FMF - Friends of Minerals Forum, discussion and message board

[Byron Anderson]

Mineral Friends

Help! I think I wrecked a specimen. I used HCl to clean up a drill core specimen that encountered a feldspar and pyrite pocket. I apparently left the specimen in too long and now have yellow feldspar (I knew I was playing with fire). I've soaked the specimen for several days in water and it seems to hold it's white color for a day or two, but then changes to that nasty yellow again. Am I screwed? Any solutions out there?

[GneissWare]

1. A photo would help.

2. Maybe screwed. Acid and pyrite is not a great idea. You could try soaking it in an 10% ammonia solution (outside) for a few days. You are getting a reaction product bleeding out of the pyrite.

[Pete Richards]

Two possible solutions are Iron Out (a commercial product) and the Waller solution (google it, but probably not economically feasible for this project unless you are a professional chemist). They are good at removing iron, and will not damage most (but not all) other materials. Another possibility is oxalic acid, which can be obtained as a wood bleach in some hardware stores.

Treat all of these materials as dangerous - use outside if at all possible and avoid fumes; be aware of possible damage to other stuff in the area where you do the treatments. None of these are highly dangerous, I would say, but all are to some extent. In all cases, soak for a long time with frequent changes of water after treatment to remove the remnants of the treating chemicals.

We usually say "Try this out on a specimen you can afford to lose", but since you've already screwed this one up, you can probably go right to work on it!

[Byron Anderson]

Thank you for the suggestions! I have both iron out and oxalic acid. I think I'll try the 10% ammonia solution first though, since that is one I haven't used and this is an excuse to add it to the bag of tricks for cleaning field collected specimens. If it doesnt work I'll move on to the other two options. Thanks again gentlemen.

[John Betts]

I have seen this many times and believe the yellow is caused by "hard" water used to dilute the acid and during post-acid soaking. No solution is known. Iron Out and Waller Solution are ineffective.

In the future use distilled water for the first few rinses and for diluting your acids.

[GneissWare]

I have only seen a yellow discoloration from hard water when using oxalic acid. It forms calcium oxalate which has a very distinctive yellow color. I have used lots of HCl and when you get a discoloration, it is usually from not soaking in water long enough--and it leaves a weird greenish-yellow color.

A photo would really help focus the discussion.