EM Ch 3

• Atomic (Ionic) substitution


  • Different ions occupy the same structural site if they have similar size, similar charge, and available in the environment where the mineral is forming
  • Favors ions of similar radius and similar charge
  • Limited to 15%-30% of the radius at surface temperatures and pressures
  • Substitution of ions with larger differences can occur at higher temperatures
• Substitution


  2 or more ions of similar radius and charge can substitute for one another
• Simple complete substitution


  When 2 elements can freely substitute for one another in any proportion
• Complete solid solution series


  Simultaneous substitution of ions of different charges in two different structural sites that preserve the electrical neutrality of the crystal lattice
• Example of complete solid solution series

  • Plagioclase feldspar series
• Coupled ionic substitution


  Ions of substantially different size limit the amount of substitution
• Example of coupled ionic substitution


  • Calcite (CaCO3) and magnesite (MgCO3)

  
• Miscibility gap


  Potential compositions that do not exist in nature due to limited amount of substitution
• Example of miscibility gap

  • Ms25 to Ms40 in the calcite-magnesite series
• Phase diagrams


  • Display the stability fields for various phases separated by lines representing conditions under which the phase changes can occur
• Liquidus


  Phase boundary (line) that separates melt from field that contain some solids (crystals)
• Solidus


  Phase boundary (line) that separates the all-solid field from the field that contain some liquid (melt)
• Eutectic


  Condition under which liquid is in equilibrium with two different solids
• Peritectic


  Condition under which a reaction occurs between a pre-existing solid phase and a liquid to produce a new solid phase
• Phase


  Mechanically separable part of a system (can be solid, liquid, or gas)
• Invariant melting


  Occurs when a melt of the same composition are produced by melting of rocks of different initial composition
• Incongruent melting


  Occurs when a solid mineral phase melts to produce a melt and a different mineral with a composition that is different from the initial mineral
• Discontinuous reaction


  Mineral crystals and melt react to produce a completely different mineral, with negligible solid solution between the minerals
• Continuous reaction


  Mineral crystals and melt react continuously and incrementally change the composition of both mineral and melt, requiring a solid solution series
• Solvus


  Phase boundary (line) that separates conditions in which complete solid solution occurs within a mineral series from conditions under which solid solutions are limited

  
• Phase rule/ Gibbs' Phase rule (1928)


  F = C + 2 - P, where F is the number of degrees of freedom or variance, C is the minimum number of chemical components required to define the phases, and P is the number of phases present in the system

  
• Stishovite created at deep mantle regions, coesite created at meteor impacts and thermonuclear bomb sites
• Stishovite and coesite are high pressure varieties, tridymite and cristobalite are high temperature-low pressure varieties, quartz is the stable polymorph that is why it is abundant in the crust

  
• Low (alpha) quartz is more stable in low temperature environments
• Lever rule


  The proportion of the tie line on the solidus side of the system composition represents the proportion of liquid in the system, and the proportion of the tie line on the liquidus side represents the proportion of crystals (solids) in the system
• What happens in an albite-anorthite diagram as a system cools

  1. Crystal composition evolves down the solidus

  2. Liquid composition evolves down the solidus during continuous melt-crystal reaction and additional crystallization

  
• The smaller the amount of partial melting that occurs in a system, the more enriched are the melts in low temperature components
• Larger percentages of partial melting progressively dilute the proportion of low-temperature components
• Which mineral crystallizes first from a magma depends on the specifics of melt composition (if magma is basaltic, ferromagnesians like olivine and pyroxene will be first to crystallize)
• Separation of crystals from the melt generally causes the melt composition to change
• Multiple minerals can crystallize simultaneously from a magma
• Perthite


  1. feldspars containing Na-feldspars due to initially orthoclase-rich solid solutions

  
• Antiperthite


  Na-feldspars containing K-feldspars due to initially albite-rich solid solutions
• Silica oversaturated


  Occurs when there is more than enough silica (more than 2/3) to convert nepheline to albite, evidenced by the presence of tridymite or quartz alongside plagioclase feldspars

  
• Silica undersaturated


  Occurs when there is not enough silica to convert nepheline to albite, evidenced by presence of feldspathoids (ex: nepheline)
• Silica saturated


  Occurs when there is exact (2/3) amount of silica to convert nepheline to albite, evidenced by the presence of feldspar and absence of silica and feldspathoids

  
• Isotopes


  Varieties of the same element characterized by difference in the number of neutrons
• Oxygen has 3 isotopes: 16O, 17O, and 18O. 16O constitutes >99.7%, 18O constitutes ~0.2%, while 17O is a relatively rare isotope
• Carbon has 3 isotopes 12C, 13C, and 14C. 12C constitutes >98.9% while 13C constitutes 1.1%
• Alpha decay


  Release of 2 protons and 2 neutrons, decreasing mass number by 4 and atomic number by 2