B. PLAGIOCLASE FELDSPAR (BH-250-6)
The plagioclase feldspars form the solid solution series albite, oligoclase, andesine, labradorite, bytwonite, anorthite. They characteristically show lamellar twinning which follows the albite twin law (this twinning is responsible for the striations on plagioclase that you may have seen in hand samples). Under crossed polars this twinning appears as narrow, parallel bands of alternating darker and lighter gray interference colors. On the flat microscope stage the extinction angles of these albite twins can be used to roughly estimate the plagioclase composition. You will use the Michel-Levy technique, a relatively easy, efficient method, to find the composition (see Kerr page 294-5). Use it on one of the thin sections, and try to find at least 2 valid measurements. Be sure to include all your measurements (even the ones you'd rather not). Use relief indications if your measurement falls in the ambiguous 0-20 degree range.


Plagioclase Feldspars:
Plagioclase feldspars are common minerals in some igneous rocks such as basalt and granite. They form the solid solution series, albite, oligoclase, andesine, labradorite, bytownite and anorthite. One of the characteristic optical property that these minerals show lamellar twinning which follows the albite twin law. It is always difficult to find the end members of the series, most of the time what you find is solid solution that contains certain amount Na and Ca . Albite is common to find but anorthite is not a common mineral. The albite twinning is used to estimate the percentage of Na and Ca in the plagioclase, and there are two methods that you will learn in this lab.

Michel-lévy method
Find a plagioclase that shows albite twinning on 010 section. You can recognize these sections by the fact that they show uniformly illumination of all lamellae when parallel to the vibration planes of the polars, and the equality of extinction angles for twin sets rotated to the left and right.

When using Michel-lévy method six to eight measurements will be needed for each feldspar, and a difference between Left and Right angle measurements in the order of 6° is allowable. If the average angle is less than 20° then you have to know the refractive index of the plagioclase with respect to the balsam. If the plagioclase has a lower refractive index use the smaller curve on page 295 in Kerr, and if the balsam has higher refractive index use the large curve to estimate the composition of the plagioclase.

Wright method
The Wright method can be used in a case where a single crystal displays both Carlsbad and albite types of twinning. Here four possible extinction positions can be studied and from one crystal you can estimate the plagioclase composition. Page 293-297 in Kerr provides an explanation of this method, as well as some discussion. Please, read these pages.

The compositions that you are going to find using the above two methods are just approximations, to find the exact composition of the feldspar it useful to use other techniques such as electron microprobe.

 

 

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