FMF - Friends of Minerals Forum, discussion and message board

[kushmeja]

Hello All,

I recently acquired several specimens from the new amethyst find in Windham County, CT, USA. Many of the specimens exhibit complex crystal forms and I could use some help determining the type of morphology / twinning that they have. Some specimens show an onion-like crystal structure with concentric layering at the base of the crystals. Others like this one I've included pics of below, exhibit what appear to be twinning in a near circular pattern, where there are a series of double-terminated crystal faces connected (and overlapping each other) at the sides.

My question is what kind of twinning does the Specimen below show? Would this be considered a gwindel, since the crystals appear to have grown sideways? I'm quite new to crystal morphology, so I'm not sure of some of the terminology and specifics, so please forgive my ignorance if I'm way off.

Also, I apologize in advance for the pictures. I had extreme difficulty photographing this piece due to the complex crystal faces and my lack of skill with a camera.

Thanks in advance for your help. Best,

Kush

[Tobi]

Hi Kush,

I'm sorry that I can't help, but I also have no clue about crystal morphology. But you could contact our site admin Pete Richards ( https://www.mineral-forum.com/message-board/privmsg.php?mode=post&u=1190 ) who's really an expert in that field ...

Tobi

[Pete Richards]

It would be useful to have some knowledge of the geology of Windham County....

Based on my viewing of these images, my best guess is that this is faden amethyst. You can find many images and descriptions of faden crystals and their growth process on the web, I am sure. I have never seen faden amethyst, but faden quartz in general is quite common in certain environments - specifically alpine vein environments. Hence the importance of knowing whether this kind of environment is found in Windham County.

If these are faden crystals, the distortion of habit due to the faden growth process is not as extreme as it often is elsewhere, but this seems the easiest way to explain the "stacked" crystal structure.

Gwindels have a very characteristic twisted morphology, with the twist being around the long axis of the elongated crystal. These do not appear to have such a twist.

In any case, I don't think these crystals are twinned. Twinning does not typically produce stacked or crystallographically inappropriate elongated crystals without some part of the aggregate having a distinctly different orientation from another part. This is a simplification to be sure, but I think it is an appropriate distinction in your case. Neither faden crystals nor gwindels are twins, but rather they are systematically distorted single crystals.