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[Roger Warin]

Hi calcite lovers
Always for beginners.

This crystal is remarkably simple, the main form is the negative rhombohedron {04.1}.
But a complication is added, this specimen is twinned.
Four laws of twins are possible in calcite. That occurring here is the rarest, and much as it does not exist in Belgium, where limestone is abundant yet.

[Roger Warin]

But it’s a twin which we can easily identify the law, simply by observing the internal clivages, many into calcite. Some traces of cleavage planes are parallel to the median plane of composition of the twin.
The composition plane of this twin is a face of a rhombohedral cleavage {10.1}.
The twin can be by contact (reflection) or by interpenetration. In the latter case, a cross is obtained with each individual of the twin being biterminated as the following picture.
Roger.

[Roger Warin]

But on the same specimen, we can also find another interesting twin, the twin {02.1}. In Belgium, this is called a bishop's miter twin.

Enjoy this rare twin law on {10.1} found in calcite from Leiping Mine, Guiyang Co., Chenzou Pref., Hunan Prov. – China.
Let me know if you have any questions.
Roger.

[Don Lum]

Roger, thanks for the pictures, drawings and clear explanation.

Don

[gemlover]

And i can easily see the bishop's mitre. great posting, thanks. i still have great difficulty visualizing crystal planes, your graphics are a wonderful help.

john