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Techniques for Collectorsremoving calcite from apatite

19th Jul 2006 15:12 UTCPhil B.

Any tips on removing calcite from (very fragile) apatite? I want to expose the crystals as much as possible.


Thanks, Phil.

19th Jul 2006 16:18 UTCAlfredo Petrov Manager

I use weak citric acid buffered with sodium citrate. (Citric acid by itself will etch apatite.)


I've heard that some collectors expose the long fragile Bancroft area apatites from its coarse calcite by mechanical means after first reinforcing the fragile fractured apatites with glue.

19th Jul 2006 17:04 UTCPhil B.

where can i get sodium citrate (or nitrate?)


Phil.

19th Jul 2006 17:45 UTCPeter Haas

Dilute acetic acid (e.g. white vinegar) is the better choice here, because:


- First, it is a weaker acid - a dilute solution will not attack apatite.

- Second, it acts only as an acid. Citrate also acts as a chelant (i.e. a multidentate complex ligand) with a pronounced selectivity towards Ca2+, Mg2+, Fe2+ and other ions of similar size. In general, complexation - especially when multidentate ligands are involved (entropy gain) - is independent on the pH value. Citric acid/citrate will attack apatite after prolonged exposition.


Please note, that calcite is very slowly dissolved by acetic acid. On the other hand, this helps to control even a delicate etching job.

20th Jul 2006 01:16 UTCPhil B.

sry, im only at the 9th grade science level (soon to be 10th) could u explain that a bit simpler plz? And does that mean acetic will have NO effect on the apatite?


Thanks, Phil.

20th Jul 2006 03:42 UTCPaul L. Boyer

A chelant (in our laboratory we call them chelating agents) are usually organic chemicals with negatively charged groups on them. These negative charges are attracted to the positive charges of a metal ion, such as Fe2+. If the chelator is large enough to wrap around the metal ion, it can remove it from solution. Citric acid has six carbons and three carboxylic acid groups which will be negatively charged, so it would be attracted to metals. I am in HIV research, and we use a chemical called ethylene diamine tetra acetic acid (EDTA) to pull Mg2+ out of solutions. This compound used to be used to treat lead poisoning in people because it wrapped around the lead ion and shielded it from the body. The EDTA:lead complex was then passed through the kidneys. Acetic acid is fairly mild, but it will be up to somebody more familiar with this to tell you if it will etch apatite.

20th Jul 2006 13:02 UTCPhil B.

thanks for clarifying


Phil

29th Jul 2006 08:26 UTCZbynek Burival Expert

Try acetic acid about 10%, its really weak acid. Dont like citric - complicated about "side effects" like making complexes.

My friend was cleaning Slyudyankas apatites with dilluted hydrochloric acid and they stay undamaged if they are not washed too long. If they are in contact with acid too long they loose color to very slightly blue or white. But HCl is not the way to follow;)
 
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