Lecture 5 (9/20/2006)

Crystal Chemistry

Part 4: Compositional Variation of Minerals Solid SolutionMineral Formula CalculationsGraphical Representation of Mineral Compositions

Solid Solution in MineralsWhere atomic sites are occupied by variable proportions of two or more different ionsDependent on: similar ionic size (differ by less than 15-30%)results in electrostatic neutralitytemperature of substitution (more accommodating at higher temperatures)

Types of Solid Solution1) Substitutional Solid SolutionSimple cationic or anionic substitutione.g. olivine (Mg,Fe)2SiO2; sphalerite (Fe,Zn)SCoupled substitutione.g. plagioclase (Ca,Na)Al(1-2)Si(3-2)O8(Ca2+ + Al3+ = Na+ + Si4+)2) Interstitial Solid SolutionOccurrence of ions and molecules within large voids within certain minerals (e.g., beryl, zeolite)3) Omission Solid SolutionExchange 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 AnalysesChemical analyses are usually reported in weight percent of elements or elemental oxidesTo calculate mineral formula requires transforming weight percent into atomic percent or molecular percentIt 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

Next LectureCrystal Chemistry V:Crystallization under Igneous, Metamorphic, and Near-surface Conditions

Read p. 104-115