Solubility of minerals

Before anyone gets on my case, I am well aware that there is a need to quantify what solubility means with respect to its use in identifying a mineral. I am also well aware that solubility in most cases cannot be used to positively identify a mineral, however for those of us who do not have the resources to resort to sophisticated and expensive methods, it can be a useful tool in narrowing down the possibilities. Trouble is that most of the data on the solubility is not quantified and if it is, not in a practical way. However I am hoping that some agreement can be reached on at least one term, insoluble.
Anyone have any ideas on what that term means in a practical sense. Say for example, how long a sample can be in a solution before it shows any changes in order to be called insoluble? Or? Someone must have some idea since that term is and has been so commonly used in mineralogy.

Hi Reiner,

do a mineral in a fluid, e. g. water: If a day later Your spezimen is still there and You can not see any damage/solution traces/etched away edges or something like that, it is "not soluble" in the test fluid. That´s my practical definition.

Greetings from Goslar

Georg

Hello georg,

I would agree with that but what about 12 hrs later? When mineralogists do their solubility tests how long do they wait? Is there a standard period of time for that?

I'm sure this won't help, but a chemist defines solubility in terms of amount of solute per volume of solvent, at a given temperature and pressure. We (the chemists) define solubility as an equilibrium phenomenon, and not a kinetic (i.e., rate at which it happens) phenomenon.

Hello Steve,

That would be the scientific way of doing it but it would not be very practical.

Hello,

Interesting thread...

A question for the experts - how soluble is lanthanite-(La)? I have nice micro crystals of this mineral, and they often seem to have a dusting of clay on them. I'm tempted to soak & rinse them, but have read (somewhere) that this mineral is fairly soluble. I've already discovered that ultrasonic cleaning is NOT recommended - the crystals mysteriously disappeared, even after only a few seconds of treatment.

Any thoughts?

Thanks in advance,

Hans

Hello Johannes,

This summer I found what I think is Lanthanite-Ce with a high La content (EDS analyzed) and I washed it with water and did not observe any change. Dana says it is not soluble in water so I would say go ahead and wash it. However you may loose some of it if it is not well attached to the matrix, mine was well attached so there was no problem.

Hi Reiner,

chemists, mineralogists and others define solubility as a quotient: max. mass of mineral soluted in 1 l fluid, mostly water.
They do not attend how long time is necessary to reach this max. value of soluted substance. They obtain this values mostly by the invers way: They mix a solution from which the "mineral" is precipitated; and analyze the fluid.

There are minerals not soluble in water, as Quarzt. - Easy solubable in water, as halite. After 1 day in water your spezimen is "disapeared" = soluted in water. Or at least You can see etched away edges e. c. t. - Minerals as Selenite, Anglesite e. c. t. are "a little bit" soluble in water. So far as I know, there is no standart procedure to test this. You can wash them in water without any damage. You can find them also in a climate with rain, as in the Harz Mountains. In good books for mineral collectors You find no list about solubility of minerals. But at some minerals, as Halite, a note: "soluble in water"; at some a note "soluble in acetic acid by generation of CO2-gas", as Calcite; at some "soluble in KOH-solution" as Anglesite.

My proposal: Test Your spezimen 24 h. But why not just 12 h; or 48 h: I can not tell You any reason. - A further possibility: Make a test with a surely analyzed mineral; and compare Your doubtly mineral by operating it in exactly the same way.

Hope this helps a little bit. Merry Chistmas and good luck in the new year! Greetings from Goslar

Georg

Hello Georg,

The reason I am asking is to try and make some sense out of the published information. If a paper says it is insoluble what does that mean? I would agree that if there is no change after 24 hours it is insoluble but does everyone do it that way? Maybe someone who has published such information can enlighten us. Unless one has a confirmed sample it is not possible to compare and I have very few confirmed samples. RUFF would be the perfect project to provide standardized practical solubility information.

Hi Reiner,

in the strictest sense "insoluble" means: 0,0000000000000 g are soluble in 1 l water. I think, diamond and graphite are in this sense "insoluble" in water. - Baryte, which is normaly called "insoluble" is a very little bit soluble in water. But normaly You can not realize that solubilty. -Maybe Your literature is not exact at that point. Is the author still alive? Ask him, what he wants to say. Maybe the author himself do not know, how inexact the term "insoluble" can be. Is/was he a chemist, or an amateur?

Or give me the chemical formula, and I tell You what I think about. Or can You post the literature You are thinking about?

Best wishes for the new year and greetings from Goslar

Georg

Hi Reiner,

if the formula/literature is a "secret of state", write me a private mail.

Greetings

Georg

Actual measurements of solubility are usually done on finely powdered material, which will be much more noticeable than for an unpowdered chunk of a mineral specimen. A substance might be noticeably soluble in water (as a powder), and nevertheless a crystallized mineral specimen of it could easily be washed for a couple minutes without showing any visible alteration - Gypsum and fluorite, for example. Whether the mineral crystal has microscopic cracks or incipient cleavages that water could get into is also going to affect the outcome. And a "slightly" soluble mineral specimen that will easily survive a brief wash in cold water under the tap might not survive so well if using hot water, or using an ultrasonic bath.

Incidentally, I'm as frustrated as Reiner is by the lack of quantified information about solubility of minerals under different conditions. But I suppose in many cases this information will never be available unless serious amateurs undertake to do some experiments and publish the information. We can't expect professional mineralogists to do it - they're paid these days to describe and manipulate atomic structures, not publish information on cleaning, or kitchen identification techniques. The era of descriptive mineralogy is over. We serious amateurs can shed a tear for the bygone era, and then get off our arses and publish some of this 19th Century-style descriptive stuff ourselves.

This got me thinking.

Is Ice soluble in water? Or does it just melt?

The problem for amateurs is access to confirmed samples. Most of what is available from dealers is not confirmed so even if you do the necessary solubility tests the question remains is this actually a sample of that mineral ( case in point, bobdownsite) If I had access to confirmed samples I would gladly establish a data base on solubility, it only takes a tiny speck the size of this "." under a microscope to determine solubility.



Edited 1 time(s). Last edit at 12/28/2011 12:46PM by Reiner Mielke.

Jolyon, the answer to your question is definitive.

A phase change is always required for solubility. A solid cannot exist as a solid in dissolved state. For example, NaCl (solid) becomes Na+ (aqueous) and Cl- (aqueous) when it dissolves. Ice also suffers a phase change (solid to liquid) before dissolving.

Any substance is miscible with itself (i.e., will always form a homogeneous solution in any ratio). When ice melts it (of course) becomes water, which is identical with its surrounds (i.e., more water). Therefore ice melts, then dissolves.

Would quantifying mineral solubility be of much use to most people interested in minerals? I think not.

I don't think we need tables of data giving solubility in grams of mineral per liter of water at 25oC. Rather what would be useful is a more qualitative approach. For example: "When gypsum is placed in water at 25oC for one hour, we typically see..." or "when diamond is placed in water at 25oC for one hour we typically see..." ("...honey are you playing with my wedding ring again?")

The solubility of chemical compounds is expressed as a solubility product. It is the equilibrium concentration of the compound in one liter aqueous solution at a specific temperature and pressure. It is a number. You can find more explanation than you probably want at http://en.wikipedia.org/wiki/Solubility_equilibrium and a table of values at http://www.ktf-split.hr/periodni/en/abc/kpt.html.

For barite, BaSO₄, the K_sp = [Ba⁺⁺][SO₄^--] = 1.08 x 10^-10 That is, the equillibrium molar concentration of barium multiplied by the molar concentration of sulfate in water is equal to 1.08 x 10^-10 at 25^oC and one atmosphere of pressure. If we are considering the equillibrium solubility of pure barite in pure distilled water, the molar concentration at 25^oC and one atm of pressure will be the square root of the K_sp or about 1 x 10^-5 moles per liter. That equates to 2.33 x 10^-7 grams per liter.

The upshot is that we can express solubility in a number. And for simple molecular formulas we can find them in tables. However, I believe that for most minerals the values have not been determined and are not likely to be done. Solubility constants in acids can be calculated also, but are a bit more complex.

Foot note: my chemistry was long ago; so, Peter (or anyone) if I have made an error please correct me,



Edited 1 time(s). Last edit at 12/28/2011 04:57PM by Donald Peck.

I think Steve's suggestion makes some sense, as long as temperature and the fineness of powdering (screen mesh?) is stated. Both would have a significant effect on the result.

I agree that the lack of info on such a basic mineral property as solubility in water is very frustrating. Alfredo is right: measuring solubility is not as sexy as determining/refining the crystal chemistry of a mineral. On the other hand, knowing the crystal chemistry shall provide clues to understanding the solubility (in terms of the crystal lattice strength), but certainly not a substitute for experimentally measured solubility data.

Reiner,
In absence of the solubility data for a particular mineral, our best guess is to access the solubility based on the mineral chemistry.
Bobdownsite may be considered a mixed Ca-Mg orthophosphate (PO4) - fluororthophosphate (PO3F).
From the chemical properties tables for the relevant chemical compounds, we can see that
(a) Ca-fluororthophosphate is slightly soluble in water at room temperature, more so than the Ca-orthophosphate (apatite); and
(b) Mg compounds tend to be more soluble than the corresponding Ca compounds, so the presence of Mg is likely to increase the solubility.
Therefore, it is reasonable to assume that bobdownsite shall be slightly soluble in water at room temperature (25C). I would expect it to be slightly/slowly "affected" by water. That is roughly the same category of solubility as gypsum.
In practical terms: one shall use as quick a wash as possible and water as cold as possible.

Steve,
IMHO, calling ice melting in water a dissolution is a biiiiiiiiiiiiiiiiiig stretch. Dissolution process shall involve a heterogeneous interaction, i.e. the solute and the solvent shall be different compounds. Melting is a physical process; dissolution is a chemical process, where the solute and the solvent interact chemically. An incorporation of the water from melting ice into the surrounding water is a simple mixing, not a dissolution.
BTW, if the resulting combined water is frozen again, you are not gonna call it a solid solution, aren't you?

Don,
One comment to what you wrote:
The solubility product is used for insoluble or poorly soluble compounds. You were right to use BaSO4 as an example.
For the soluble compounds, the solubility is expressed in terms of the solubility limit = solute concentration in a concentrated solution at a given temperature, and could be expressed in different numbers/units (g/L, M, N, etc.).