Plagioclase Series Members
The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab), and readily determined by measuring the plagioclase crystal's refractive index in crushed grain mounts, or its extinction angle in thin section under a polarizing microscope. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages.
| Name | % An | % Ab |
|---|---|---|
|
|
90–100 | 10–0 |
|
|
70–90 | 30–10 |
|
|
50–70 | 50–30 |
|
|
30–50 | 70–50 |
|
|
10–30 | 90–70 |
|
|
0–10 | 100–90 |
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