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Heat changes some minerals - how?
  
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Tom Mazanec




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PostPosted: Jan 29, 2021 10:32    Post subject: Heat changes some minerals - how?  

How does heat change a mineral? It should not change the chemical composition (unless the stuff burns). If it is temperature that does it you would expect the change to revert when it cools.
What happens when heat alters a mineral?
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lluis




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PostPosted: Jan 29, 2021 10:39    Post subject: Re: Heat changes some minerals - how?  

Dear Tom,

In case composition should not be altered, then it is simply a change among different forms stable at different temperatures....

If you raise the temperature, you would get the form that is stable at that temperature..
Change could be quick or slow, depending on species (for tin, it is slow,.,.. and is called tin cancer.... That, as far as I know, is not reversible (caused by low temperatures....)

With best wishes

Lluís
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Peter Megaw
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PostPosted: Jan 29, 2021 11:28    Post subject: Re: Heat changes some minerals - how?  

Tom...depends a lot on the mineral...in some cases changes in structure may be reversible when it cools off...although in some minerals the changes might be metastable and the reversion might take a long enough time that they appear permanent.

But for virtually any hydrous mineral, and there are lots, heating will drive off some or all of the H2O or OH and you will wind up with a new mineral....or a decrepitate. Some of these reactions are reversible, but not many.

A great example is all around us. Gypsum CaSO4 2H2O is mined and then heated to drive off most of the water....the product is Plaster of Paris. Adding water creates a paste we can mold into whatever shape we want and when it sets it is close to being gypsum again. Sandwich it between two sheets of paper and they call it sheetrock or wallboard.

Portland cement is another everyday example of what happens when you heat a mineral...in that case calcite CaCO3.

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Jesse Fisher




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PostPosted: Jan 29, 2021 11:29    Post subject: Re: Heat changes some minerals - how?  

Heating minerals below their melting point can induce any of a number of changes, depending on the structure and composition of the mineral. With minerals with structural water (zeolites for example), heating to a certain point will drive off the water causing the structure of the mineral to alter or break down. For minerals who's color is dependent on structural defects rather than chemical chromophores, heating can anneal these defects and bleach the color out of the mineral. Examples would be amethyst, kunzite, sherry-colored topaz, and many fluorites, to name a few. If the mineral contains any fluid inclusions, heat will cause the inclusions to expand, fracturing the crystal. This is quite common with fluorite and quartz from low temperature deposits. There are likely many more examples that others will think of.
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Riccardo Modanesi




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PostPosted: Jan 29, 2021 11:49    Post subject: Re: Heat changes some minerals - how?  

Hi to everybody!
Some years ago in Idar-Oberstein my work group and myself made an experiment with three gemstones: a brownish topaz, a gray zoisite and an amethyst quartz. After heat treatment they turned respectively to a rose pink topaz, a vivid blue tanzanite and a citrine quartz. Heat treatment is a very common one, as the biggest majority of us should know.
Greetings from Italy by Riccardo.

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lluis




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PostPosted: Jan 29, 2021 13:28    Post subject: Re: Heat changes some minerals - how?  

Dear Peter

If mineral lose water, well, maybe pickish chemist, but changed also composition,
Which is one of the things Tom said that should not be..
Loosing water, reduction, oxidation are things that may happen on heating. But all them, well, changes composition. Changing from quartz to cristobalite, same composition, is only done heating (same as with achantite and argentite....)

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Lluís
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alfredo
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PostPosted: Jan 29, 2021 15:37    Post subject: Re: Heat changes some minerals - how?  

Not only water can be lost on heating. When Bolivian sodalite was first heated for analysis, the weight loss was attributed to an "anomalous water content" - until they noticed that it was NaCl that was being lost - They were losing common salt as vapor. Heating sulphates like alunite strongly can result in sulphur oxides or sulphuric acid vapor being lost. Pyrite can lose sulphur and turn into pyrrhotite. So when you say "heating" it's important to state how much heating is being applied. Heating a mineral red hot can certainly induce chemical changes other than just mere water loss.
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lluis




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PostPosted: Jan 29, 2021 15:52    Post subject: Re: Heat changes some minerals - how?  

Dear Alfredo

Losing salt, NaCl, common salt, kitchen salt, by heating, well, it is very hard....
Boiling point for it is 1465ºC. And if in a compound, probably higher....

On the other hand, as far as I understood, question was about changes without changing composition. Losing water, or sulphur, or sulphur dioxide, or whatever, well, is changing composition.

with best wishes

Lluís
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alfredo
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PostPosted: Jan 29, 2021 16:53    Post subject: Re: Heat changes some minerals - how?  

Lluis, The water in the mud puddles outside my house has all evaporated, despite the temperature being much less than 100 C. ;))
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lluis




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PostPosted: Jan 30, 2021 09:24    Post subject: Re: Heat changes some minerals - how?  

Dear Alfredo,

Good point!
But, that is simply vapour pressure... Water evaporates because it tries to get the equilibrium with humidity in air. Done that air moves, it continues evaporating trying to find the equilibrium. If water was placed inside a closed room, it would simply evaporate till equilibrium is reached (that would probably mean, depending on size of room and quantity of water, that a quantity of water would not evaporate.
But talking about vapour pressure of salt, if you take a look at this graph, you will see that it is negligible until around 1200ºC, and then, still very low unless near its boiling point....
https://webbook.nist.gov/cgi/cbook.cgi?ID=C7647145&Mask=4&Type=ANTOINE&Plot=on
(Link normalized by FMF)

Then, to “evaporate” salt, say that it sounds odd to my eyes (just for physical reasons...)
Anyway, may you be so kind to send me a link where I may read the work and conditions?
On the other hand, as I said before, as far as I understood, the question was about changes in minerals through heat without changing composition. Losing anything changes composition, if my comprehension is not that bad...
With best wishes

Lluís
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Riccardo Modanesi




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PostPosted: Jan 30, 2021 13:11    Post subject: Re: Heat changes some minerals - how?  

lluis wrote:
Dear Peter

If mineral lose water, well, maybe pickish chemist, but changed also composition,
Which is one of the things Tom said that should not be..
Loosing water, reduction, oxidation are things that may happen on heating. But all them, well, changes composition. Changing from quartz to cristobalite, same composition, is only done heating (same as with achantite and argentite....)

With best wishes

Lluís


Hi to everybody!
Lluis, you are right! A common example is chalks for blackboards, which are made by heating gypsum at a 120°C temperature.
Greetings from Italy by Riccardo.

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PostPosted: Jan 30, 2021 21:26    Post subject: Re: Heat changes some minerals - how?  

And let's not forget all the carbonates that lose CO2 when heated! Siderite changes into wûstite, magnesite into periclase, aragonite into lime, etc. (Peter Megaw already mentioned previously about calcite turning into lime.)
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