calcite v. argonite

[LTrain]

From last month's article:
In normal seawater, equation 3 holds (supersaturation). The product of calcium and carbonate is about 3 times the Ksp* of aragonite and 5 times that of calcite (aragonite and calcite are different crystal forms of calcium carbonate; these differences will be discussed more in future articles).
...
I may be jumping the gun so to speak, but I am interested in the argonite/calcite distinction as far a substrate is concerned. As a grad student reefer on a extremely limited 'about to be married' budget, would other calcium carbonate forms be able to dissolve minimum levels of calcium in the water? Can calcium requirements be met with substrate alone? I can get ahold of granular 'calcium carbonate,' but this is probably just land mined limestone in calcite form. Although I am cost conscious, doing the "right" thing for my future corals and gaining a true knowledge of what is going on in my tank are more important, so if this will be covered in 'future articles,' I can wait with patience...
Thanks!

 

[randy holmes-farley]

Calcite as a substrate will dissolve to a lesser extent than aragonite. I'm not aware of any tanks, however, with substantial numbers of calcifying organisms that can maintain calcium and alkalinity via substrate alone.

If you want to use calcite as a substrate, I see no reason not to. After all, many people use things that don't dissolve any calcium or alkalinity at all (like silica sand) and still have fine tanks. The particle size distribution is probably much more important to the health of the tank than is the crystalline form. There are many threads that discuss optimum sizes on the various bulliten boards, but in general the larger sizes are not preferred.

That said, if you live on the East coast, you can probably get aragonite sand from Home Depot for about $3.50 per 50 (40?) pound bag. It is sold by Southdown.

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[CraigBingman]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by LTrain:
<strong>From last month's article:
In normal seawater, equation 3 holds (supersaturation). The product of calcium and carbonate is about 3 times the Ksp* of aragonite and 5 times that of calcite (aragonite and calcite are different crystal forms of calcium carbonate; these differences will be discussed more in future articles).
...
I may be jumping the gun so to speak, but I am interested in the argonite/calcite distinction as far a substrate is concerned. As a grad student reefer on a extremely limited 'about to be married' budget, would other calcium carbonate forms be able to dissolve minimum levels of calcium in the water? Can calcium requirements be met with substrate alone? I can get ahold of granular 'calcium carbonate,' but this is probably just land mined limestone in calcite form. Although I am cost conscious, doing the "right" thing for my future corals and gaining a true knowledge of what is going on in my tank are more important, so if this will be covered in 'future articles,' I can wait with patience...
Thanks!</strong><hr></blockquote>

In my opinion, the amount of calcium and alkalinity that a system gets from dissolving substrate is relatively low compared to the calcium carbonate demand of a typical reef system with appreciable stony coral cover. In the model that I have for estimating reef tank calcification rates, I assign 0 kg m-2 y-1 as the net calcification rate for sand areas.

Calcite is less soluble than aragonite at reef tank temperatures. The discrepancy becomes less at higher temperatures (well above reef tank temps.) If you find aragonite that has recrystallized in a sedimentiary rock, it is a safe bet that it happened in very warm conditions.

In a reef tank, both calcite and aragonite become rapidly fouled with organics. Once "shrink wrapped" by lipids, it probably doesn't make much difference to the solution in the tank whether the substrate is made of calcite or aragonite. Until the surfaces are fouled, it is possible that you might experience slightly higher rates of abiological (chemical) precipitation on the surfaces of the grains. This is expected to be a short-lived phenomenon.

A potential issue with non-marine sources of calcareous substrate is that the particular bits that you have might have been enriched in toxic stuff by some chemical process over time. Geologists call this "diagenesis" and chemical species definitely can redistribute in sedimentary rocks over the geological timescale.

Another potential issue is that you may have a difficult time finding mined calcites milled down to an appropriate grain size for use in a reef aquarium. In my opinion, that is probably the largest issue in substrate choice. It is just easier to find aragonite in the right particle size through retail channels.

Adey feels that Halimeda hash (a particular type of biogenic aragonite derived from decomposed Halimdea) is especially soluble. There is probably something to that. However, even if your sand bed is pure Halimeda hash, I strongly doubt that it would be capable of making much of a dent in the calcium/alkalinity demand of a typical reef aquarium.

Craig

 

[randy holmes-farley]

Craig:

What's the basis for thinking the halimeda aragonite might be more soluble?

[ April 22, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[Anonymous]

hi.
Since I did not see the Adey's article (hate it when experts throw out names and expecting you to find the reference!), I would have to guess.

Aragonite is aragonite, and so the solubility constant will be the same. What Adey mean by "more" soluble, I guess, is the faster rate at which the saturation is reached because some particular property related to how Halimedia build aragonite within the "leaves."

It is like saying that baker's sugar (powder) is "more" soluble than rock sugar because of particle size...

 

[CraigBingman]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Randy Holmes-Farley:
<strong>Craig:

What's the basis for thinking the halimeda aragonite might be more soluble?

[ April 22, 2002: Message edited by: Randy Holmes-Farley ]</strong><hr></blockquote>

Randy,

Dissolution of a particular substrate is likely to involve both kinetic and equilibrium phenomena. In the case of Halimeda hash, the particles are extremely finely divided, so it has a much higher surface area to volume ratio than other substrate materials. Additionally, not all "aragonite" is the platonic ideal of chemically pure single-crystalline aragonite. Crystals with strain, defects or impurities can have significantly higher chemical potentials than ideally crystalline aragonite.

I don't want to make too much of an issue of this. From a practical standpoint, a number of the commercial aragonite substrates have a non-trivial Halimeda hash component, so if people have bought them, they more than likely have some of this material in their sand already.

Craig

 

[randy holmes-farley]

Thanks, Craig.