FMF - Friends of Minerals Forum, discussion and message board

[Bob Carnein]

One of my special enthusiasms is twinned crystals. Despite the fact that it's a bit on the homely side, I recently purchased this aragonite twin that had been in the Kay Robertson collection. The label says it's from Molina de Aragon, Spain. When I received it, I was surprised to find that it has "drill bit" terminations (it's doubly terminated), unlike other Spanish aragonite twins I've seen. I have not found photos of any similar aragonite from this locality on mindat, but Kay Robertson's catalog number is on both the label and the specimen, so I'm sure the label goes with the crystal. Does any Forum member know whether the locality is plausible? How common are such twinned aragonites?

[Pete Richards]

That's a strange one! Most aragonite twins (trillings, really) do a pretty credible job of mimicking six-fold symmetry, but this one does not. It's not really clear how many lobes there are, and the overall appearance is more triangular than hexagonal.

I wonder if there are ever aragonite contact (vs. penetration) trillings?

I hope someone who knows the Spanish aragonites better than I do can help sort this out!

[rweaver]

One thing to keep in mind with the Kay Robertson collection is the fact she was collecting long before most of us were born.This would have led her to be able to collect from locations no longer around today. I knew Kay from spending time at the MSSC shows over the years and she always (from what I can remember) put in some very interesting cases at the shows.

[Bob Morgan]

Interesting!
In Crystal Habits of MInerals, Kostov states both contact and penetration twinning (p 324).
He also has crystal drawings of both along with a molecular drawing of the contact twin boundary (p 37).
In Goldschmidt's Atlas check out drawings #22, #276, and #279.
So there should be more.
As a fellow collector of twins I'll be looking for another.

[Jordi Fabre]

The Aragonite was named in 1797 by Abrahan Gottlob Werner for the type locality, the village of Molina de Aragón, Spain. That is the main place for these unusual twins. That's not so well known, that's why Kay Robertson had a wrong label, labeling it as from the most popular place of them. So, your Aragonite most probably is from Molina de Aragón and probably from a place named "Morro Gorrino."

[Johan Kjellman]

Actually, your twin resembles very much figures 234 and 235 on plate 86, the last one in Haüy's atlas from 1801.

cheers

[Bob Carnein]

Thanks. Can you possibly scan that figure? Or is it available on-line (I did a search but was unable to find it).

[Johan Kjellman]

[Jordi Fabre]

[Bob Carnein]

Thanks, Johan and Jordi. What I'm seeing in the figure appears to have 2-fold symmetry (2 crystals intergrown). My specimen definitely has 3-fold symmetry around the twin axis. The forms present appear to be similar to those in the figure (essentially 2 sets of 4 prism faces), but my specimen is more like the "normal" aragonite cyclic twins with 3 crystals intergrown at 60/120 degrees..

[Bob Morgan]

Fig. 36 in Goldschmidt is one of Huay's drawings - his fig. 234. I'm attaching a poor quality photo of the drawing.

[Bob Carnein]

That looks like a match! Thanks.

[Johan Kjellman]

[Johan Kjellman]

Also, note that Haüy always publish individual images of the separate contributing crystals for the twin formations - each one in its actual orientation. In this case, figures 236-239.

cheers

[Bob Carnein]

Aha! Now I see why both Fig. 237 and 238 are present in the plate. Unfortunately, I can't see the "groove" clearly on my crystal (there's a sort of "messed up" area where Kay Robertson's (and I) put our catalog numbers.