FMF - Friends of Minerals Forum, discussion and message board

[James Catmur]

This seems to be an open Gwindel from Spain. We found this single specimen in 2007 in a small outcrop next to a vein of quartz. There were other quartz crystals but none like this. The crystal faces are parallel but there is a slight twist between the two crystals, which I only noticed today.

See what you think? Is it a Gwindel or just two almost totally parallel crystals

The locality where it was found is mentioned in Volumen VIII, Cuarzo y otros minerales de silice, Miguel Calvo Rebollar, p 243, top of the page where it says "La Presa".

[Bob Morgan]

Does this specimen have several Japan Law twins?
https://www.mineral-forum.com/message-board/download.php?id=72376

It resembles one that Jaroslav Hyrsl found with many twins.

[Bob Morgan]

The above post was in reference to Enrique LLorenz's Peruvian Specimen at the first of this thread.
https://www.mineral-forum.com/message-board/download.php?id=72376

As to the question about whether the double crystal immediately above is a Gwindel, I don't think so. The attachment would be on a prism face.

[James Catmur]

Bob

The attachment is a prism face - the two are attached to each other by their prism faces. That is why it had me confused - every face in each crystal has a parallel face in the other one, but there is about a 4 degree twist between them. So it looks like two totally parallel crystsls with a twist (clockwise) between them.

[Bob Morgan]

I can see your point and agree the divergence is reasonable as a gwindel. I was referring to the attachment to the matrix, which is usually along a prism face of one crystal. Still...?

[Josele]

[Bob Morgan]

I agree it's not a gwindel, even though it looks like there might be an a axis alignment at some spot. Very open gwindels can have some notched separation, but this would be too extreme.
There does appear to be a point of attachment on one crysrtal's pyramid, but gwindels grow out of the matrix along the a axis.

As to the Peruvian quartz, the possible twinning would be small deposition crystals on larger ones. That is very common from that locality, but can't be discerned from the photo.

[Josele]

Bob, you are right, many characteristics are not clearly visible in a photo, that's why I like to say "I think that ..."

Is true that high end and fine gwindel specimens usually show both terminations free of contact, with "c" axis more or less parallel to matrix surface, but there are also many samples that seem to be attached to matrix at terminations, with "c" axis penetrating into matrix, as can be seen in Gwindel Mindat Gallery

[Amir Akhavan]

Sorry to interrupt ;-) but I'd like to point out that a gwindel is not a composite or aggregate.
It's not made of stacked crystals and it has nothing to do with parallel growth.
Each gwindel started as a s i n g l e c r y s t a l that is elongated along and twisted around an a-axis.
That many gwindels and in particular open gwindels show sutures and that their tips fan out is a secondary effect of being overgrown by a layer of macromosaic quartz.
Amir

[Tobi]

A few more of my personal favourites from my collection that I want to add:

[Tobi]

I guess that quartzes from the Alps are favourites for many quartz collectors. I really love alpine quartzes, so this one is definitely one of my personal all-time favourites:

[Jacqui Parsons]

[Bob Harman]

Blue quartz can be caused by any of a number of inclusions, both microscopic and macroscopic. The blue quartz might come from a wide variety of environments and worldwide localities.

These 2 examples, from an under appreciated situation, were self collected from Indiana, are of microcrystalline quartz, BLUE CHALCEDONY, and apparently derive their color from water existing in the micro spaces between the micro crystalline quartz. If kept dry, they seem to gradually pale out to very pale blue-gray over a period of years. Keeping fresh blue examples under water seems to help preserve the blue color. BOB

[Bob Harman]

From a relatively new find, these hematite included quartz specimens are marketed by the name "red amethyst or pink amethyst". They are from a gypsum mining district in Patagonia, Argentina and first appeared on the mineral market about 2016 - 2017. The quartz varies in color from pink thru bright red to a mauve shade.

I have 3 examples, acquired at the 2018 Denver Show. My 2 larger examples have calcites atop the quartz. The photo is a close up of the examples from my display case at last week's Cincinnati mineral show.

[Tobi]