Mg Calcite as a "Buffer"???

[ReefDiver]

Hi Randy: There is a thread located on MD forums that was started by Anthony: http://forum.marinedepot.com/Topic50785-13-2.aspx

Just wanted to get your professional opinion of Mg Calcite as a buffer? Would this be a good substitute for a RDSB? Thanks.

Steve

[bertoni]

Calcite (calcium carbonate) is not useful as a buffer additive except in a CO2-driver calcium reator. The same would apply to magnesium carbonate. I'm assuming you mean a mixture of calcium carbonate and magnesium carbonate when you say "magnesium calcite".

[Boomer]

Steve

True Mg Calcite is more soluble than calcite or aragonite but still not enough to do anything. What are you calling Mg Calcite ? It only grows as a "surface film" on calcium and magnesium carbonates.

Eric is lost and does not know what he is talking about. Anthony is more on target to a point. Neither understand what Mg Calcite really is and are miss-using mineralogical terms.

I have discussed this issue before here as most, 99.9 99%, do not know what Mg Calcite is. Many call Magnesite, Magnesium Calcite. Well, it is not, it is Magnesium Carbonate, just like Calcite is Calcium Carbonate. And Dolomite is not Magnesium Calcite either. It is Calcium, Magnesium Carbonate.

Mg-Calcite is Calcite, that has a specific ratio of Mg++ substituting some of the Ca ++ in a Calcite crystal lattice.

Here is my discussion


http://reefcentral.com/forums/showthread.php?s=&threadid=380514&highlight=mg+calcite


http://reefcentral.com/forums/showthread.php?s=&threadid=603680&highlight=mg+calcite

[ReefDiver]

Thanks Boomer.

Not certain as to the chemistry behind all of this but I don't think that any of these substrates dissolve much in our reef systems. Hence the reason for dosing kalk, Ca++, and doing large water changes. At least that's what I make of it. I did post the link to that discussion on the MD forum. The follow up to that should be rather interesting!

Steve

[Boomer]

Yes I would agree with you. I find hard to believe Anthony's remarks on his sand dissolving as he claims.

Tell Anthony this is from Boom for him :D Also tell him that if he wants to be sure his sand is Mg-Calcite to take it to a local universtiy Geology Department and have them run it through an XRD . I have doubts he really has Mg-Calcite sand as it is kinda rare but it is possible

What Mg-Calcite is

High-magnesium calcite' (CaCO3 containing more than 4% MgCO3 ) and 'Low-magnesium calcite' (CaCO3 containing less than 4% MgCO3 ). Pure Calcite less than 1%. MgCO3. Magnesite, less than 1% CaCO3. Dolomite ( the mineral), on the order of 50 % Calcite and Magnesite.

Calcium Carbonate as a Supplement (Aragamight; Liquid Reactor)
http://www.advancedaquarist.com/iss...ly2002/chem.htm


MODERN HIGH-MAGNESIUM CALCITE ORGANISMS PRODUCE LOW-MAGNESIUM CALCITE IN EXPERIMENTAL MID-CRETACEOUS SEAWATER
http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_60423.htm


Low-magnesium calcite produced by coralline algae
in seawater of Late Cretaceous composition
http://www.pnas.org/cgi/reprint/99/24/15323.pdf

The role of cyanobacteria in crystallization of magnesium calcites,
Paleontological Journal, Volume 40, Number 2 / March, 2006

Abstract Laboratory experiments showed the effect of the cyanobacterium Microcoleus chthonoplastes on the formation of magnesium calcites, using model solutions (2.14M MgCl2-0.05M CaCl2-0.6M NaCl-0.18M NaHCO3). The conditions of existence of cyanobacteria in such solutions in light or darkness significantly alter the structure of the sediment and the shape and size of the carbonate crystals. Cyanobacteria slow down crystallization due to the formation of exometabolites with a chelating effect, which leads to the precipitation of high-magnesium calcites. In the photosynthetic environment the presence of huntite (CaMg3(CO3)4), possible forerunner of dolomite, is prominent.

CO2− 3 concentration and pCO2 thresholds for calcification and
dissolution on the Molokai reef flat, Hawaii
http://www.biogeosciences.net/3/357/2006/bg-3-357-2006.pdf


Also, High-magnesium calcite ooids from on the Great Barrier Reef and the sediments of Great Baham Banks platform is dominated by aragonite (up to 95%), but also contains up to 5% high-magnesium calcite and minor low-magnesium calcite (max. 1%)



Something for Eric

Halley, R.B., and Yates, K.K., 2000, Will reef sediments buffer corals from increased global CO2? (abs.): 9th International Coral Reef Symposium, Bali, Indonesia, Abstracts, p. 249.

[bertoni]

I've always thought that dissolving sand was really a case of bacterial consumption. That should happen with silica sand, too, although at a slower rate, I think.

[Boomer]

Silica sand Jon is not even close :)