What Are The Key Differences In The Crystal Structure And Optical Properties Of Staurolite And Chloritoid That Would Allow Me To Distinguish Between The Two Minerals In A Thin Section Of A Metamorphic Rock Sample, Particularly When They Exhibit Similar Pleochroic Colors?

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To distinguish staurolite and chloritoid in a thin section, especially when their pleochroic colors are similar, consider the following key differences:

1. Crystal Structure and Cleavage:

  • Staurolite: Monoclinic crystal system with two cleavage planes intersecting at 90 degrees, often forming characteristic staurolite twins (cross-shaped).
  • Chloritoid: Monoclinic crystal system with one prominent cleavage plane, leading to a more tabular or flaky appearance.

2. Optical Properties:

  • Relief: Staurolite has higher relief due to higher refractive indices, making it stand out more under the microscope. Chloritoid has lower relief.
  • Birefringence: Staurolite shows moderate to high birefringence with vivid interference colors under crossed polars. Chloritoid has lower birefringence, resulting in less intense colors.
  • Pleochroism: While both can exhibit pleochroism, staurolite often shows yellow to brown colors, whereas chloritoid tends towards blue or green, though overlaps can occur.

3. Habit and Inclusions:

  • Staurolite: Typically forms prismatic crystals and often includes minerals like quartz or biotite. Characteristic staurolite twins are a diagnostic feature.
  • Chloritoid: Forms tabular or lamellar crystals and may not have the same inclusion patterns.

4. Geological Context:

  • Staurolite: Common in higher-grade metamorphic rocks, such as pelitic schists in the staurolite zone.
  • Chloritoid: More typical of lower-grade metamorphic rocks or specific facies like the chloritoid zone.

5. Extinction Angle:

  • Staurolite and chloritoid may have different extinction angles relative to their cleavage, which can be measured for further distinction.

Conclusion:

By combining observations of cleavage, relief, birefringence, pleochroism, habit, and geological context, you can effectively distinguish staurolite from chloritoid in thin sections, even when their pleochroic colors are similar.