Lecture 5 (9/20/2006) Crystal Chemistry Part 4: Compositional Variation of Minerals Solid Solution Mineral Formula Calculations Graphical Representation of Mineral Compositions

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 %) similar ionic size (differ by less than %) results in electrostatic neutrality results in electrostatic neutrality temperature of substitution (more accommodating at higher temperatures) temperature of substitution (more accommodating at higher temperatures)

Types of Solid Solution 1) Substitutional Solid Solution Simple cationic or anionic substitution e.g. olivine (Mg,Fe) 2 SiO 2 ; sphalerite (Fe,Zn)S Coupled substitution e.g. plagioclase (Ca,Na)Al (1-2) Si (3-2) O 8 (Ca 2+ + Al 3+ = Na + + Si 4+ ) 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)

Recalculation of Mineral Analyses Chemical analyses are usually reported in weight percent of elements or elemental oxides 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 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 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 Spreadsheets are useful ways to calculate mineral formulas and end-member components

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