FMF - Friends of Minerals Forum, discussion and message board

[Ru Smith]

Rare quartz crystals displaying a c face show evidence of etching. Why does etching produce a flat (though pitted) c face? Who has examples?

[Ru Smith]

Ametrine from Bolivia, with 3 x 1 mm c face.

[Ru Smith]

Quartz from Diamantina with 2.5 x 1.5 mm c face.

[Pete Richards]

I have one example, on an ametrine crystal, and have seen others. The c faces always seem to be pitted, though the reason is not clear. It would be interesting to know if the c faces grow as sharp faces and then are etched, or whether they grow as rough faces in the first place.

[Ru Smith]

Thanks Pete,

I have that same question. Since I've never seen a c face on a quartz crystal that's not etched the suspicion would be that the pinacoid is produced during the etching. I'm just wondering if there's a simple explanation for why that would happen.

[Pierre Joubert]

[Ru Smith]

Agreed, Pierre, that could happen, but in each of the cases shown (and described elsewhere) the face is perpendicular to the c-axis. Hard to see that as a coincidence. Also the Diamantina example shows etching in a broadly similar style (though with varying intensity) on the other faces of the crystal.

[vic rzonca]

I wonder if this lab grown quartz could be considered example of the c face?

[Ru Smith]

Wow! Looks like it.

[Ru Smith]

I just read (Springer Handbook of Crystal Growth) that the c faces in synthetic quartz result from early termination of crystal growth such that the mature form of the crystals is not attained. I had been wondering if relative growth rates in the direction of the c axis were lower in synthetic crystals than in natural, non-seeded ones.