Calcium carbonate sample -> calcite or aragonite?

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Discussion Overview

The discussion revolves around methods to determine whether a calcium carbonate sample is calcite or aragonite. Participants explore various testing techniques, including chemical reactions and microscopic examination, to differentiate between the two forms of calcium carbonate.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that aragonite reacts more actively with weak acids like diluted acetic acid compared to calcite, questioning if this is a valid test.
  • Another participant inquires whether the sample is powdered or crystalline, implying that the form may affect testing methods.
  • Discussion includes the suggestion of using a modest power microscope (20-40x) to observe crystal forms, as calcite and aragonite have distinct crystal structures.
  • A later reply mentions the need for chemical tests or x-ray tests if crystal structures are not visible, indicating limitations in visual inspection.
  • One participant references a specific chemical test using Feigl's solution as a definitive method for distinguishing between calcite and aragonite.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of various testing methods, and no consensus is reached regarding the best approach to determine the sample type.

Contextual Notes

Limitations include the dependence on the sample's physical form (powdered vs. crystalline) and the potential need for advanced testing methods if visual inspection is inconclusive.

Picasso
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Calcium carbonate sample --> calcite or aragonite?

Is there a definite way to determine whether a calcium carbonate sample is calcite or aragonite? The weak bond of aragonite should produce an active reaction with a weak acid such as diluted acetic (vinegar), whereas calcite will require a stronger acid to produce an active reaction. Is this a valid test?
 
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Picasso said:
Is there a definite way to determine whether a calcium carbonate sample is calcite or aragonite? The weak bond of aragonite should produce an active reaction with a weak acid such as diluted acetic (vinegar), whereas calcite will require a stronger acid to produce an active reaction. Is this a valid test?

A powdered sample or a crystalline sample?
 


Consistency of sand.
 


Picasso said:
Consistency of sand.

Well you can try a look with modest power microscope e.g. 20-40x. If you can see any well defined crystal forms, it should be possible to distinguish by inspection since calcite and aragonite have different crystal forms. If you don't see any well defined crystal structures, then ... I can't help. You need a chemical test, x-ray test, areas I'm not familiar with.
 
Found a definite chemical test using Feigl's solution:

http://www.mindat.org/mesg-100-179708.html
 

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