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Aragonite "drill bit" twin
  
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Bob Carnein




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PostPosted: Mar 27, 2020 12:56    Post subject: Aragonite "drill bit" twin  

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?


IMG_9163 (2).JPG
 Mineral: Aragonite
 Locality:
Molina de Aragón, Comarca Molina-Alto Tajo, Guadalajara, Castilla-La Mancha, Spain
 Dimensions: 3.5 cm long
 Description:
Aragonite "drill-bit" twin
 Viewed:  8524 Time(s)

IMG_9163 (2).JPG



IMG_9160 (2).JPG
 Mineral: Aragonite
 Locality:
Molina de Aragón, Comarca Molina-Alto Tajo, Guadalajara, Castilla-La Mancha, Spain
 Dimensions: 3.5 cm long
 Description:
"Drill-bit" termination.
 Viewed:  8526 Time(s)

IMG_9160 (2).JPG



IMG_9154 (2).JPG
 Mineral: Aragonite
 Locality:
Molina de Aragón, Comarca Molina-Alto Tajo, Guadalajara, Castilla-La Mancha, Spain
 Dimensions: 1 cm across
 Description:
"Drill-bit" termination on Spanish aragonite.
 Viewed:  8528 Time(s)

IMG_9154 (2).JPG


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Pete Richards
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PostPosted: Mar 27, 2020 13:28    Post subject: Re: aragonite "drill bit" twin  

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!

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rweaver




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PostPosted: Mar 27, 2020 13:48    Post subject: Re: aragonite "drill bit" twin  

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.
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Bob Morgan




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PostPosted: Mar 27, 2020 17:00    Post subject: Re: aragonite "drill bit" twin  

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.
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Jordi Fabre
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PostPosted: Mar 27, 2020 17:17    Post subject: Re: aragonite "drill bit" twin  

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."


Aragonito - Molina de Aragón.jpg
 Mineral: Aragonite
 Locality:
Morro Gorrino, Molina de Aragón, Comarca Molina-Alto Tajo, Guadalajara, Castilla-La Mancha, Spain
 Dimensions: 3,0 x 2,9 x 2,5 cm
 Description:
A good example

Piece and photo: Martí Rafel
 Viewed:  8429 Time(s)

Aragonito - Molina de Aragón.jpg


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Johan Kjellman




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PostPosted: Mar 30, 2020 06:22    Post subject: Re: aragonite "drill bit" twin  

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

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Bob Carnein




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PostPosted: Mar 30, 2020 12:38    Post subject: Re: aragonite "drill bit" twin  

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




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PostPosted: Mar 30, 2020 15:46    Post subject: Re: aragonite "drill bit" twin  

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


Bob,
I just pm'd you the link as it couldn't be posted here. PM me your email if it doesn't arrive

cheers
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Jordi Fabre
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PostPosted: Mar 30, 2020 15:56    Post subject: Re: aragonite "drill bit" twin  

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

Bob,
I just pm'd you the link as it couldn't be posted here. PM me your email if it doesn't arrive

cheers

Yes you can, please check: Links within the message forum

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

try this link:
https://archive.org/stream/traitdeminra05ha#page/180/mode/2up
(link normalized by FMF)

cheers
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Bob Carnein




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PostPosted: Mar 30, 2020 17:31    Post subject: Re: aragonite "drill bit" twin  

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..
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Bob Morgan




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PostPosted: Mar 30, 2020 18:44    Post subject: Re: aragonite "drill bit" twin  

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


Huay.jpg
 Mineral: aragonite
 Description:
 Viewed:  8171 Time(s)

Huay.jpg


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Bob Carnein




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PostPosted: Mar 30, 2020 19:21    Post subject: Re: aragonite "drill bit" twin  

That looks like a match! Thanks.
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Johan Kjellman




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PostPosted: Mar 31, 2020 06:26    Post subject: Re: aragonite "drill bit" twin  

Bob Carnein wrote:
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..


Hi Bob, what you see inHaüy's Atlas are four crystals intergrown "almost" as three. This has to do that three crystals do not suffice to close the full 360 circuit. Hence, two of the crystals, the two front ones on image 234, have a little extra groove (re-entrant angle). But the "quasi-symmetry" is three-fold, which is expressed in figure 235.
I looked in the text as well and Haüy mentions having one such crystal in his collection. I go and work sometimes in Paris with Haüy models. If there will be a next time, I will try and check out Haüy's own piece and hopefully post an image here.

cheers
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Johan Kjellman




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PostPosted: Mar 31, 2020 06:29    Post subject: Re: aragonite "drill bit" twin  

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.

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Bob Carnein




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PostPosted: Mar 31, 2020 09:18    Post subject: Re: aragonite "drill bit" twin  

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.


IMG_9164 (3).JPG
 Mineral: Aragonite
 Locality:
Molina de Aragón, Comarca Molina-Alto Tajo, Guadalajara, Castilla-La Mancha, Spain
 Dimensions: 3.5 cm
 Description:
 Viewed:  8079 Time(s)

IMG_9164 (3).JPG


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Peter Seroka




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PostPosted: Apr 01, 2020 12:19    Post subject: Re: aragonite "drill bit" twin  

Hi Bob

If you let me have your.address by PM, I'll send you a few more of those aragonite crystals with "drill bit terminations" (with visible grooves) from Molina de Aragon. As I am collecting at this classic site almost once per year, there are always a few extra specimens, which I give away to friends and institutions..

The locality, amongst the local people known as Cerro Pelado (bald mountain) at the Rio de Gallo (chicken creek), is about 2 km close to the beautiful city of Molina de Aragon, just next to a poultry-pig farm.

Having been known since more than 200 years, since its first description by the Spaniard Joseph Tuorrubia in 1754 (Aparato para la Historia Natural Espania), the Rio de Gallo-locality near Molina de Aragon has been productive in delivering uncountable specimens of fine, sometimes incredibly lustrous aragonite crystals. Most, however, are the well known "Trillings" from 3-7 cm.
Crystals with "drill bit" terminations are quite scarce; among 100+ "ordinary" twins there is one with "drill bit".
Overall, the locality is still rich and productive, especially after summer, when the grass had been cut by the farmers.

I have a collection of approx. 120 aragonites from the 65 most important occurrences in Spain. If you are looking for other crystals or oddities, please let me know.
Best regards,
Peter Seroka



Aragonite locality Rio de Gallo, Cerro Pelado, background City of Molina de Arago, Guadalajara, Spain.jpg
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 Viewed:  7840 Time(s)

Aragonite locality Rio de Gallo, Cerro Pelado, background City of Molina de Arago, Guadalajara, Spain.jpg



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Bob Morgan




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PostPosted: Apr 01, 2020 13:57    Post subject: Re: aragonite "drill bit" twin  

Peter, what a nice offer! Yes, I'd like one to go with a couple of penetrant ones I've obtained over the years from there.
I'll be seeking an answer as to why the two different habits, and what makes one favored over the other.
Are these rarer ones all 'drillbit' or do some have flattened c faces?
What a beautiful locality only to have a pig operation next door.
My address is:
2711 Mechanics Avenue
Savannah, Georgia 31404 USA

If you ever come this way, We'd love to have you visit.

Bob Morgan
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