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Solubility of minerals

Posted by Reiner Mielke  
Steve Hardinger December 28, 2011 06:27PM
I didn't say that melting of ice is a dissolution phenomenon. I said that the water (produced by the melting ice) dissolves.
Alfredo Petrov December 28, 2011 06:50PM
I interpret Reiner's original question to relate to solubilities in various acids at different concentrations too, not just water, so... lots of measurements needed. A very time-consuming task if done with any significant number of species. And although the results might be occasionally of marginal usefulness in mineral identification, they could be very useful to people doing mineral cleaning and preparation.
Reiner Mielke December 28, 2011 07:52PM
The solubility product is difficult to determine and not very practical. Like Alfredeo said I am also talking about solubility in the common acid and alkali solutions. I am thinking of a particular size of particle say 0.5mm and how long it takes to dissolve in a specific solution at say 20C. What puzzles me is why mineralogists even bother to provide such sloppy solubility data, if you are going to do it then do it right!
For example: Bobdownsite, it is reported by; Tait, K.T., Barkley, M.C., Thompson, R.M., Origlieri, M.J., Evans, S.H., Prewitt, C.T., Yang, H. (2011) Canadian Mineralogist, 49, 1065-1078. that "Bobdownsite is insoluble in water, acetone, or hydrochloric acid." Well I placed a 0.5mm piece of Bobdownsite ( unconfirmed) in concentrated HCl at 20C and it completely dissolved in 20 minutes! Now if someone with a confirmed piece could repeat my experiment and let us know the results that would be just wonderful ( or send me a piece and I will test it). If everyone would spend a little time doing this we could come up with a useful database of solubility in Mindat. However Mindat would have to make it easier to add such information to their database and it would have to be standardized.
Michael Hatskel December 28, 2011 08:25PM
If we bring in the solubility in acids/alkalies, there is even more uncertainty about the term "solubility", because in many cases it actually means "decomposition" rather than "dissolution".
It is my understanding that when the literature simply says "soluble/insoluble in acids" or "in strong acids", it means just the acidic action, e.g. calcite in HCl. But does every reader know that it should not be extended to ANY acid, e.g. does not include HF (for silicates), HNO3 (for sulfides/sulfosalts), etc.?

That's why the all-inclusive term "affected/affected to a limited extent/not affected" may seem to be less accurate but also less confusing to describe applicable cleaning methods.

The nature of the effect needs to be known, of course: dissolution, chemical corrosion, unwanted coating formation, etc.
Simplified example: Halite NaCl is not considered acid-soluble but will dissolve in the water content of a dilute HCl acid. Therefore, halite is affected by dilute acids, so using them to clean halite is not recommended.

There are tons of valuable practical info in the Mindat forum threads on specimen cleaning and identification. I hope that there is a standard procedure in place for transferring that info to the mineral species pages or at least putting a link there.
Maybe in addition to referencing the threads on the species pages, all that info could be summarized in some sort of lists (tables), at least at a qualitative level, somewhat as an update to what Sinkankas did in his book?

For example, it might be helpful to have a Mindat list of minerals whose solubility in water is decreasing with temperature. Shouldn't be too many mineral species, but may be a good start.
Reiner Mielke December 28, 2011 08:42PM
My understanding of solubility as used by mineralogists is any change in the original mineral or the solution it is in is an indication of dissolution, as such, one should note what the changes are. For example, a change in the color of the mineral, which would be extremely important for deciding how to clean a mineral.
Albert Mura December 28, 2011 09:31PM
Reiner, I'm not sure where that idea came from "solubility as used by mineralogists is any change in the original mineral or the solution it is in is an indication of dissolution", however, that is not the meaning of solubility. Solubility is measured, as other have said above. mass of a solute dissolved in a certain vol. of solvent or in certain cases a solubility product. In any case, a lot of pure chemicals (some of which are minerals) have their solubilities reported in the chemistry literature. As to water solubility I think you would find that most minerals, even as a powder, would not be very soluble and therefore not easily measured. Acids and bases are a different matter. An important issue in solubility are the conditions under which the rest is run (temp. etc.) and if they are not kept constant from one person to the next, you have a bunch of useless data that you can not be compare.
Alfredo Petrov December 28, 2011 09:53PM
"An important issue in solubility are the conditions under which the rest is run (temp. etc.) and if they are not kept constant from one person to the next, you have a bunch of useless data that you can not be compare." ...exactly, Albert. This is why solubility as currently reported in mineralogical literature by mineralogists is useless.

And of course, Michael, partial solution, decomposition, gelification, etc, will require additional description and explanation. And for purposes of ID, decomposition can provide more useful information than simple complete dissolution, especially if a coloured precipitate is left behind.
Albert Mura December 28, 2011 09:56PM
Adding to the confusion is the fact that most minerals are not pure substances and therefore one sample will not necessarily have the same solubility as another. I think that the difficulties in measuring solubilities of minerals speaks more to the lack of data in that field than other less scientific reasons.
Michael Hatskel December 28, 2011 10:02PM
Change in color is not necessarily caused by dissolution.
Leaching of a color-causing component from the specimen surface may qualify as a partial dissolution.
An opposite example: when IronOut solution is too concentrated and hot, the iron stain coating may change its color from yellow to green because the ferric ions (Fe3+) get reduced to ferrous ions (Fe2+) while remaining in place as opposed to getting dissolved into the IronOut solution. Same color change to green may happen to the Fe-bearing silicates in the specimen. Here you have it: color change caused by a chemical redox reaction, not dissolution.
As I suggested before -- affected, not soluble.
Reiner Mielke December 28, 2011 11:44PM
Where I get my notions on solubility is from Dana's textbook of Mineralogy. In the section on solubility there is reference to changes in the solution. "partial solution is often shown by the color given to the liquid etc. ". This could only come about if something is removed from the mineral by dissolution. I cannot find any requirement in any mineralogy book that restricts the meaning of solubility to complete dissolution. Besides such a narrow definition would not be very useful. I suppose for what I have in mind a be better described would be the behavior of a mineral in a solution. This would provide far more useful information than strictly solubility.
Donald Peck December 29, 2011 04:31PM
Michael, I agree. I don't think there is a problem with the readily soluble minerals. I think (but am not sure) that Reiner is more concerned about the solubility of the slowly soluble or slightly soluble minerals. Thanks for the comment.

OOPS! I wrote this before reading page 2.

Edited 1 time(s). Last edit at 12/29/2011 04:40PM by Donald Peck.
David Martz June 18, 2012 05:56PM
Being from the western U.S. the subject of solubility is rather significant to me for a couple of reasons. Most importantly, if you consider mineral identification to include keys from the environment in which a specimen was found, solubility is sometimes critical. Turquoise is a good example where the worldwide distribution reflects dry climates, due to its solubility and the way water is involved in its formation. Hint: Don't look for it in a jungle, and if you find something resembling turquoise in a wet climate, focus on other blue copper minerals first, in particular the less soluble ones. Second, in terms of pseudomorphs. Near the surface, less soluble minerals replace more soluble minerals as a rule, and this is occasionally an important clue when trying to determine why a mineral has taken a crystal form it is not supposed to exhibit. It is also useful for rejecting certain replacements, and therefore mineral possibilities. I have seen a few pseudomorphs described where the replacement was not feasible, hence the original mineral was not what the collector thought. Finally, solubility is vital in the ordering of some minerals in a deposit. All that being said, the laboratory use of solubility for identification has limits, but also unexplored potential. As an example, if you had tiny crystals of something you thought was a carbonate, you could dissolve equal amounts of it it water and in carbonated water. If it is a carbonate, it should dissolve slowly or not at all in the carbonated water. Similar tests could be done using saturated epsom salt water (for sulfates), but some standard runs with known minerals would be needed for comparisons.
Alfredo Petrov June 18, 2012 06:14PM
David, I fear that solubility is a much more complex topic than that... Turquoise, for example: The 2 best specimens I have are from Virginia, which is horribly humid in summer, and Belgium, where it rains every day, so one cannot say that this is just a dry climate mineral. And the most common carbonate, calcite, will dissolve much better in carbonated water than in pure water.
Uwe Kolitsch June 18, 2012 07:12PM
"The 2 best specimens I have are from Virginia, which is horribly humid in summer, and Belgium, where it rains every day, so one cannot say that this is just a dry climate mineral."

The climate may have been completely different while the mineral has formed (hundreds, thousands, millions of years ago).
Alfredo Petrov June 18, 2012 07:29PM
Indeed, Uwe, that's right, but it doesn't dissolve away and disappear when wet now, so not identifiable by solubility, which is apparently what David would like to use as a classification tool. Gypsum would perhaps be a better example of a slightly soluble mineral that forms equally well in dry and wet climates. (The first gypsum crystals I ever found, as a kid, were in a clay pit in England.)
Reiner Mielke June 18, 2012 08:06PM
From the perspective of a mineral, climate is irrelevant, it is the conditions of the fluid that the mineral is in contact with that is important. Clay has a very low permeability and therefore allows little movement of fluids, this preserves the gypsum crystals regardless of the climate. Gypsum that grows in a salt flat is in water saturated with gypsum so in fact they are wet even though it may be a dry climate. Climate is only important in that it may facilitate the right conditions for fluids from which the minerals can form, however these same conditions can occur in different climates.
Bart Cannon June 18, 2012 11:31PM
I would love to know how to dissolve barite.

In forty years of fussing, I've never even been able to even frost a crystal face with anything.

I have some silvers enclosed in white barite.

Alfredo Petrov June 19, 2012 12:03AM
Try molten sodium carbonate. I don't think it will hurt the silver, but I'm not sure about that. :-)
Don Saathoff June 19, 2012 12:39AM
Bart, be careful though......Sodium Carbonate melts at ~851C and silver at ~961C.......if you have a muffle furnace in which you can control the temp you should be OK.

Tom Tucker June 19, 2012 12:51AM
Bart, so much for dissolving barite, oops, baryte - how do we move enough solute saturated with barium and sulphate through something as impervious as the Pierre shale and its equivalents to precipitate those magnificent crystals at Elk Creek, and elsewhere?
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