I assume that apatite mineral in the lunar material is reacting with HCl at 75 degrees when I leach samples to remove any terrestrial contamination. The reason for this assumption is that the oxygen isotope compositions of unleached and leached bulk materials are different, i.e., leached are heavier than the unleached and it could happen if apatite reacts with HCl during leaching process causing -PO4 to go out into the water and ultimately out of the material to be analyzed for oxygen isotopes. Yet another assumption that it could happen only if apatite has heavy oxygen isotopes than the bulk material. I'd appreciate your feedback and suggestion if I could use any other chemical to remove terrestrial contamination (mostly carbonates and oxides) without disturbing the apatite in lunar samples. Thanks!
Thanks Kenneth for your input! The lunar or other planetary materials contain Ca5(PO4)3(F,Cl,OH) type of apatite. So the possible chemical reaction might be:
Ca5(PO4)3(F,Cl,OH) + HCl = CaCl2 + H3PO4 + by products
The critical point here is the formation of phosphoric acid which is water soluble and will take away the oxygen from the apatite causing oxygen isotope composition to change for acid-leached lunar samples.
I would also think that apatite dissolves in warm HCl. For leaching out calcite contamination, you could try acetic acid - that is supposed not to attack apatite, as far as I know. But I doubt it would do much with your other contaminating 'oxides' though….
Quite simply yes.
Leaching apatite is a standard way to separate the REE fraction of the phospahet from the silicate.
If you want to be sure that apatite is leached, simply analyse the leachate by ICPAES.
The stoichiometry of the leachate (particularly its molal Ca/P ratio) will tell you if apatite is the main contributor.
Yours
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