FMF - Friends of Minerals Forum, discussion and message board

[RussEpp]

This large green beryl has a termination that looks like hundreds of small mountain peaks. However, what I find really interesting is that 3 contiguous sides are smooth and 3 have a much rougher but distinctive texture. How could that happen and has anyone seen anything comparable. I believe the beryl is either from Madagascar or Mozambique. The first picture shows the termination and 2 of the rougher textured sides, the second picture a closeup of the texture of the rougher sides and the third picture, two of the smooth sides.

[James Tate]

The surface appears to be possible melt-features to me. I've studied impactites from Africa near the Mozambique border area. This could be a sample involved within a high-energy event process that left a signature of likely impact-related temperature involvement, and mild shock features.

Certainly a possibility...and reason for the odd melt tektite-like texture.

[Duncan Miller]

I think the rough texture is just natural etching of exposed surfaces, with the smoother faces protected by whatever mineral the crystal was embedded in originally.

[RussEpp]

Thanks for the responses, the melting and etching are both plausible and interesting ideas. I initially doubted both, because it had appeared to me that the 2 textures were perfectly aligned with the faces, but after re-examining the specimen realized that the 2 different textures were not precisely aligned with the edges of the 3 faces, actually the smooth texture continued a few millimeters over the 4th face and likewise the rough "melted" texture is not perfectly aligned, (although very close).

[lluis]

Hi, All

I think, if that serves for anything, that what says Duncan, etching, is the answer. Just I maybe differ in I think that could be an etching in preferential faces, as is seen in many other minerals.... Not necessarily covered by another mineral, just preferential etching...

With bets wishes

Lluís

[Bob Morgan]

I concur with etching but not selective etching based on crystal structure. All prism faces are equivalent. It seems more likely that the crystal was partially exposed to the etchant, probably by being horizontal in a fluid with the smooth surfaces above the surface. What dissolution appears on them would be because of crystal flaws as in the deep crevasses of the smooth surfaces. And the edge between smooth faces would be more vulnerable to etching.

What a 'cool' crystal, far more interesting than a uniform beauty.

[John S. White]

Definitely etching. I differ with Bob however as I prefer to think the smooth faces were protected. They were either in contact with another crystal or they were lying upon a surface that shielded them from the etchent.

[Fiebre Verde]

[RussEpp]

[Sante Celiberti]

Definitely, a beautiful and interesting alternative to the usual smooth and lustrous pinacoid.
It's a pity you don't have information on the locality.

[RussEpp]