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Carbonates form on ponds in Oman like ice
  
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Matt_Zukowski
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PostPosted: Apr 27, 2018 11:50    Post subject: Carbonates form on ponds in Oman like ice  

There is an interesting article in the NY Times about using Omani rocks to take CO2 out of the air. Although the article is interesting in its own right, i am posting this because there is a picture of a pond with a carbonate crust that if broken reforms in a couple of days - just as ice would if the temperature remained below freezing. Look for the caption "When the water comes back into contact with air, a thin layer of carbonate hardens across its surface."

https://www.nytimes.com/interactive/2018/04/26/climate/oman-rocks.html?hp&action=click&pgtype=Homepage&clickSource=www.spam.orgheading&module=second-column-region&region=top-news&WT.nav=top-news
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PostPosted: Apr 28, 2018 12:21    Post subject: Re: Carbonates form on ponds in Oman like ice  

I once did some work on a property (in Texas) where the owner had a windmill flowing into a cattle watering trough. He never cut the windmill off and where the water overflowed at one end was a carbonate deposit that was about a meter wide at the base and a meter tall.

It had formed in only a few years. It was very friable though. It is interesting how fast some slow processes may occur.

When i asked why he never cut the water off he said the overflow provided water for wildlife.

Dale
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PostPosted: Apr 28, 2018 17:54    Post subject: Re: Carbonates form on ponds in Oman like ice  

Thinking about this further, i wonder why the carbonate appears to be floating on the water. It seems like this could be either because 1) the carbonate crust is floated by entrained gasses or low specific gravity biologic materials; 2) the brine has a high enough SG; or 3) some sort of mechanical bridging is occurring that suspends the carbonate over the liquid.

Anyone have an idea?
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Vinoterapia




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PostPosted: Apr 30, 2018 09:44    Post subject: Re: Carbonates form on ponds in Oman like ice  

I have seen this happening in small ponds in caves, where the carbonate form a thin crust or "islands" supported by surface tension. I guess that eventually, these flocculate to the bottom.
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PostPosted: Apr 30, 2018 13:02    Post subject: Re: Carbonates form on ponds in Oman like ice  

Vinoterapia - were the crusts as thick as the Omani crusts? Do they look the same?

I boil water in a pot to raise the humidity level in my house during the winter. In years past i have tried to prevent precipitate forming in the pot by pouring out the brine before it precipitated and cleaning. This year i realized that keeping precipitate out of the was not worth it, so i went the other way: I kept adding water to the pot so the brine would concentrate and a couple of times let the pot boil dry. The result is the attached picture.

The precipitate is probably mostly calcite (given the water chemistry and the fact that it fizzes readily), with some reddish brown coloration (iron oxides/hydroxides? siderite?). I am bringing this up because i watched xtal formation as the brine boiled to nothing. The xtals formed a hexagonal framework, not unlike cerussite "snowflakes," with lots of open area within the structure. These particles did float, but that was in a boiling environment with water vapor bubbles to attach to.

My situation is probably very different that the Omani floating crusts since i am forming carbonates through evaporation and in the Omani ponds, the carbonate is forming as a reaction between the water and air at the air/water interface.

This all still leaves the question of why as precipitate forms and agglomerates into larger particles, that these particles do not sink through flocculation as Vinoterapia suggested.



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PostPosted: Apr 30, 2018 16:26    Post subject: Re: Carbonates form on ponds in Oman like ice  

Hi Matt, the problem with the Omani example is that we do not have any measurement or scale to know how thick and extend those crusts are.

In caves, the floating crust does not look as dull as in the article photo, although it could be a difference in the point of view/perspective. As well, in the caves, as the crust grow, they tend to attach to the rim of the pond thickening and developing well-formed calcite crystals.
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AntonioL




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PostPosted: Jul 02, 2018 10:28    Post subject: Re: Carbonates form on ponds in Oman like ice  

Do you think that surface tension of the water might play a factor. I can imagine a scenario where as the first tiny crystals begin to form, that they float due to surface tension. As more crystals are formed and as the crystals grow eventually they interlock and lock into place at the surface of the water. As the layer of crystals is not water/airtight, evaporation of the water below the top layer of crystals continues to cause more crystals to form and grow on the underside of the existing crystals.

Just a thought.
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