Reef Chemistry Question of the Day #266 Calcite vs Aragonite

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #266

The two most common forms of calcium carbonate in reef aquaria are calcite and aragonite.

What is the fundamental defining difference between calcite and aragonite?

A. The amount of magnesium incorporated in place of calcium
B. The amount of strontium incorporated in place of calcium
C. The relative amounts of calcium and carbonate present
D. The three dimensional arrangement of calcium and carbonate ions

Good luck!


























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[Js.Aqua.Project]

Taking a stab and going D, as the chemical formula for both is CaCO3.

 

[Sleepydoc]

D

 

[Jose Mayo]

D

 

[Marcio Gielfi]

D

 

[Chuk]

D different crystal structures.

 

[Randy Holmes-Farley]

And the answer is....

What is the fundamental defining difference between calcite and aragonite?

A. The amount of magnesium incorporated in place of calcium
B. The amount of strontium incorporated in place of calcium
C. The relative amounts of calcium and carbonate present
D. The three dimensional arrangement of calcium and carbonate ions


All forms of calcium carbonate can have some amount of magnesium and strontium in them. Calcite formed in a reef aquarium can often have a lot more magnesium in it than does aragonite, but that's not the difference that defines the name calcite compared to aragonite.

All forms have 1:1 moles of calcium and carbonate, except to the extent that particular ions substitute into the structure. For example, magnesium or strontium swapping in for calcium, phosphate swapping in place of carbonate, etc.

The defining difference is the way the calcium and carbonate are arranged in the crystals.

It can be hard to describe and visualize the differences, but here is a comparison of the arrangements:

https://www.researchgate.net/figure...gonite-and-calcite-Aragonite-a_fig1_305654328

 

[MnFish1]

Reef Chemistry Question of the Day #266

The two most common forms of calcium carbonate in reef aquaria are calcite and aragonite.

What is the fundamental defining difference between calcite and aragonite?

A. The amount of magnesium incorporated in place of calcium
B. The amount of strontium incorporated in place of calcium
C. The relative amounts of calcium and carbonate present
D. The three dimensional arrangement of calcium and carbonate ions

Good luck!

























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A