Optical Microscopy of Meteoritic Metal

Home The Micrographs Field of View Techniques Contact Me

TECHNIQUES- Etching

Sample Selection | Sectioning | Epoxy Mounting | Grinding & Polishing | Etching | Microscopy


Metal microstructures are revealed by etching in Nital (2 volume % HNO3, balance ethanol).  The optimal etching time varies from one meteorite to another, and can range from 30 to 300 seconds. The etched specimen is washed with ethanol and then dried using cold, compressed air.

 

 

 

This "time-lapse" video shows metal in Tuxtuac (LL5) during Nital etching (horizontal field of view = 450 microns).  The metal was photographed prior to etching, and then re-photographed at 30 second etch intervals for a cumulative etch time of 270 seconds.  I used "CombineZP" freeware to align and animate the images.  Note that taenite darkens whereas kamacite stays relatively bright.  Taenite FCC annealing twins also become visible as etching proceeds.

 

Warning! Nital etchant is highly flammable and potentially explosive, especially at acid concentrations above 5 vol.%.  Bretherick's Handbook of Reactive Chemical Hazards describes nitric acid-alcohol mixtures as "rocket fuel".  Even at low acid concentrations, I  have witnessed Nital explode to produced the toxic gas NO2 (not good).  Always prepare this etchant in small quantities, and discard your etchant immediately after use because dissolved metal seems to catalyze the decomposition process.