FMF - Friends of Minerals Forum, discussion and message board

[Matt_Zukowski]

The attached pictures show Chalcopyrite epitaxic on Tetrahedrite that I obtained from our fearless leader last Tucson. I welcome any comments (esp from Pete Richards) describing how the various textures on different faces of the tetrahedrite form. What is it about the structure of these two minerals that causes a "basket weave" chalcopyrite pattern on {111} faces and smoothly deposited chalcopyrite on {221} faces of the tetrahedrite?

BTW - does anyone know yet where these specimens are coming from? Someone told me that Bert Ottens was going to look into it.

[Pete Richards]

It's hard to determine why the patterns on the different faces have different textures without seeing the specimen first hand. But generally I would guess that this occurs because the chalcopyrite has a different habit than the tetrahedrite, in such a way that on one set of tetrahedrite faces ({221}) the chalcopyrite faces are parallel, whereas on the other set of tetrahedrite faces ({111}) the chalcopyrite faces are inclined, forming a regular but non-planar surface.

Imagine a cubeoctahedron overgrown epitactically by many tiny cubes. On the cube faces of the host, the overgrowing cubes will present a flat surface that may very well fuse into a continuous plate. But on the octahedral faces of the host, the cubes will all be represented as little "pyramids", creating a rough surface. A tremendous amount of growth would have to occur selectively in the middle of these faces in order for the overgrowing cubes to fill out and become the corner of one large cube. Unlikely to happen!