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[Ru Smith]

Does anyone have idealized drawings of chalcopyrite twins to help with deciphering the attached example?

The overall shape is close to a tetrahedron and each of the three observable corners shows an interpenetrating twin with what looks to be a 180 degree rotation relative to the central tetrahedron.

Help welcomed and it would also be good to see more examples.

[gemlover]

I don't have what you want; just wanted to say -- beautiful specimen.

John

[Pete Richards]

It's presumably a twin on (111), a pseudo-spinel twin (this is relative to the old morphological unit cell with c slightly less than a). The modern cell has c about twice a, so the twin law would be (112).

However, I can't figure out the form that accounts for the "flanges", so sorry, no drawing. There is actually similar morphology in calcite, where it is formed by rhombohedra that are twinned on (0001), but flanges from one end extend outward into space to form a very similar habit.

[alfredo]

"Flanged" chalcopyrite penetration twins used to be quite common in various mines in Akita prefecture, Japan. Japanese collectors call these "mimi-tsuki" chalcopyrite, which means "chalcopyrite with ears".

[Ru Smith]

Many thanks John, Pete and Alfredo.

I thought I understood this for a moment yesterday, but then realized that I didn't.

Some more photos.. from various places, with a tetrahedral edge lined up vertically, hoping that this will be illuminating.

If this were an isometric form I'd call the crystals tristetrahedra - look for the triangular striations associated with shallow triangular pyramids. For each of the twin elements at the ends of the tetrahedron edge shown in this view we are looking nearly straight down on the triangular pyramids. In both cases striations on one of the three pyramid faces run horizontally in the picture - these are the faces that form the "flanges".

If I'm right, we should be looking straight down one of the crystallographic axes in this view, passing through the midpoint of that edge.

I'll add similar photos from other localities in a moment.

[Ru Smith]

Here's an equivalent view of a twin from Silverton, Colorado.

[Ru Smith]

One from Naica,

[Ru Smith]

and this is a detail from an old German specimen, showing an individual with nice "ears".

[Ru Smith]

Some illustrations of twinned chalcopyrite crystals in Dana (6th edition). Note the "flanged" form.