Pavestone Limestone Sand: Calcite or Aragonite?

[Sokretys]

I was doing some reading up on RC about this...

The take i got was that the Pavestone pulverized limestone sand is in fact Calcium Carbonate (CaC03), but that there are several forms of caco3.

calcite, dolomite, azurite and aragonite are all forms of caco3.

http://encarta.msn.com/encyclopedia_761573488/Calcite.html

"...Calcite is generally more stable than aragonite. Over several years, most aragonite will change to calcite. Thus, newly created calcium carbonate deposits may contain some or even a lot of aragonite, but ancient deposits contain little or no aragonite."

so unless we know where exactly pavestone mines this limestone...we cant be sure about what we are really using. Im assuming both calcite and aragonite will have the same denitrifying capabilities...but will differ greatly as far as buffering ability goes...

A question we must ask ourselves...is if aragonite will begin buffering our systems at a pH of 8.2..when will calcite begin buffering if at all. also...

IF:"Calcite is only slightly soluble in pure cold water. However, rainwater or other water in contact with air will absorb carbon dioxide from the air. Carbon dioxide reacts with water to form carbonic acid. Calcite reacts with carbonic acid to form calcium bicarbonate. Calcium bicarbonate is quite soluble in water. Thus, the solubility of calcite is sensitive to the acidity of the water. The solubility of calcite is also sensitive to the temperature of the water, being quite soluble in cold water but not in hot water. Calcite frequently dissolves in one location and precipitates in another location when the water becomes less acidic or cool"; THAN,

Perhaps aragonite will buffer from the surface out? and the pH a few inches down in a DSB will be much lower than that of the water body and therefore allow the breakdown of the calcium carbonate to take place and then be able to precipitate back into our water body. Will the sand then turn into brick at this point?

Note: If according to Encarta...which like wiki has to be taken with a grain of salt...but according to them; calcite is much more abundant than aragonite, and therefore cheaper in cost. id doubt a landcaping company such as limestone would pay extra money for a less abundant form of limestone...

Im not too concerned about having calcite or aragonite in my reef system since it will be housing a ca reactor. but would much prefer aragonite...for the fish only system...the buffering ability of the aragonite would be most appreciated.

Like everyone looking for 100's of pound of sand...im trying to do it cost effectively...but at the same time trying to use the right tool for the right job.

Any thoughts BRS?

Thanks,

-Nate

 

[Greg Hiller]

Pretty sure what you will be getting is calcite. The argonite sand that used to be available as Southdown sand looked like tiny spheres. It would form from natural precipitation in certain places in the ocean. I'd bet that the stuff you are getting now if you look at it with a magnifying glass will look like jagged pieces. It's okay to use in a tank (assuming it's clean and the source material is clean, no promises there), but you're right that it might not be as good a buffer.

 

[alpha0r]

It is highly debated as to if aragonite sand provides and buffer at all. The people on the RC chemistry forum told me the effect was so negligible to be practically non-existent.

 

[Sokretys]

How could aragonite not offer any buffer if its calcium carbonate and will precipitate into the tank at 8.2? Do you happen to have a link to that RC chem thread?

 

[Randy Holmes-Farley]

Neither calcite nor aragonite will dissolve into seawater unless the pH falls well down into the 7's. That is why it provides no buffering until the pH drops that low, and is why CaCO3/CO2 reactors need to run at pH 6.5 or so instead of pH 7.5.