Microscopically, it is easy to tell the difference between biotite and chlorite, but in field work we don't have the luxury of microscopy.
Dear Paul,
You know very well that the terms 'biotite' and 'chlorite' include two very complex mineral groups that can be colored both very similar, and on the other hand, may occur in close parallel intergrowth.
In my experience, biotite and chlorite are distinguishable in the field work only in very specific cases:
1) The crystals (leaves) may not be strongly deformed (crumpled).
2) The crystals (leaves) may not be subparallel intergrown with each other.
3) The crystals (leaves) may not be strongly altered (discolored by oxidation, etc.).
4) The crystals (leaves) must be large enough so that you can stick with your fingertips or with a pair of tweezers.
If these conditions are met, then there is a trick in the field work to distinguish biotite- and chlorite leaves reliably from each other:
Biotite (and also muscovite) leaves jump into the starting position when trying to bend it or folding (elastic deformation).
Chlorite leaves remain in the bent position (plastic deformation).
Try it, you'll see: it works!
Best regards,
Guenter
Dear Guenter,
I vote your answer thumbs up. Thanks so much with sharing this very useful field technique.
Best wishes,
Paul
Color is a clue but is not definitive. Chlorite is generally green, but can be other colors. Biotite is generally dark brown to black but can also be greenish gray to greenish black if it has a high ferrous iron content.
Chlorite is flexible but not elastic. Cleavage flakes will bend and remain bent.
Biotite is flexible and elastic. Cleavage flakes will bend and snap back.
Mineral assemblages can also provide a clue. Biotite tends to be associated with minerals that are stable at relatively high temperatures, whereas chlorite tends to be associated with lower-temperature mineral assemblages.
Paragenesis and mineral zoning in a hydrothermal system can be a clue. Biotite tends to be early and central, chlorite tends to be late and peripheral. But late chlorite can be superimposed on early biotite as a hydrothermal system collapses in on itself.
Chlorite is commonly a retrograde metamorphic mineral after prograde biotite.
Finally, if the chlorite and biotite are too fine grained for the elasticity test, and everything else is ambiguous, you might need to make a grain mount or get a thin section and use a polarizing microscope to test for birefringence. Biotite has high birefringence (bright multi-colored tints) and birdseye-maple extinction in thin section. Chlorite has low birefringence (dull monochrome tints) and normal extinction.
Dear Guenter,
thank you very much for this field trick :))
kind regards
Andreas
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