江威德 礦物學(一)_江威德_Lecture 12 12 2009/11/24 Pauling’s Rules 國立成功大學地球科學系 Distance effect ◎ Shortening of shared edges: → Reduction of cation-cation repulsion. Screening effect cation→ Indication of the presence of charged ions in an ionic bond. bond. ◎ Lack of shortening of shared edges. → Predominantly covalent bond. Predominantly bond. 國立成功大學地球科學系 物學(一) 江威德 Rule 4: The principle of independence of polyhedra with small cations cations of high charge. (高電荷、低配位數陽離子配位多面體之特性)。 In a crystal containing different cations, those with large valence and small coordination number tend not to share polyhedral elements small coordination number tend not to share polyhedral elements with with each other. High charge Small coordination polyhedron carbonates 礦物學(一)_江...
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