(Lab11)_Twiining,_Zoning,_Exsolution_F10

(Lab11)_Twiining,_Zoning,_Exsolution_F10 - Optical...

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1 Optical Mineralogy Lab 11 – Fall, 2010 Feldspars: Exsolution, Zoning, and Twinning
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2 Feldspars Feldspar minerals are the most common minerals in the earth's crust They are aluminosilicates of potassium, calcium, and sodium The feldspars are divided into two groups, the alkali feldspars and the plagioclase feldspars
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3 Plagioclase Feldspar The plagioclase series is a solid solution series between sodium and calcium Albite NaAlSi 3 O 8 Anorthite CaAlSi 3 O 8
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4 Alkali Feldspar The alkali feldspars are mainly potassium feldspar and albite K-spar KAlSi 3 O 8 There are three distinct minerals Microcline (Low temperature) Orthoclase Sanidine (High temperature)
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5 Ex-Solution Albite and K-spar exhibit very limited solid solution at room temperature If Na and K are both present in magma, the initial mineral will be a mixture of sodium and potassium but the resulting mineral will undergo exsolution as it cools
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6 Perthites Two phases will be formed Such mixtures are called perthites (albite or other sodic plagioclase in orthoclase) or antiperthites (microcline in plagioclase)
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7 Perthite Photo Perthite, as a result of exsolution Microperthitic demixing of high temperature mixed crystals with chemical composition (K, Na)AlSi 3 O 8 into Albite, NaAlSi 3 O 8 (light) and Orthoclase, KAlSi 3 O 8 (dark)
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(Lab11)_Twiining,_Zoning,_Exsolution_F10 - Optical...

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