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[Roger Warin]

Butterfly or heart twin, it’s the same twin of calcite. Very uncommon.
The composition plane (mirror plane between the two crystals) is vertical. It’s a cleavage plane of calcite. The two individuals have this cleavage plane {10.1} in common.
This is the rarest of the four calcite twins.
In this specimen, each individual is a prism {10.0} surmounted by an obtuse rhombohedron {01.2}. Very common.
Moreover, the twinned individuals have their c-axes nearly at right angles.
The heart-shaped twin is accompanied by distortion of shape. This one results generally results in suppressing the re-entrant angles during growth.
Roger.

[Ru Smith]

I enjoy these too. Here's another English one.

[Ru Smith]

and a monster, 13 cm, "The Shark Fin", from Dalnegorsk.

[Roger Warin]

Hello
This crystal is very simple to understand. A common rhomboedron {04.1} as secondary face is here predominant and does control the habit. This is much less common.
This negative rombohedron is wearing another very common rhombohedron {01.2}.
Prism {10.0} is present as minor faces.
This twin law is very rare. The composition plane is the cleavage plane {10.1}. This corresponds to the heart or butterfly twin.
Roger.