Reef Chemistry Question of the Day #285: Aragonite vs Calcite

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #285

Reefers often hear that organisms in their aquaria may deposit aragonite or calcite, but do not always pay attention to what those words imply.

What is (are) the difference(s) between aragonite and calcite?

A. Aragonite contains one carbonate ion for every two calcium ions, while calcite contains one carbonate ion for each calcium ion
B. At room temperature, aragonite is more stable than calcite
C. At high pressure and temperature, aragonite transforms into calcite
D. In seawater at low pH (say, pH 7), calcite is less soluble than aragonite

Pick all that you think apply.

Good luck!

Previous Question of the Day:

Reef Chemistry Question of the Day #284: Freezing Seawater

Reef Chemistry Question of the Day #284 The freezing point of 35 ppt seawater is described as being -2 °C (or 28.4 °F). https://oceanservice.noaa.gov/facts/oceanfreeze.html# Suppose you take a sample from your aquarium with a salinity of 35 ppt and send it off through the mail for analysis on...

www.reef2reef.com

 

[taricha]

If you take a little bit of the precipitation off your heater, and crush it in a drop of water on a microscope slide - you can see some really nice calcium carbonate crystals.

I don't know if the crystals are calcite or aragonite.


And I don't know what I want to answer for the question yet. But I think it's cool.

 

[Randy Holmes-Farley]

bump for answer later today

 

[Randy Holmes-Farley]

And the answer is...

What is (are) the difference(s) between aragonite and calcite?

A. Aragonite contains one carbonate ion for every two calcium ions, while calcite contains one carbonate ion for each calcium ion
B. At room temperature, aragonite is more stable than calcite
C. At high pressure and temperature, aragonite transforms into calcite
D. In seawater at low pH (say, pH 7), calcite is less soluble than aragonite

The best answer is D.

Aragonite has higher solubility in seawater at normal temperature and pressures, and is why folks typically use aragonite in calcium carbonate/carbon dioxide reactors.

Answer C is a second best answer. As temperature and pressure are increased on calcite, it goes through a few changes to different structures of calcite and then to aragonite, which is not what C states. BUT, at VERY high temperature and pressure, that aragonite that formed at lower temps and pressures can change back into a structure that is called calcite IV or V (disordered calcite). At high enough temp and pressure it liquifies.

Answer C has no relevance to reefkeeping, but anyone interested in details around answer C can find it here:

https://rruff-2.geo.arizona.edu/uploads/AM86_997.pdf

Happy Reefing!

 

[taricha]

calcite IV or V (disordered calcite)

Hah. I thought only water ice had that name scheme for all of the different distinct crystal structures it can form.
"Ice V", "Ice IX" etc.
I think there are like 20 different "ice" forms....
Amazing chart here.

 

[Dburr1014]

I would say c and d.

Isn't calcite ancient reef structure?
Buried for thousands of years under pressure?

 

[Randy Holmes-Farley]

I would say c and d.

Isn't calcite ancient reef structure?
Buried for thousands of years under pressure?

It seems that process is currently thought to be driven by dissolution of the aragonite and reprecipitation as calcite, not a pressure driven change:

The transformation of aragonite to calcite in the presence of magnesium: Implications for marine diagenesis

Magnesium (Mg) in natural waters plays a critical role in governing carbonate mineral formation, dissolution, and diagenesis. Previous laboratory expe…

www.sciencedirect.com

". Thus, the diagenetic conditions under which aragonite-rich sediments convert to calcite-rich limestones are poorly understood. "

"Aragonite is the most abundant mineral in modern-day, shallow-marine carbonate sediments (Gischler et al., 2013) and has been for a large proportion of the Phanerozoic Eon (Hashim and Kaczmarek, 2019). Because aragonite is a metastable phase under most Earth surface conditions, aragonitic sediments tend to dissolve during diagenesis, and their dissolution may be accompanied by low-Mg calcite (calcite) precipitation, which is the more stable calcium carbonate polymorph (Morse et al., 2007)."

 

[Gretchacha]

Reef Chemistry Question of the Day #285

Reefers often hear that organisms in their aquaria may deposit aragonite or calcite, but do not always pay attention to what those words imply.

What is (are) the difference(s) between aragonite and calcite?

A. Aragonite contains one carbonate ion for every two calcium ions, while calcite contains one carbonate ion for each calcium ion
B. At room temperature, aragonite is more stable than calcite
C. At high pressure and temperature, aragonite transforms into calcite
D. In seawater at low pH (say, pH 7), calcite is less soluble than aragonite

Pick all that you think apply.

Good luck!

Previous Question of the Day:

Reef Chemistry Question of the Day #284: Freezing Seawater

Reef Chemistry Question of the Day #284 The freezing point of 35 ppt seawater is described as being -2 °C (or 28.4 °F). https://oceanservice.noaa.gov/facts/oceanfreeze.html# Suppose you take a sample from your aquarium with a salinity of 35 ppt and send it off through the mail for analysis on...

www.reef2reef.com

Well this is a complicated topic because there are more variables directing formation and dissolution of these minerals than pressure, temperature and pH. A) calcite and aragonite are polymorphs, or different crystal structures, of the same composition, calcium carbonate. B) this difference in structure causes different physical properties between the minerals. In most domains, aragonite is less favored than calcite. At normal pressure and temperature (PT) aragonite is only metastable and will eventually be replaced by calcite if the mineral were to remain in sea water. C) the phase transition between aragonite and calcite is a straight line. However, if pressure increased faster than temperature, aragonite would be favored, and if temperature increased faster, calcite would be favored. It is possible to transform to calcite at a higher PT regime. D) aragonite is almost always more soluble than calcite, and pH is not the most significant factor controlling the shift in solubility gradients.

Now for the interesting bits as these two minerals relate to reef building. Aragonite and calcite are unique to minerals precipitation in general. Usually when we think about dissolving a solid, we would heat and agitate it. However, calcium carbonate minerals precipitate under these conditions, and dissolve under low temperatures and still water until their surrounding water is in a saturation state. That is why calcium carbonate precipitates on your heater, powerhead and other high energy high temp areas of your aquarium and why hot springs are surrounded by travertine and also why florida is collapsing from groundwater dissolution.
So why does aragonite form and persist if it is not thermodynamically favored? Three reasons that impact your aquarium. 1- it is bioprecipitated. The animals can control the formation of calcite and aragonite in their tissues. Calcite solubility increases with the presence of amino acids, but aragonite solubility is unchanged by amino acids. 2) the needle-like crystal form of aragonite bioprecipitated is more resistant to dissolving. 3) magnesium interferes with calcite precipitation, but not with aragonite precipitation, making aragonite the favored mineral form in high magnesium solutions. 4) bonus- it takes a really long time to dissolve and replace these minerals once formed.
One other critical factor in coral growth and aquarium chemistry is phosphorus. Phosphorus is the critical element necessary for life and what geologists look for in space samples to see if life elsewhere is possible. Phosphate in the aquarium is both necessary and harmful. Why us that? If phosphate bottoms out, corals rapidly decline. If phosphate increases, the same is true. Well it turns out that phosphate saturation and its ratio to hydrogen phosphate controls aragonite formation. As phosphate increases, aragonite formation decreases.

Take aways: favorable conditions to help corals grow their skeletons would be high magnesium and low phosphate with enough calcium carbonate in solution and warm temperatures.

if you are interested in the papers discussing these factors, I can provide links.

 

[Randy Holmes-Farley]

Take aways: favorable conditions to help corals grow their skeletons would be high magnesium and low phosphate with enough calcium carbonate in solution and warm temperatures.

I understand that, like magnesium, organics, and even whole bacteria, phosphate can get onto calcium carbonate surfaces and interfere with abiotic precipitation of calcium carbonate, and I agree that there is evidence in the literature about this, but I'm not certain the data is clear that elevated phosphate deters coral calcification in typical reef tanks.

FWIW, I've seen some fabulous hard coral tanks with very high phosphate (such as Richard Ross'):

Skeptical Reefkeeping 9: Test Kits, Chasing Numbers and Phosphate

By Rich Ross and Chris Jury There are many standard parameters in the reefkeeping world that aquarists strive to match in their home reefs – water quality,

reefs.com

Rich’s 150 gallon display, on a 300 gallon system, is running a phosphate level of 1.24 ppm, a level at 24.8 times higher than the often recommended .05 ppm. Photo by Richard Ross.

 

[Gretchacha]

I understand that, like magnesium, organics, and even whole bacteria, phosphate can get onto calcium carbonate surfaces and interfere with abiotic precipitation of calcium carbonate, and I agree that there is evidence in the literature about this, but I'm not certain the data is clear that elevated phosphate deters coral calcification in typical reef tanks.

FWIW, I've seen some fabulous hard coral tanks with very high phosphate (such as Richard Ross'):

Skeptical Reefkeeping 9: Test Kits, Chasing Numbers and Phosphate

By Rich Ross and Chris Jury There are many standard parameters in the reefkeeping world that aquarists strive to match in their home reefs – water quality,

reefs.com

Rich’s 150 gallon display, on a 300 gallon system, is running a phosphate level of 1.24 ppm, a level at 24.8 times higher than the often recommended .05 ppm. Photo by Richard Ross.

Anecdotally, there may be many reef tanks surviving and several even thriving with less than optimal phosphate levels. Biology can overcome many obstacles when the coral are healthy and other conditions are normal. However, the favored environment for aragonite is seawater without phosphate. So when people have problems with the health of their corals and phosphate is high, it’s easy to see the value in controlling phosphate to reduce the stress on the corals. Similar to how we put saltwater fish in hyposaline water when sick and the opposite for freshwater fish. We want to reduce the work the animal must do to overcome thermodynamics.
I think there is value in understanding the role phosphates play in calcification, just like we talk about magnesium’s role.

The role of pH in phosphate inhibition of calcite and aragonite precipitation rates in seawater

Seeded growth experiments were used to investigate carbonate precipitation kinetics in natural seawater spiked with 5 to 100μmol/kg dissolved inorgani…

www.sciencedirect.com

 

[Randy Holmes-Farley]

Anecdotally, there may be many reef tanks surviving and several even thriving with less than optimal phosphate levels. Biology can overcome many obstacles when the coral are healthy and other conditions are normal. However, the favored environment for aragonite is seawater without phosphate. So when people have problems with the health of their corals and phosphate is high, it’s easy to see the value in controlling phosphate to reduce the stress on the corals. Similar to how we put saltwater fish in hyposaline water when sick and the opposite for freshwater fish. We want to reduce the work the animal must do to overcome thermodynamics.
I think there is value in understanding the role phosphates play in calcification, just like we talk about magnesium’s role.

The role of pH in phosphate inhibition of calcite and aragonite precipitation rates in seawater

Seeded growth experiments were used to investigate carbonate precipitation kinetics in natural seawater spiked with 5 to 100μmol/kg dissolved inorgani…

www.sciencedirect.com

I understand what you are saying, and have cited a variety of such papers in the past, but I just do not know if there is evidence to support the idea that lower phosphate yields higher calcification rates in a reef tank. .

Corals get nutrients very differently in a reef tank than in the ocean, where particulate foods are more plentiful, and over the decades, reefers have learned that trying to mimic the ocean with respect to nutrients is often suboptimal.

 

[Gretchacha]

I understand what you are saying, and have cited a variety of such papers in the past, but I just do not know if there is evidence to support the idea that lower phosphate yields higher calcification rates in a reef tank. .

Corals get nutrients very differently in a reef tank than in the ocean, where particulate foods are more plentiful, and over the decades, reefers have learned that trying to mimic the ocean with respect to nutrients is often suboptimal.

The limitations of texting like this get in the way of complete understanding. I do not mean to imply that low phosphates improve calcification. I posit only that high phosphates can contribute to lower calcification rates than optimal. That is a subtle but important difference.

 

[Gretchacha]

I understand what you are saying, and have cited a variety of such papers in the past, but I just do not know if there is evidence to support the idea that lower phosphate yields higher calcification rates in a reef tank. .

Corals get nutrients very differently in a reef tank than in the ocean, where particulate foods are more plentiful, and over the decades, reefers have learned that trying to mimic the ocean with respect to nutrients is often suboptima

I understand what you are saying, and have cited a variety of such papers in the past, but I just do not know if there is evidence to support the idea that lower phosphate yields higher calcification rates in a reef tank. .

Corals get nutrients very differently in a reef tank than in the ocean, where particulate foods are more plentiful, and over the decades, reefers have learned that trying to mimic the ocean with respect to nutrients is often suboptimal.

I think we are both agreeing that phosphorus does not increase calcification. Allow me to elaborate and clarify. Some level of phosphorus is necessary for biological function, however the specific biological task of mineralization of aragonite is at odds its presence.

 

[Gretchacha]

I’m sorry, my reply was cut short. Let me try that again:
I think we are both agreeing that phosphorus does not increase calcification. Allow me to elaborate and clarify. Some level of phosphorus is necessary for biological function, however the specific biological task of mineralization of aragonite is at odds with the presence of phosphates in the system. Formation of aragonite is inversely proportional to phosphate concentration, therefore phosphate can only ever interfere with aragonite formation. It can never improve calcification. Phosphate levels are one of several factors that corals must overcome in order to create aragonite, which is neither favored nor stable. All we can hope to do with phosphates is to limit their impact on biomineralization while providing enough to support other biological functions.

 

[Randy Holmes-Farley]

I’m sorry, my reply was cut short. Let me try that again:
I think we are both agreeing that phosphorus does not increase calcification. Allow me to elaborate and clarify. Some level of phosphorus is necessary for biological function, however the specific biological task of mineralization of aragonite is at odds with the presence of phosphates in the system. Formation of aragonite is inversely proportional to phosphate concentration, therefore phosphate can only ever interfere with aragonite formation. It can never improve calcification. Phosphate levels are one of several factors that corals must overcome in order to create aragonite, which is neither favored nor stable. All we can hope to do with phosphates is to limit their impact on biomineralization while providing enough to support other biological functions.

I don't agree with some of your assertions above and I'd be reluctant to extrapolate them to setting of desirable vs undesirable phosphate target levels in a reef tank.

i do agree that there is some evidence in the literature that suggests corals calcify slower at higher phosphate, but there is also evidence that they do not do so to a degree important to reefkeeping, and since reefers often elevate alkalinity, they may more than offset any small phosphate issues.

This paper from 2021 shows that the calcification of coral frags at elevated N and P calcified at nearly the same rate as controls (Figure 2a):

https://link.springer.com/content/pdf/10.1038/s41598-021-92276-y.pdf

The concentrations they used:

High N and P: "ammonium and phosphate were both permanently elevated to 14 and 10 µmol L−1 respectively."

That is 0.2 ppm ammonia and 0.95 ppm phosphate

Control N and P:
"During this study, seawater concentrations of ammonium and phosphate averaged 0.55 and 0.32 µmol L−1 respectively across the experimental period,"

That is 0.009 ppm ammonia and 0.03 ppm phosphate.

This is exactly the range of phosphate values one might consider normal for a reef tank (0.03 ppm) and high (0.95 ppm), and yet the drop in skeletal surface area at the end of the study was quite small (23.23 cm2 for the high NP and 26.14 cm2 for the control). Even more striking, the pulsed high N and P (as opposed to the continuous high NP) was even HIGHER than control at 28.27 cm2).

 

[jda]

We should at least note that Ross himself has indicated that the corals that survived in his tank were a subset and many others did suffer and die along the way. He does not recommend po4 levels that high for people who are not willing to accept a subset of tank inhabitants. He has since lowered his po4 levels. People see posts about this tank in singularity and think that po4 does not matter instead of the more important point that it did not matter to those particular corals... with an expert level keeper.

I could send you a handful of acros out of my tank that would mirror that study with virtually the same growth in .03 and .95, but also send many others that would have long died before you hit .95.

FWIW - they used Acropora Intermedia in that paper, which is a pretty easy to grow stag.

 

[jda]

Back to Calcite... does it usually have other traces that corals uptook when they formed skeleton like aragonite does? Many of us have had to switch to calcite to be used in CaRx media and I have not needed to dose any magnesium or anything any differently than when I used aragonite. I have always wondered this and never found a good answer online. Other than needing more co2 to do the same job (usually), it appears to be an adequate substitute.

Edit: does coralline leave calcite or aragonite? I have seen both in papers, but more say calcite.

 

[Randy Holmes-Farley]

Back to Calcite... does it usually have other traces that corals uptook when they formed skeleton like aragonite does? Many of us have had to switch to calcite to be used in CaRx media and I have not needed to dose any magnesium or anything any differently than when I used aragonite. I have always wondered this and never found a good answer online. Other than needing more co2 to do the same job (usually), it appears to be an adequate substitute.

Edit: does coralline leave calcite or aragonite? I have seen both in papers, but more say calcite.

Coralline deposits calcite, and it generally has more magnesium in it (at least when deposited by coralline).

https://onlinelibrary.wiley.com/doi/10.1111/jpy.12559

"In this paper, we explore the ultrastructure and cellular scale Mg-content variations within four species of North Atlantic crust-forming Phymatolithon. The cell wall has radial grains of Mg-calcite, whereas the interfilament (middle lamella) has grains aligned parallel to the filament axis. The proportion of interfilament and cell wall carbonate varies by tissue and species. Three distinct primary phases of Mg-calcite were identified: interfilament Mg-calcite (mean 8.9 mol% MgCO3), perithallial cell walls Mg-calcite (mean 13.4 mol% MgCO3), and hypothallium Mg-calcite (mean 17.1 mol% MgCO3)."

 

[Randy Holmes-Farley]

We should at least note that Ross himself has indicated that the corals that survived in his tank were a subset and many others did suffer and die along the way. He does not recommend po4 levels that high for people who are not willing to accept a subset of tank inhabitants. He has since lowered his po4 levels. People see posts about this tank in singularity and think that po4 does not matter instead of the more important point that it did not matter to those particular corals... with an expert level keeper.

I could send you a handful of acros out of my tank that would mirror that study with virtually the same growth in .03 and .95, but also send many others that would have long died before you hit .95.

FWIW - they used Acropora Intermedia in that paper, which is a pretty easy to grow stag.

I think the interpretation that some corals died off due to phosphate has to be conjecture rather than a demonstrated fact since one cannot know why they died and many folks lose corals over time. It certainly may be a correct conjecture, however.

Let's see if we can get his current thinking: @Thales

 

[jda]

I just re-read some of our correspondences. Beyond limited subsets, slower growth did not concern him but was observed, His po4 levels were down to something like .50ish in 2019... or 4 years ago. He also indicated that many times that he did not suggest that anybody did what he does just to do it.

I agree with all of these things. I don't recommend that anybody keep stuff the way that I keep them just to do it either... and I can find all kinds of reasons of why the best color is with lots of wider spectrum lighting, residual building block levels kept low with high availability, sandbed to chew up no3, fuges and super heavy skimming to keep po4 from climbing, ignoring any parameter except for the big 3 and changing a bit of water with IO. I can grow anything, anytime, anywhere and sensitive inverts breed (or try to) in my tanks and I have super diverse life and microfauna. This is harder and I know that it is not for everybody.

I think that there is zen and a very mature level of self awareness to know what you want to keep and only do what is necessary to achieve it. This will keep many more people in the hobby and happy, most likely. I just started a smaller Z&P tank and I plan to let no3 get to 20-25, if it will, and po4 to .1 to .2. I don't have a sump and have a bad HOB skimmer mostly for gas exchange and I am just going to change water for even calcium and carbonate. This will make this tank more fun for me. However, I cannot do this for my acropora tank.

My main point it that suggesting that the Ross tank from years ago is a sign of success for the masses is as bad of an idea of showing a well-run ZeoVit tank driving residual no3 and po4 well below even ocean levels in the other direction. It is a bad idea on a message board, IMO, since most won't dig in to understand the nuance of why it works. At least in the past, he has agreed with me.

One study on an easy to keep coral is not persuasive, IMO. It is similar to Dana's studies on porites and light when many people kept porites under incandescent clip-on lights from the hardware store. We can all pick a study or two that shows something that we want to see. I had a highly decorated professor in my Engineering School that had like 20 honorary pHd and a few other earned ones once tell us that he could commission a study to prove that your mom was your dad and your dad was your mom... and get it past peer review. His whole point was to stop and think. That old SOB scared the heck out of us when we were 18/19 years old, but I really started to appreciate him later on and moreso now.