Smith College - Geology 222b - Petrology

Petrographic Data File

Formula NaAlSi3O8-CaAl2Si2O8 Solid solution series between Na-rich end-member (albite) and Ca-rich end-member (anorthite).

Click here for solid solution phase diagram

Crystal System Triclinic Triclinic shape contributes to interesting optical angles.
Crystal Habit Euhedral or anhedral grains  
Cleavage (001) is a perfect cleavage, and (010) is also quite good. (110) is a very poor cleavage and is not usually seen in thin section. Cleavages intersect at about 93-94o angles. The cleavage may not be obvious in thin section; the best examples are often seen along the edge of the slide. Conchoidal to uneven fracture. Brittle.

Colorless in thin section. No pleochroism.

White and grey are common in hand sample; albite tends to be lighter and anorthite darker. Iron inclusions can produce reds.
Under cross-polars, first-order colors, mostly gray. Yellow interference colors generally imply a thick section.
Optic Sign Biaxial (+) or (-) The sign varies depending on composition.
2Vx 45-102o (high) 
75-102o (low) 
2V depends on composition. It is not a reliable indicator of composition, but can help confirm a composition determined by other techniques. Twinning and exsolution lamellae may make it difficult to get a 2V measurement.
Optic Orientation Varies

Due to the solid solution series, changes in the optical orientation can be a good indicator of composition.

Click here for Michel-Levy method
Click here for Carlsbad-albite method

Refractive Indices
alpha = 
beta = 
gamma = 

Variation is due to composition of the mineral. Greater amounts of anorthite correlate to higher refractive indices.
Max Birefringence 0.007-0.013  Maximum interference colors in thin section are usually first-order grey or white. First-order yellow colors are seen with anorthite.
Elongation Elongate parallel to the c or a axis  
Extinction Inclined As this is a triclinic mineral, the indicatrix axes will not line up with the crystal axes on any regular basis, and as a result extinction is almost invariably inclined.
Distinguishing Features

Hardness (6-6.5) and cleavage are good indicators of a plagioclase as opposed to other similar silicates. In thin section or grain mount, the general lack of pleochroism and the twinning are good indicators. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from K-feldspar. Can be confused with quartz if there is no twinning, however the optic axis figure will resolve this. Often a grey color in comparison to other feldspars. Chemical zoning is common - the calcic core has a larger extinction angle than the sodic rim.

Occurrence One of the most common minerals on Earth. Occurs in nearly all igneous rocks - very common in granites, rhyolites, pegmatites, and similar rocks. Albite tends to be more common than anorthite, with the exception of metamorphosed carbonates and amphibolite. Also very common in metamorphic rocks, with albite being common in low-grade metamorphic rocks. Also seen in sedimentary detritus.
References Nesse, W. D. (2012) "Plagioclase" in Introduction to Mineralogy. Oxford University Press, New York. p. 245-250.
Editors Raquel Blonshine ('11), Michelle Cortrite ('13), Wanda Feng ('15), Helena Tatgenhorst ('16)

This rollover shows the extinction in plagioclase. The twinning makes for different extinction angles depending on the angle of the crystal, but favors inclined extinction.
This rollover image shows the contrasting visibility of Plagioclase when viewed under plain polarized light versus crossed polars (second image).
Plagioclase.Extinction.Thumbnail This rollover demonstrates the inclined extinction of Plagioclase as the microscope stage is rotated approximately 45ยบ. The characteristic twinning of Plagioclase is also visible.
Plagioclase.Extinction.Thumbnail Photomicrograph of plagioclase in anorthosite (BV106) in plane light. Select image and then move cursor over image to view in crossed polarized light, where olivine and pyroxene inclusions may be seen. Magnification 1.5x; field of view 7.45 mm x 5.65mm.
Plagioclase.Extinction.Thumbnail Photomicrograph of plagioclase in anorthosite (BV106) in crossed polarized light. Select image and move cursor over image to view twinning at extinction, then rotated 45o counterclockwise. Magnification 5x; field of view 2.25mm x 1.7 mm.
plag PPL This rollover demonstrates the ability to see twinning when the plagioclase mineral is under crossed polars. The first image is in plane polarized light and no twinning is seen. The rollover is the same image under crossed polars and twinning is seen. The slide is from rock sample K-84.
This rollover demonstrates extinction within the plagioclase mineral. The twins have opposite extinction. In the first image, some twins are white and the opposite ones are dark because they are extinct. The second image is the same slide rotated 45 degrees clockwise and the dark twins from the first image are now white while the white twins from the first image are now dark. The slide is from rock sample K-84.
WWW Images

UCL Catalogue of Minerals in Thin Section

UCLA Petrographic Workshop

UNC Atlas of Igneous and Metamorphic Rocks, Minerals, and Textures

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