Optical Mineralogy Lab 11 Fall, 2018 Feldspars: Exsolution, Zoning, and Twinning 1 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 2 Plagioclase Feldspar The plagioclase series is a solid solution series between sodium and calcium Albite NaAlSi3O8 Anorthite CaAlSi3O8
3 Alkali Feldspar The alkali feldspars are mainly potassium feldspar and albite K-spar KAlSi3O8 There are three distinct minerals Microcline (Low temperature) Orthoclase Sanidine (High temperature)
4 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 exsolve as it cools 5
Perthites Two phases will be formed Such mixtures are called perthites (albite or other sodic plagioclase in orthoclase) or antiperthites (microcline in
plagioclase) 6 Perthite Photo Perthite, as a result of exsolution Microperthitic demixing of high temperature mixed crystals with chemical composition (K, Na)AlSi3O8 into Albite,
NaAlSi3O8 (light) and Orthoclase, KAlSi3O8 (dark) 7 Zoning Some plagioclase feldspars will have one composition in the interior of the crystal, and a gradually or sharply changing composition toward the outer
edge of the crystal This is called zoning There are several types of zoning 8 Normal or Continuous Zoning Normal zoning connotes the gradual transition during the growth of a crystal (from core to rim) to a relatively lowtemperature composition in a crystalline solution series It is the anticipated result of fractional
crystallization where equilibrium has failed to keep up with falling liquidus composition 9 Discontinuous Zoning Discontinuous zoning (example: calcic plagioclase with a narrow rim of sodic plagioclase) might occur when convection currents within magma carry a calcic crystal into a much more sodium-rich environment, from a sudden change in
physical conditions, or from erosion followed by deposition 10 Reverse Zoning Reverse zoning connotes the transition, generally abrupt, to a higher temperature outer zone in a crystal Some hiatal event such as an addition of fresh magma to a magma chamber undergoing fractional crystallization or sudden loss of
volatiles from a sub-volcanic magma chamber is responsible for reverse zoning 11 Oscillatory Zoning Plagioclase (An35-50) occurs Oscillatory zoning is a large number of thin shells of different
compositions as rectangular grains with euhedral oscillatory-zoned cores and irregular, serrate borders - Thin Section SN3, May Lake Tonalite 12 Twinning Feldspars are either monoclinic (sanidine,
orthoclase) or triclinic Twinning, a very common phenomenon in the feldspars, varies according to the composition and the crystal system There are at least seven different twin laws for the feldspars Of these, only three are common 13 Carlsbad Twinning Seen in either monoclinic or triclinic feldspars
Carlsbad twins are growth twins (that is, they form while the crystal is growing) Carlsbad twinning is a type of penetration twinning Carlsbad twinning is common in igneous rocks, but very rare in metamorphic rocks They are seen as a pair of individual crystals, separated by a single line, in thin section 14 Carlsbad Twin Photo
Carlsbad twins are seen as a pair of individual crystals, separated by a single line, in thin section 15 Albite Twinning Seen only in triclinic feldspars Albite twins may be growth, deformation, or transformation (formed when the crystal is transformed from mono- to
triclinic) twins Albite twinning is polysynthetic contact twinning 16 Albite Twin Photo Plagioclase, unzoned, in a hornblende diorite
Note the strong, parallel sets of albite twins, and the less visible set of pericline twins inclined almost at right angles to the albite twins Crossed nicols, 40x It is seen as numerous individual twins, parallel to each other, in thin section
The twins may be wedge-shaped (thicker at one end) if the twinning results from deformation 17 Pericline Twining Seen only in triclinic feldspars Pericline twinning is quite similar to albite twinning
It may result from growth, deformation, or transformation It is a polysynthetic contact type of twinning and shows a similar appearance to albite twinning, although with a different crystallographic orientation 18 Twin Combinations It is also possible for crystals to exhibit compound twinning in which two twin laws
are expressed simultaneously 19 Carlsbad-Albite The two halves of the Carlsbad twin may show albite twinning The albite twins are oriented parallel to the Carlsbad twins
20 Albite-Pericline During transformation, both albite and pericline twins may form The combination results in a "cross hatch" effect 21
Albite-Pericline Photo Cross hatch twinning is particularly common in microcline because microcline often forms by transformation from orthoclase Cross hatch twinning is also called "gridiron" or tartan twinning 22
Observation of Twinning Twinning can be observed only under crossed-nicols (CN) Usually low or medium power is used The iris should be open 23
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