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Name: David
Status: Other
Grade: Other
Location: KS
Country: United States
Date: June 2008

I have a block of salt, from a salt mine here in Hutchinson KS. Inside the salt is a empty area, like a bubble inside the block, there is a bubble of air inside. It acts like a level. I was wondering if this is rare, and how old that air could be? What would the composition of the air be?

Inclusions in crystals are not spectacularly rare, but they are not common either. Inclusions can occur in many kinds of crystals, including calcite, halite (your salt) and even quartz. You say it acts like a level - so I presume you are saying there is liquid inside. That liquid is a sample of the liquid from which the crystal formed - a solution of salt. The bubble is probably not air, but is more likely to be a near vacuum void, with water vapour as the dominant gas present.

Halite, and many other similar minerals, are produced from strong brine solutions at high temperatures. As the solution slowly cools, the salt is lost from the solution and crystallizes. There is evidence to suggest that the size of the bubble is indicative of the temperature of the salt solution at the time the crystal was formed. There was no air present at the time of the crystal formation, just hot salty water deep underground. The water that is trapped in the crystal will also be hot. When it cools, the water shrinks, and is no longer able to fill its cavity - a bubble is left. This bubble is like a thermometer. Theory is that if you heat the crystal to the same temperature at which it was formed, the bubble should disappear. Testing seems to support this theory.

Nigel Skelton

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