Polymorphism - Illustrating Mineral Structures Ions...

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Illustrating Mineral Structures • Ions represented as spheres – drawn to scale • Stick and ball method • Polyhedron method • Hybrid: Sick and Ball, plus polyhdron • Map view – unit cell dimensions Isostructural Minerals • Many minerals have identical structures, different compositions - example: Halite (NaCl) and Galena (PbS) - differ in many physical properties - identical symmetry, cleavage, and habit • Isostructural group - Several isostructural minerals - Have common anion group - Much substitution between cations - E.g Calcite group Polymorphism 1
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• The ability for compounds to crystallize with more than one structure - Polymorphs - Polymorphic groups • Caused by balance of conflicting requirements: - attraction and repulsion of cations and anions - fit of cations in coordination site (size) - geometry of covalent bonds • Polymorphs controlled by P and T conditions: - High P favors tightly packed lattice, high density - High T favors open lattice, low density, wide substitution • Presence or absence of polymorphs provide information on P and T conditions • Four types of polymorphism: - reconstructive - displacive - order-disorder - polytypism Reconstructive Polymorphism • Requires breaking of bonds – major reorganization • May not have symmetry of structural elements in common, although may because of composition • Example: C • C: diamond and graphite 2
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- Diamond: all covalent bonds - Graphite: covalent bonds in sheets, van der Walls bonds between sheets Graphite stable at earth surface T and P • Diamond stable at high P and T
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Polymorphism - Illustrating Mineral Structures Ions...

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