I wish to enhance the visibility of the successive growth phases of opaque crystals (mostly pyrite ; 5-200µm) in optical microscopy thin sections. I understand I have to use nitric acid etching. Any suggestions as to how?
It's not that hard. Put HNO3 (4-7N or so) in a beaker and then use tweezers to hold the thin section in the acid for roughly 20s. It reacts faster with different minerals and this is great for pyrite, but may cause the chalcopyrite or other phases to be tarnished (a simple solution for the latter is to re-polish by hand on a cloth with alumina or diamond paste). After you etch, put the sample in a vat of water for about five seconds to remove any residual acid and dry it with a kimwipe before using on the scope. You should probably do all the above in under a fume hood for safety. Hope this helps.
Etching pyrite is quite tricky. You can make a thin section for sacrifice, and try various nitric acid concentrations/time/temperature combinations.
More easily is to look at thin section in infra red light microscope. At this wave length some pyrite crystals are “transparent” and you can view fluid inclusions and other internal features. Here is some references:
http://dx.doi.org/10.2113/gsecongeo.97.3.603
http://www.dynamicearth.de/Projects/geoprocesses/mitarbeiter/marti/paper/lue_zie.pdf
An more robust method is to do element mapping using an electron microprobe and search for the elements (first using the EDS spectra) that reveal the zoning of the pyrite and then do WDS on that elements. You can also use a backscattered electron (BSE) image on thin section, if the composition variation in growth layers provides sufficient contrast on BSE image. I hope this will help your study. Good luck.
I would try a HNO3 solution (1:1 volume concentration), dropping it on the mounts in cycles of 20 minutes (times may vary) and repeat till textures are revealed. As already suggested, make a section to be sacrificed and try till you find the best times, remembering that you will probably lose other phases.
Just a thought from some time ago working to reveal micro crystals embedded in glass... rather than put the specimens in the acid I suspended them directly above concentrated acid (sorry, I forget how "concentrated") and allowed the vapor to do the etching. This worked well for me and gave me more control over the rate of etching.
Hum. Interesting technique. I will try that also.
Thank you very much!
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