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I notice that most optical calcite for sale seems to be from natural (mined) sources.

Also, I know in World War 2, mining optical calcite was a strategic objective.

Is there some reason why high-grade optical calcite cannot be lab grown?

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2 Answers 2

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The crystal structure is trigonal, so has very low symmetry and high chance of twinning or other line defects forming during, say, Czochralski type growth methods. My guess would be the good optical quality stuff sat and annealed in situ for a long time to clear most of the twins/dislocations out.

Support for this comes from A.J. Gratz et al., Geochimica et Cosmochimica Acta 57 491-495 (1993) where they observed, with AFM, calcite growth occuring primarily on screw dislocations.

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This paper from 1974 seems relevant:

Optically clear single crystals of calcite up to 2 cm in length have been pulled from a Li2CO3 flux in the range of 50–75 wt% CaCO3 under a pressure of 1–15 atm of carbon dioxide. The necessary phase equilibria for this growth was experimentally verified. The best growth parameters were a pull rate of 0.25 mm/hr with a rotation rate of 28 rpm. Seed orientation and perfection are critically important.

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