Posted 08 September 2013 - 09:45 AM
Gibeons are certainly a very stable iron, they can generally do without a corrosion inhibiting treatment. Muonionalustas have been subjected to a very different environment, and as a result are more weathered, and have absorbed a greater amount of terrestrial contaminants. While they are both fine octahedrites, there is a significant difference in hardness. Gibeons cut very well, while Muons are much tougher and frequently dull a blade after a single cut.
There are two main components that contribute to the appearance of the etched surface. The type of etchant and its strength (concentration), and the final grade of the abrasive used to finish the surface, produce great variation in the contrast. I prefer nitric acid in methanol for etching, and though some recommend ferric chloride, I decline to introduce any additional source of chlorine into a specimen. The final abrasive grade can vary from around 320 grit to a mirror polished surface. The degree of the final grinding or polishing varies from type to type of iron, I find that 320 - 400 grit is appropriate for Campos, 600 for fine octahedrites like Gibeons or Muons, and a high polish to a mirror-like surface works best for Canyon Diablos.
There are differences between specimens of the same variety as well. Heat and/or shock can seriously affect the underlying crystal structure, which will be very apparent after etching. I once cut up a fist sized Gibeon that showed a typical etch most of the way through, and then came to an area of lesser contrast and diminishing bands, and finally a golf ball sized portion that lacked any Widmanstatten pattern at all. Kind of like a self contained ataxite in an otherwise typical specimen.
You generally won't feel the lines of an etch, the surface is affected only a few molecules deep. That's why the type and angle of the lighting can have such a profound effect on the appearance. Some specimens can benefit from a more aggressive etch process though, and enough kamacite is removed to leave the brighter acid resistant taenite or other bright bands unaffected. This is common in Canyon Diablos which require a longer period in the acid solution