FMF - Friends of Minerals Forum, discussion and message board

[Josele]

Yesterday I uploaded some pictures of this crystal to Mindat gallery. I wrote to Amin C. Akhavan, Mindat supervisor of Nepalese specimens, asking if is acceptable crystallographic identification in this case. I just received the answer: Yes, this specimen is currently approved as albite in Mindat gallery.
I appreciate very much the help of Pete Richards to solve this identification, thank you very much Pete.

[Pierre Joubert]

These are 2 stunning specimens! I love that quartz!!!

[Pete Modreski]

Albite seems reasonable--but the size, extraordinary! I also thought of gypsum when I saw the first photo; perhaps your friend could confirm for sure, that it is not a soft mineral?

I also thought of, perhaps, euclase for this crystal.

Pete

[Josele]

At first, to wish you a healthy and happy new year.

Thanks for your inputs. I will ask Eric to check the hardness.

I was thinking that gypsum is not compatible with alpine-vein type minerals, but I'm not sure if can exist exceptions to this rule. Anyway, gypsum crystallographic parameters are, to me, different than I can see in these pictures.
Euclase yes is compatible with alpine-veins geological setting, but euclase crystal forms are, to me, also different.

I think that a good crystallographic examination of the pictures do not let many space to doubt: it has all parameters very like to a plagioclase and the transparency points to calcium absence. We must also bear in mind that plagioclases are the most common partner of Ganesh Himal quartz.

Fifty years ago, when XRD and microprobe were not easely available, a crystallographic checkup was the best way to identify a mineral specie when the crystal shows enough parameters, why not at present?

[Pete Richards]

I agree that gypsum seems most unlikely, and that euclase is more possible but the typical morphology is rather different from this crystal. My main misgivings about identifying this as albite are the lack of any indication of cleavage, even as minor cracks, and the apparent lack of twinning, which is very commonly present in albite crystals, either as simple contact twinning (on 010) or as repeated twinning.

It is true that fifty (or perhaps eighty) years ago crystallographic examination was one of the main means to identify minerals. But this was done with quite precise measurements made using a two-circle reflecting goniometer, able to measure angles to ± 2' of arc. If we could put this crystal's little brother on a goniometer, we could easily verify whether its interfacial angles correspond to those known for albite. That's a level of precision that cannot be obtained from studying two-dimensional images by eye, however.

[Jordi Fabre]

Is very easy to do a simple hardness test, the Albite don't scratch the glass. As we work only with photos it could confuse, the photos are 2D and the crystallographic measures prefer 3D ;-)

BTW, 50 years ago the mistakes happens frequently and they were corrected later by the modern analytic techniques. Ask to the Museums! ;-)

[bugrock]

Hello,

Do not have expertise in mineralogical devices but once a mineralogist helped me identify a polished nodule using a hand held light refractometer. I came away with the idea that when the choices are few such a devise may be useful. Given the size of the specimen perhaps it could be tested in this way, provided the minerals being considered differ significantly in their RI's.

George

[alfredo]

Josele, I too am convinced that your spectacular crystal is indeed albite, but I must disagree with your statement that plagioclase transparency has any relation to Ca content. Ca-rich plagioclases can be completely transparent, more than enough to use as gemstones, like for example bytownite from Mexico, or almost end-member anorthite from Japan.

[Josele]

Thanks for all comments. I agree with all your opinions:
- is strange there is not any cleavage indicator, nor inner "phantoms" parallel to (001),
- is not possible to take accurate crystallographic measures just from pictures,
- a hardness test will help to narrow the possibilities,
- a refraction test should be very useful too.
Then we must wait to these test or analysis to be absolutely sure, scientifically sure. Nevertheless, perhaps you will agree that as there is very, very, very much possibilities that is albite, is acceptable to name it as albite when noting it is "visually crystallographically identified"

[Josele]

At last there is a test result of this amazing specimen. The test was done in a Japanese gemological laboratory and gives a stunning result (for me): Quartz

I'm not a crystalography expert but I seem to observe that:
- The angles between faces are imposible in quartz (even at naked eye).
- Striation here is parallel to c axis, never seen in quartz.

I think there is a mistake in this test: They have analyzed the quartz supporting the albite crystal.
Or I'm completely wrong...

What you think about?
Very thanks for your opinion.

Can see Test result in a high resolution image here.


Can see HD photos of the specimen here.

[Mark Ost]

So tough to say from a picture but just at a glance if you said quartz for the support I would not blink. But if you say quartz for the clear mineral I would think about it twice or three times. I would try the hardness test as that is fairly indicative and you don't need a lab to do it.

[Josele]

Dear Mark, thanks for your input. This piece is right now in Nepal and I am in Spain. The owner of the specimen is not expert in mineral testing and he do not want to damage this exceptional crystal. That's why he did not a hardness test himself.

Certainly, from a picture we can not do firm statements but spending some time to see the photos on the previous page, I can not recognize a quartz but it seems I'm wrong.

Perhaps somebody can help to clarify this matter.
Thanks for your comment.

[lluis]

Good evening, Josele

I am maybe odd, but I thought always as Jordi.
An odd quartz (meaning odd that we are not used to see it; could be somehow abundant in some places)

I doubt that Japaneses analyzed the support.
I could not read Japanese, but they state " Nippon "gemstone" laboratory ".
If in a gem lab, I would analyze the "gem" portion.

Maybe I am wrong, but I should say that I am not surprised by the labo result.
Maybe I am too naive

With best wishes

Lluís

[Pete Richards]

How unfortunate that we cannot hold this crystal and twist it in any and all directions. If it is indeed quartz, there should be a set of six prism faces with parallel edges (well, part of the set might be missing), they should all have striations perpendicular to their parallel edges, and other faces should not be striated. If these could be identified, we could know the orientation of the crystal and perhaps understand the details of how it is distorted. I see striations on two faces, but cannot determine if they are all parallel or not.

[Pete Modreski]

This has been very interesting. I think it points out the great difficulty in being certain of a "sight identification" of a crystal, especially when it is of an unusual habit and appearance.

I can certainly understand the reluctance to do a scratch test which might put excessive pressure on the specimen--being uncertain of how much stress the crystal could tolerate without snapping off. Especially when the handler is not terribly familiar with 'the feel" of exactly how to do a scratch test.

Wondering about the test by the Japanese lab--does it say anywhere, what method they actually used to make this identification? Just by sight and morphology? XRD? XRF? Something else? Many tests a lab might do would require removing a fragment from the specimen--again, I'm sure they would be reluctant to try to do this.

Looking at the photos, one keeps flip-flopping back and forth from the alternatives. The thin tabular overall shape, certainly looks like feldspar. But quartz can be tabular too--it COULD be quartz! Looking at the base of the crystal, it does not seem continuous with the quartz matrix--it looks like a separate growth of a different mineral. And so it goes! Here is one idea. If someone has access to photographing the specimen, perhaps they could also take a short video sequence—if a camera is available that can do this, close up with good resolution—showing the crystal as it is rotated and twisted to different positions. Such a video clip, sent to some of our crystal “experts” (Pete Richards, others), might lend some new insight. Just a thought.