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Exploring Mineral Solid Solutions and Formulas

Learn about solid solution in minerals, calculate mineral formulas, and represent compositions graphically. Understand types of solid solution and how to recalculate mineral analyses. Dive into crystal chemistry topics.

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Exploring Mineral Solid Solutions and Formulas

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  1. Lecture 5 (9/20/2006)Crystal ChemistryPart 4: Compositional Variation of Minerals Solid SolutionMineral Formula Calculations Graphical Representation of Mineral Compositions

  2. Solid Solution in Minerals Where atomic sites are occupied by variable proportions of two or more different ions Dependent on: • similar ionic size (differ by less than 15-30%) • results in electrostatic neutrality • temperature of substitution (more accommodating at higher temperatures)

  3. Types of Solid Solution 1) Substitutional Solid Solution Simple cationic or anionic substitution e.g. olivine (Mg,Fe)2SiO2; sphalerite (Fe,Zn)S Coupled substitution e.g. plagioclase (Ca,Na)Al(1-2)Si(3-2)O8 (Ca2+ + Al3+ = Na+ + Si4+) 2) Interstitial Solid Solution Occurrence of ions and molecules within large voids within certain minerals (e.g., beryl, zeolite) 3) Omission Solid Solution Exchange of single higher charge cation for two or more lower charged cations which creates a vacancy (e.g. pyrrhotite – Fe(1-x)S)

  4. Recalculation of Mineral Analyses • Chemical analyses are usually reported in weight percent of elements or elemental oxides • To calculate mineral formula requires transforming weight percent into atomic percent or molecular percent • It is also useful to calculate (and plot) the proportions of end-member components of minerals with solid solution • Spreadsheets are useful ways to calculate mineral formulas and end-member components

  5. Next Lecture Crystal Chemistry V: Crystallization under Igneous, Metamorphic, and Near-surface Conditions Read p. 104-115

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