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Melt-crystal equilibrium 1

Melt-crystal equilibrium 1. X. Magma at composition X (30% Ca, 70% Na) cools  first crystal bytownite (73% Ca, 27% Na) This shifts the composition of the remaining melt such that it is more Na-rich (Y) What would be the next crystal to precipitate?

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Melt-crystal equilibrium 1

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  1. Melt-crystal equilibrium 1 X • Magma at composition X (30% Ca, 70% Na) cools  first crystal bytownite (73% Ca, 27% Na) • This shifts the composition of the remaining melt such that it is more Na-rich (Y) • What would be the next crystal to precipitate? • Finally, the last bit would crystallize from Z Y Z

  2. monalbite anorthoclase 1100 high albite sanidine 900 intermediate albite Temperature (ºC) 700 orthoclase low albite microcline 500 Miscibility Gap 300 10 70 30 50 90 Orthoclase KAlSi3O8 Albite NaAlSi3O8 % NaAlSi3O8 Melt-crystal equilibrium 2 - miscibility • 2 component mixing and separation  chicken soup analogy, cools and separates • Fat and liquid can crystallize separately if cooled slowly • Miscibility Gap – no single mineral is stable in a composition range for x temperature

  3. Ternary Diagrams A20B50C30 • Describe variations of 3 components in crystal group(s) • X=A50B30C20 • Y=A10B10C80 • Z=A30B30C40 Y Z X

  4. Muscovite KAl2(AlSi3O10)(OH)2 No micas Miscibility Gap Biotite series Phlogopite KMg3(AlSi3O10)(OH)2 Annite KFe3(AlSi3O10)(OH)2 Combining phase and composition diagrams for mineral groups Mica ternary

  5. Feldspar Group • Most abundant mineral in the crust  6 of 7 most common elements • Defined through 3 end-members  • Albite (Na), Anorthite (Ca), Orthoclase (K) • Comprised of 2 series: • Albite-anorthite (Na-Ca) • Albite-orthoclase (Na-K)

  6. Tectosilicates Substitute Al3+ for Si4+ allows Na+ or K+ to be added Albite-Orthoclase Feldspars Substitute two Al3+ for Si4+ allows Ca2+ to be added Albite-Anorthite Albite: NaAlSi3O8

  7. Feldspar Group – Albite-Anorthite series • Complete solid solution Plagioclase Feldspars • 6 minerals • Albite (Na) • Oligoclase • Andesine • Labradorite • Bytownite • Anorthite (Ca) • Albite-Anorthite double duty • End-members (Pure Na or Ca) • Minerals 90-99.99% Na or Ca • Notation: • AnxAby An20Ab80=Oligoclase

  8. Feldspar Group – Albite-Anorthite series • Optical techniques to distinguish between plagioclase feldspars: • Michel-Levy Method – uses extinction angles of twinned forms to determine An-Ab content • Combined Carlsbad-Albite Method  uses Michel-Levy technique for both sides of a twin form

  9. Michel-Levy chart for K-feldspar optical ID

  10. Staining technique • Stains that attach to K really well (Like Co(NO3)2 ) will higlight the K-feldspars quickly and easily in hand specimen or thin section

  11. Tectosilicates Substitute Al3+ for Si4+ allows Na+ or K+ to be added Albite-Orthoclase Feldspars Substitute two Al3+ for Si4+ allows Ca2+ to be added Albite-Anorthite Albite: NaAlSi3O8

  12. monalbite anorthoclase 1100 high albite sanidine 900 intermediate albite Temperature (ºC) 700 orthoclase low albite microcline 500 Miscibility Gap 300 10 70 30 50 90 Orthoclase KAlSi3O8 Albite NaAlSi3O8 % NaAlSi3O8 Feldspar Group – Albite-Orthoclase series • Several minerals – Alkali Feldspars • High – T minerals • Sanidine • Anorthoclase • Monalbite • High Albite • Low Temperature exsolution at solvus • Chicken soup separation • Forms 2 minerals, in igneous rocks these are typically intergrowths, or exsolution lamellae – perthitic texture

  13. Liquid 1100 monalbite anorthoclase 900 high albite sanidine intermediate albite Temperature (ºC) 700 orthoclase low albite microcline 500 Miscibility Gap 300 10 70 30 50 90 Orthoclase KAlSi3O8 Albite NaAlSi3O8 % NaAlSi3O8 Alkali Feldspar Exsolution • Melt cools past solvus (line defining miscibility gap) • Anorthoclase, that had formed (through liquidus/solidus) separates (if cooling is slow enough) to form orthoclase and low albite • In hand sample – schiller effect  play of colors caused by lamellae

  14. Alkali Feldspar lamellae

  15. Feldspathoid Group • Very similar to feldspars and zeolites • Include Nepheline, Analcime, and Leucite • Also framework silicates, but with another Al substitution for Si • Only occur in undersaturated rocks (no free Quartz, Si-poor) because they react with SiO2 to form feldspars

  16. Feldspathoids, Cont. • Nepheline • Important feldspathoid mineral • Indicates undersaturated magma

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