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

Melt-crystal equilibrium 2 - miscibility l 2 component mixing and separation  chicken soup analogy, cools and separates l Fat and liquid can crystallize separately if cooled slowly l Miscibility Gap – no single mineral is stable in a composition range for x temperature Miscibility Gap microcline orthoclase sanidine anorthoclase monalbite high albite low albite intermediate albite Orthoclase KAlSi 3 O 8 Albite NaAlSi 3 O 8 % NaAlSi 3 O 8 Temperature (ºC)

l Describe variations of 3 components in crystal group(s) l X=A 50 B 30 C 20 l Y=A 10 B 10 C 80 l Z=A 30 B 30 C 40 Ternary Diagrams X Y Z A 20 B 50 C 30

Combining phase and composition diagrams for mineral groups Mica ternary Biotite series Annite KFe 3 (AlSi 3 O 10 )(OH) 2 Phlogopite KMg 3 (AlSi 3 O 10 )(OH) 2 Muscovite KAl 2 (AlSi 3 O 10 )(OH) 2 No micas Miscibility Gap

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

Tectosilicates Feldspars Albite: NaAlSi 3 O 8 Substitute two Al 3+ for Si 4+ allows Ca 2+ to be added Albite-Anorthite Substitute Al 3+ for Si 4+ allows Na + or K + to be added Albite-Orthoclase

Feldspar Group – Albite-Anorthite series l Complete solid solution  Plagioclase Feldspars l 6 minerals F Albite (Na) F Oligoclase F Andesine F Labradorite F Bytownite F Anorthite (Ca) l Albite-Anorthite double duty F End-members (Pure Na or Ca) F Minerals % Na or Ca l Notation: F An x Ab y  An 20 Ab 80 =Oligoclase

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

l Michel-Levy chart for K- feldspar optical ID

Staining technique l 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

Tectosilicates Feldspars Albite: NaAlSi 3 O 8 Substitute two Al 3+ for Si 4+ allows Ca 2+ to be added Albite-Anorthite Substitute Al 3+ for Si 4+ allows Na + or K + to be added Albite-Orthoclase

Feldspar Group – Albite-Orthoclase series l High – T minerals F Sanidine F Anorthoclase F Monalbite F High Albite l Low Temperature exsolution at solvus F Chicken soup separation l Forms 2 minerals, in igneous rocks these are typically intergrowths, or exsolution lamellae – perthitic texture Miscibility Gap microcline orthoclase sanidine anorthoclase monalbite high albite low albite intermediate albite Orthoclase KAlSi 3 O 8 Albite NaAlSi 3 O 8 % NaAlSi 3 O 8 Temperature (ºC) l Several minerals – Alkali Feldspars

Alkali Feldspar Exsolution l Melt cools past solvus (line defining miscibility gap) l Anorthoclase, that had formed (through liquidus/solidus) separates (if cooling is slow enough) to form orthoclase and low albite l In hand sample – schiller effect  play of colors caused by lamellae Miscibility Gap microcline orthoclase sanidine anorthoclase monalbite high albite low albite intermediate albite Orthoclase KAlSi 3 O 8 Albite NaAlSi 3 O 8 % NaAlSi 3 O 8 Temperature (ºC) Liquid

Alkali Feldspar lamellae

Feldspathoid Group l Very similar to feldspars and zeolites l Include Nepheline, Analcime, and Leucite l Also framework silicates, but with another Al substitution for Si l Only occur in undersaturated rocks (no free Quartz, Si- poor) because they react with SiO 2 to form feldspars

Feldspathoids, Cont. l Nepheline l Important feldspathoid mineral l Indicates undersaturated magma