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EducationInclusions of chalcopyrite in calcite

14th Mar 2018 18:28 UTCJennifer Cindrich

I found a layer of chalcopyrite crystals in coarse crystalline calcite, that i uncovered to expose the crystals (by grinding and careful etching). I assume they grew on the surface of calcite crystals and have been included while the calcite continued growing.

There are some "giant" crystals, respectiverly skeletons of them, that are outstandingly larger than the others. It is obvious that they have an empty halo around them, where they are at, no small crystals are grown.

Can anyone come up with an explanation for that phenomenon?

I could obseve that effect several times, see the attached photos. On photo 3 is a crystal and a skeleton close together and, logically consistent, the halo is oval shaped.

Thank you all ahead of time for any contributions on this topic.

14th Mar 2018 19:47 UTCMatt Neuzil Expert

How have you been etching? I am not sure what is meant by this halo. If you are using acid, it is possible that you have etched thru where there use to by crystals other than calcite?

14th Mar 2018 21:04 UTCWayne Corwin

I suggest the big ones sucked up all the iron and sulfur around them.

14th Mar 2018 21:08 UTCAmir C. Akhavan Expert

I believe this to be an excellent example of Ostwald ripening.

The bigger crystals grew at the expense of the small ones.

https://en.wikipedia.org/wiki/Ostwald_ripening

The big ones may simply be older and thus larger than newly formed crystals or they might have a structural flaw that promotes growth, like dislocations or twinning.

Both, size and certain structural flaws, will give them an advantage in gathering material.

If the big ones just sit deeper in the calcite, they will be older. They may have started to grow when nucleation rate was low (that is, the rate of formation of molecule aggregates that may grow to crystals), so there are just a few of them.

Later conditions changed, and nucleation rate increased, and many crystals formed. In the vicinity of the already present crystals it is easier to add atoms to these than to form new crystals.

If given enought time, small crystals will even dissolve and the material will get deposited on big crystals.

Very perceptive of you to spot this, Jennifer!

14th Mar 2018 21:30 UTCJennifer Cindrich

Matt,

I did it as follows. First i ground the calcite down to close to the chalcopyrite layer (the crystal faces of a calcite crystal are usually not identical to the cleavege planes). The rest of the coverage i removed with 10% HCl.

With "halo" i mean the empty space around the large crystals.

I'm not sure how to understand your last question. There is no other stuff other than only calcite and chalcopyrite. And yes, i am sure that i did not soved too much of the calcite and no chalcopyrite had fallen off

14th Mar 2018 21:39 UTCJennifer Cindrich

Amir,

Thank you so much for this informaion! Very fascinating! And thank you also for the compliment.

14th Mar 2018 22:54 UTCJennifer Cindrich

"Thank you Amir for your great answer. Indeed, there are actually crystals existing as skeletons only, "eaten" by the others. That would have been my next question :)

Am i understanding right, there must be a kind of diffusion process in the solid state, a matter transport of Cu, Fe and S through the calcite? A balance of vagrant atoms/ions in the calcite lattice? "

15th Mar 2018 00:36 UTCAmir C. Akhavan Expert

I don't think this could work by diffusion inside the calcite. And it doesn't make sense - the chalcopyrite (or the ions) would have to diffuse into the calcite and precipitate at certain spots, and the calcite would have to retreat or else the crystal would crack. You can have a phase separation from a solid solution when a lattice "tolerates" other ions at elevated temperatures. But I doubt that you can dissolve any sulfide in calcite or any other carbonate.

The fact that the chalcopyrite occurs in layers indicates a deposition on a growing calcite crystal face.

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