FMF - Friends of Minerals Forum, discussion and message board

[James Catmur]

An interesting article about speed of growth of Dolomite and how to increase it at ambient conditions

https://www.science.org/doi/10.1126/science.adi3690
(link normalized by FMF)

[marco campos-venuti]

The "dolomite problem" does not refer to the difficulty of growing crystals in the laboratory, but to its abundance in nature in low temperature environments since dolomite crystallizes in hydrothermal systems at temperatures higher than 100 °C. This ancient geological problem was solved years ago when it was understood that dolomite precipitates thanks to the presence of a bacterial fauna (Vasconcelos et al, 1995). So the mechanism by cycling the solution does not solve the dolomite problem at all. The authors simply discovered a trick to precipitate dolomite in the laboratory.

Vasconcelos, C., McKenzie, J. A., Bernasconi, S., Grujic, D., & Tiens, A. J. (1995). Microbial mediation as a possible mechanism for natural dolomite formation at low temperatures. Nature, 377(6546), 220-222.

[Roger Warin]

This reminds me of the opposite sequence in an Oregon lake!

[James Catmur]

I am not sure 'possible'='solved' but adds to the discussion. Thank you

https://www.nature.com/articles/377220a0
(link normalized by FMF)

[Pete Richards]

I think it is appropriate to ask "what really is The Dolomite Problem"? I think the term is attractive enough and vague enough that different people may have different answers because they interpret the question differently.

A quick search of the web led to a recent paper by C.M. Pena and others, which states: "Dolomite, CaMg(CO3)2, is an abundant carbonate mineral in ancient rock formations, but it is extremely scarce in modern sedimentary environments. This intriguing observation has been repeatedly referred to as the “dolomite problem” in the geological literature." I do not cite this to establish "the" correct answer to this question, but to suggest that we as mineral collectors may be misunderstanding what at least some geologists have meant by the question.

I think the meaning of dolomite in this question may not be the nice rhombohedral crystals we find in vugs in various geological environments, but the large expanses of dolomite rock - dolostone - which are found in the sedimentary record. These are composed of interlocked sand-sized and finer grains of dolomite in crystals too small to see with the eye and too intergrown to be of much interest to mineral collectors. Dolostones are very often quite porous, which is not insignificant.

In this context, one answer to the dolomite question is that the dolomite of these dolostones formed by slow replacement of original calcite (limestone) either during diagenesis or later. Some would say dolostones are old because the replacement process takes a long time at geologically modest temperatures, especially the ordering of calcium and magnesium in the crystal lattice necessary to form true dolomite. An intersesting point here is that the one-for-one replacement of calcite "molecules" by dolomite molecules involves a decrease in volume, and can explain the porosity typical of dolostones.

Like many aspects of geology, questions like this one can be understood in a number of different ways, each of which may have several different plausible (and sometimes incomplete) answers!

[marco campos-venuti]

Pete I have to contradict you. Your explanation of the replacement of calcite by dolomite during diagenesis was the ancient solution to the origin of dolomite formations such as the Dolomite Mountains (1 km thick without one single crystal of calcite remain). Numerous pieces of evidence have always contradicted this hypothesis and this was the "Dolomite problem". Today it is known that dolomite precipitates as a primary mineral thanks to the binding of the Mg2+ ion on bacterial EPS (Extracellular polymeric substance). Numerous well-documented cases of primary dolomite associated with bacterial faunas in sabkha environments are known.

Bontognali, T. R. (2019). Anoxygenic phototrophs and the forgotten art of making dolomite. Geology, 47(6), 591-592.

[Kevin Schofield]

[Linus]

I like to hear of your three methods of dolomite formation. I'm particularly a fan and supporter of the third method you describe, which I have seen in the field in several important mineral districts - places where a dolomite "front" can be mapped on the scale of 100's of meters, often immediately peripheral to ore-mineral deposition. I know of many geologists who simply write off dolomite presence as a regional effect and I strongly believe that this is not always correct.

[Roger Warin]

The Marche-les-Dames dolomite deposit is one of the purest in the world. However, the composition of the rock is not homogeneous.

[Kevin Schofield]

Following on from my post above, by an odd coincidence an article came up on my LinkedIn feed, pointing out the cover article from the magazine SCIENCE VOLUME 382| ISSUE 6673|24 NOV 2023 entitled "The Dolomite Problem. Resolving an epic geologic mystery".

The short LI abstract says:

Dolomite (CaMg(CO3)2) is a key mineral in stunning geological formations like the Dolomite mountains, Niagara Falls, and Hoodoos (those funny rock-capped spires in Utah). However, Dolomite is essentially impossible to grow in the laboratory. The abundance of dolomite in nature, coupled with its inability to be grown in the lab, is an apparent contradiction that has stymied scientists for over 200 years.

I am pleased that our work in resolving the Dolomite problem was published today as a cover article in Science. ( https://lnkd.in/grPrxE8W [link normalized by FMF]). Not only did we explain why Dolomite can't be grown in the lab, we also explained how Dolomite forms in nature, and furthermore, directly grew Dolomite in a liquid-cell electron microscope for the first time.

The mechanism is counterintuitive--in order to grow dolomite, you first have to *dissolve* it. This dissolution eliminates Ca/Mg disorder which blocks further crystal growth. In nature, Dolomite forms where there are frequent rain or tidal cycles. The freshwater washes away the disordered atoms, and when the water evaporates, the dolomite forms in the recently vacated sites. The new surface is still disordered, but slightly more ordered than before. After hundreds of millions of years, this 'two steps forward, one step back' mechanism can lead to the Dolomite Mountains.

This isn't just a story about geology, it is a fundamental new theory for the crystal growth of defect-free materials. We are already building new collaborations to grow higher-quality LEDs and semiconductors, with fewer defects, by introducing pulses of dissolution during growth.

I think his claim of "solving the problem" is maybe a little hyperbolic, as he's basically restating the reflux/mixing model that has been around (as noted above) for some time. But the final paragraph on defect-free crystal growth is an interesting evolution...