MgCO3 solubility

[jfinch]

Hey boomer, I'd like to restart our conversation here (since what we were talking about has little to do with the subject of the other thread) :D

That thread is here: http://www.reefcentral.com/forums/showthread.php?s=&postid=2961587#post2961587 (http://archive.reefcentral.com/forums/showthread.php?s=&postid=2961587#post2961587)

I'm confused regarding the solubility of MgCO3. I was thinking that dolomite would be a poor choice for a substrate because of the (uncontrollable) Mg from dissolution. I based this opinion on the Ksp for MgCO3 found in the CRC Handbook of chem & physics (which is still at work and I'm at home). The logKsp for CaCO3 is around -8 while the logKsp for MgCO3 is closer to -6. Doesn't this mean that MgCO3 will dissolve before CaCO3? My confusion was caused when boomer posted all those links to mineral sites. MgCo3 (magnesite) is not more soluble then calcite. And it appears that dolomite, CaMg(CO3)2, is even more insoluble. One of the links boomer gave showed a logKsp of -17 to -19 for dolomite.

Is the MgCO3 listed in the CRC handbook and apparently other references different then magnesite? If MgCO3 is as insoluble as some say, how is it that seawater can have so much Mg in it? I don't think ion pairing can explain it as everything I've read still say's the majority of Mg is found in the normal ionic form, Mg++. Or is there other factors that I'm not aware of that come into play?

Randy, I've read your articles on Mg. In one of them you state that seawater is slightly undersaturated with regard to MgCO3. How did you come to that conclusion?

Thanks for reading this far :D

[Randy Holmes-Farley]

According to Millero in "Chemical Oceanography" (page 106 Table 3.7), MgCO3 is undersaturated by a factor of 27 in normal seawater.

Getting the exact solubility from Ksp values is near impossible due to activity coefficient issues in seawater. It is more soluble that CaCO3, but there is more Mg++ in seawater, and CaCO3 is quite supersaturated to begin with.

FWIW, Spotte has an extensive discussion of what happens to MgCO3 and CaCO3 (and everything in between) in "Captive Seawater Fishes". Apparently, they all arrive at a surface coating that contains both calcium and magnesium.

That said, the issues are more complicated than Spotted discusses, and he says it is quite complicated. From one of my magnesium articles:

Magnesium
http://www.advancedaquarist.com/issues/oct2003/chem.htm


If this sounds confusing, don’t feel alone. In Stephen Spotte's book Captive Seawater Fishes, Spotte says “The study of carbonate minerals involves nuances of solubility that pose some of the most difficult problems in chemical oceanography and geochemistry.�


So just because MgCO3 is understaurated does not necessarily mean that it will disoslve any more than CaCO3 being supersaturtated means that it precipitates (which it doesn't on any reasonable time frame).

[Boomer]

I'm confused regarding the solubility of MgCO3. I was thinking that dolomite would be a poor choice for a substrate because of the (uncontrollable) Mg from dissolution. I based this opinion on the Ksp for MgCO3

You can't be comparing Pure Magnesite, MgCO3 to---> Dolomite, MgCa(CO3)2 and use a Ksp for Magnesite as if it is the same for Dolomite.


Uncreasing stability in seawater

Hi-Mangesium Calcite --->Aragonite--->Low Magnesium Calcite--->Pure Calcite--->Dolomite.

There is no mention for the fit that I could find for Magnesite. Magnesium Calcites aren't Magnesite. They are overgrowths or surface posions found growing on calcium carbonates, quite often in seawater. They also grow on Dolomite. Most of the Dolomite in this hobby is not Dolomite at all but Dolomitic Limestone, where Dolomite the mineral, is between 10 -50 %. The rest is Calcite.

[jfinch]

According to Millero in "Chemical Oceanography" (page 106 Table 3.7), MgCO3 is undersaturated by a factor of 27 in normal seawater.

You must have a newer version of that book then I do. P.106 is still in chapter two in mine (1992 edition). Millero doesn't talk much about Mg in the 1992 ed other then it's role in ion pairing.

You can't be comparing Pure Magnesite, MgCO3 to---> Dolomite, MgCa(CO3)2 and use a Ksp for Magnesite as if it is the same for Dolomite.

Yes, I realize that now. I should have payed more attention in geology class. :lol: I was under the false assumption that dolomite was a heterogenious mix of MgCO3 and CaCO3 and that the MgCO3 in the dolomite would behave like the MgCO3 listed in my reference. Sorry.

I'm just wondering why magnesite (the mineral) doesn't dissolve like the MgCO3 listed in the CRC?

If Mg is undersaturated by a factor of 27, then the Ksp should be:
[53 mmol/l][0.2 mmol/l] x 27 = 3 x 10^-4
That is vastly different then the Ksp of magnesite isn't it?

So what is formed if I take seawater and add MgCl2 until I see a precipicate? From Bingman's article on magnesium, http://web.archive.org/web/20030624142251/http://www.animalnetwork.com/fish2/aqfm/1997/jul/bio/default.asp I can't tell if it would be magnesite (stable) or huntite (metastable). BTW, what does metastable mean in that context?

Is this compound different then if I were to take a sample of fresh water, add Na2CO3 and MgCl2 until a precipitate was formed? And wouldn't this compound be MgCO3(s) and have a Ksp similar to the one in my CRC handbook?

I know this has less and less to do with aquaria and probably more to do with geology, but I'm curious...

[Boomer]

You must have a newer version of that book then I do

Me too Jon :lol:

I'm just wondering why magnesite (the mineral) doesn't dissolve like the MgCO3 listed in the CRC?

You are not the only one confused on this, me too:D. I wonder if it is really Magnesite and maybe really real Magnesium Calcite. I have seen in print a few palces this; MgCO3 ( Magensium Calcite) :confused: Some interesting links below

BTW, what does metastable mean in that context?

Minerals that crystallize at a certain temperature and can only retain the initial crystal structure at certain temperatures.

Huntite is CaMg3(CO3)4. It is a very common mineral in caves

Hunitie vs Magnesite theroies
http://users.skynet.be/infolib/dolomite/files/12_Chapter6.pdf

Thermodynamics
http://ctdp.ensmp.fr/species/Huntite.html

Low Temp Formation. SEE # 6
http://users.skynet.be/infolib/dolomite/bookprospectus.html

[Boomer]

Jon, I you trying to become a geologists :D I just noiced your avatar :lol: Looks like two geology hammers;) I see you are still half in the red :rollface:

[jfinch]

Boomer thanks for the links. I would have never guessed that MgCO3/Mg(OH)2 chemistry was that darn complicated! (do you just find those links spur of the moment or do you have a favorites folder that requires a shirpa guide to navigate :lol:)

I just noiced your avatar Looks like two geology hammers I see you are still half in the red

:lol: In grade school I actually wanted to be a geologist... but when I got older and found out there was no money in it I opted for engineering :lol: As a matter of fact I spent part of last weekend digging up geodes in the desert west of Salt Lake. But the hammers are more representative of my musical likes...

[Boomer]

(do you just find those links spur of the moment or do you have a favorites folder that requires a shirpa guide to navigate :

:lol:

Both, but I just found the first two links.

I have a folder on links called Aquariums and one on Geology. There are about 20 subfolders in the Aquaium folder, one on "Water_Test Kits_Chemistry_Equip" It is huge, I often get lost when lookin for things :lol: It scrolls for about 3 feet:rollface:

Here is my latest link

http://users.rcn.com/patwilde/ocpertbl.html

Showed this one to Randy last week

Ok, ok, I got lost finding it

http://www.chemistrycoach.com/periodic_tables.htm