Smith College - Geology 222b - Petrology

Petrographic Data File

Formula KAlSi3O8 High temperature (above 700°) sister to orthoclase and microcline.  Solid solution of sodium (albite) possible, but not common. The difference between sanidine, orthoclase, and microcline is its random order of placement of Al within bonding sites. Microcline and Orthoclase have ordered placement of Al.
Crystal System Monoclinic Another way to tell the difference between sanidine, orthoclase, and microcline is that sanidine and orthclase are monoclinic and microline is triclinic.
Crystal Habit prismatic, tabular Zoning is common, expressed in varying birefringence and extinction angles.
Cleavage (001) perfect;  (010) good  
Twinning Carlsbad twinning with  (010) composition plane dividing crystal into two segments. Manebach and Baveno twins also possible.
Color/Pleochroism Colorless in thin section.  
Optic Sign Biaxial (-)  
2V 0-40° Lower temperatures produce an increase of Al and Si in tetrahedral sites, increasing 2V values.  Na substitution can also increases the 2V.
Optic Orientation X^a = +5 to +9 °
Y^c  = +21 to +17°
Z = b
O.A.P. normal to (010)
High temperature sanidine has a differently oriented optic plane: 
X^a = +5°
Y = b
Z^c = +21°
O.A.P. parallel to (010)
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 

1.514 - 1.526
1.518 - 1.530
1.521 - 1.533
0.005 -
Birefringence 0.005 to 0.008 No higher than first order white
Elongation somewhat elongate parallel to the a axis  
Extinction Can be parallel  
Dispersion r > v  
Distinguishing Features Generally  distinguished from orthoclase by smaller 2V.  High sanidine indicated by orientaion of optic plane.
Occurrence Formed in high temperature volcanic and hypabyssal igneous rocks.  Can also be present in high tempereature contact metamorphic rocks.
Editors Amanda Trotter (AC), Kristin Abel (02), Robyn Bluestein (03), Lucy Eckert (05), Roxanne Renendo (09), Alyssa Pascuzzo (15)

Thin section of twinned sanidine crystal in crossed polarized light.  At 10x magnification the sanidine is too large for a complete view of the crystal, but the fine matrix surrounding it allows clear distinction of its boundaries and shape.  Click on image to view the slight zoning visible on the underside of the crystal.  For a different view,  see the U.C.L.A. sample in the link below.
This is a thin section of Sanidine, shown is crossed polarized light. The contrast of light and dark demonstrates the Carlsbad twinning, which is often seen in Sanidine. Click in the picture to see it in plane polarized light.
Thin section rollover showing sanidine crystal in plane polarized light and cross polarized light with Carlsbad twinning.
Interference figure of a Sanidine crystal. Figure is off center and shows the southeast quadrant of the biaxial interference figure. The rollover image shows the interference figure with the gypsum plate inserted showing color change to blue in the SE corner and red(orange/yellow) to the NE and NW corners. This indicates that the Sanidine is biaxial (-).
Photomicrograph of Sanidine (x20). Example of a non-twinng, well formed crystal face (010) with the x-axis parrallel to the plane running northeast to southwest. In this image you can see the negative/low relief that help distinguish from other K-Spars. Rollover image shows the same crystal with the gypsum plate inerted. Interference color changes from 1st order white to 1st order bright blue, indicating lenght slow.
WWW Images U.C.L.A. Petrographic Workshop

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