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Tourmaline - Quartz epitaxy?
  
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Josele




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PostPosted: Apr 22, 2022 06:48    Post subject: Tourmaline - Quartz epitaxy?  

Although I have never heard about tourmaline-quartz intimate relations, from long time ago I wonder if this could be a case of epitaxy.

Please have a look at these photos and give me your opinion.

Thanks for your attention.



P1250810.jpg
 Mineral: Tourmaline + Albite (variety cleavelandite) + Quartz
 Locality:
Paprok, Kamdesh District, Nuristan Province, Afghanistan
 Dimensions: 6 x 6 x 5 cm
 Description:
Pink tourmaline with hollowed termination and cleavelandite.
 Viewed:  1499 Time(s)

P1250810.jpg



P1250819.jpg
 Mineral: Tourmaline + Albite (var. cleavelandite)
 Locality:
Paprok, Kamdesh District, Nuristan Province, Afghanistan
 Dimensions: 6 x 6 x 5 cm
 Description:
This tourmaline sample has a black core (schorl?) covered by a pink layer (elbaite?).
 Viewed:  1493 Time(s)

P1250819.jpg



P1250807.jpg
 Mineral: Tourmaline + Quartz
 Locality:
Paprok, Kamdesh District, Nuristan Province, Afghanistan
 Dimensions: 6 x 6 x 5 cm
 Description:
Hollowed termination is lined by multiple small biterminated quartz in parallel position.
 Viewed:  1498 Time(s)

P1250807.jpg



T_Q.jpg
 Mineral: Tourmaline + Quartz
 Locality:
Paprok, Kamdesh District, Nuristan Province, Afghanistan
 Dimensions: 6 x 6 x 5 cm
 Description:
C-axis of quartz in not parallel to the tourmaline basal plane but somewhat inclined.

It seems be parallel to the flat trigonal pyramid {101} edge marked in red.
 Viewed:  1504 Time(s)

T_Q.jpg


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marco campos-venuti




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PostPosted: Apr 22, 2022 14:12    Post subject: Re: Tourmaline - quartz epitaxy?  

In a paper published years ago, I suggested the possibility of quartz epitaxy after tourmaline at an angle of 23.5°. I took these data from a group of quartz crystals probably found in Morro Redondo, Minas Gerais, all including a tourmaline crystal with a constant angle between the two individuals.
Can this angle fit with your sample?
marco



figura 6.JPG
 Locality:
Morro Redondo Mine, Coronel Murta, Jequitinhonha, Minas Gerais, Brazil
 Dimensions: 7 cm
 Description:
 Viewed:  1412 Time(s)

figura 6.JPG


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Josele




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PostPosted: Apr 22, 2022 16:54    Post subject: Re: Tourmaline - Quartz epitaxy?  

Marco, sorry I don't see any sign of epitaxy in your specimen. Tourmaline orientation looks like parallel to one of the quartz termination edges but with only one crystal included this is likely a coincidence.

Anyway the orientation is very different in my sample, where c-axis of both species form an angle a little under 90º. In yours this angle of 23º is much more acute.

Likely in my sample there is no epitaxy either although this could explain the parallel growth of the many quartz prisms.



P1250821.jpg
 Mineral: Tourmaline + Albite (var. cleavelandite) + Quartz
 Locality:
Paprok, Kamdesh District, Nuristan Province, Afghanistan
 Dimensions: 6 x 6 x 5 cm
 Description:
What can provoke this parallel growth?

Even in some prism faces there are a few quartz prisms with the same orientation.
 Viewed:  1373 Time(s)

P1250821.jpg


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Pete Richards
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PostPosted: Apr 22, 2022 18:53    Post subject: Re: Tourmaline - Quartz epitaxy?  

Josele, your specimen of quartz on elbaite is certainly interesting. The parallel arrangement of many quartz crystals on the etched elbaite definitely suggests epitaxy.

Is it possible to get some closer pictures? The ones you have posted can be magnified quite a bit, but an even closer view would be helpful.

Is it possible that all the quartz crystals are connected to each other? If so, then it is probably all one crystal, but with a very strange appearance that still begs for explanation (which I don't have).

The biggest problem with identifying this as epitaxy is that there are three equivalent directions in tourmaline, of which you have colored one. But each of these three directions should be equally likely for the orientation of the quartz. Are there any other places where quartz grows in the other two directions?

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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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marco campos-venuti




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PostPosted: Apr 23, 2022 06:02    Post subject: Re: Tourmaline - Quartz epitaxy?  

The pocket contained several hundred crystals, all with their respective tourmaline in the center and with the same orientation. I had the opportunity to measure some of them and they all gave values very close to 23.5 degrees.
Is it possible that this is the angle between the c-axis of the quartz and the edge of the rhombohedron of the tourmaline?
In my sample:
1-Tourmaline grows first.
2-During an alteration process it is dissolved and inside the crystal the quartz begins to crystallize small crystals in an oriented way (this is the case of Josele's tourmaline).
3-The small oriented quartz crystals actually behave like a single crystal. The quartz continues to grow and incorporates the tourmaline with a fixed angle.
It should be noted that in the sample, although we see the tourmaline crystal, analyses do not detect the presence of tourmaline and therefore it is clear that this is almost completely replaced by quartz.
Quartz is a product of the high temperature alteration of the tourmaline.
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Josele




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PostPosted: Apr 23, 2022 16:44    Post subject: Re: Tourmaline - Quartz epitaxy?  

Pete, new photos, two of them photomicrographs of a tourmaline prism face.
Many quartz prisms are in contact but others are not. Some of them have a small but obvious deviation from the general orientation. I think they are individuals, do not belong to the same crystalline building.
All quartz in the sample is in the same orientation, there is no quartz related to the other edges of tourmaline. I must say that my assumption that c-axis of quartz is parallel to a {101} edge is only an approximation based in what I see at naked eye, I have no way to measure properly this coincidence.
I find it hard to believe that there is an epitactic relationship between quartz and tourmaline when there are no references in the literature. Isn't everything already discovered by now?
However, I can't find any other reason either.

Marco, after seeing only that photo I am unable to give an opinion, sorry.



P1260232.jpg
 Mineral: Tourmaline + Quartz
 Locality:
Paprok, Kamdesh District, Nuristan Province, Afghanistan
 Dimensions: FOV: 20 mm
 Description:
Hollowed termination is plenty of tiny crystals.
 Viewed:  1236 Time(s)

P1260232.jpg



IMG20220423222233.jpg
 Mineral: Tourmaline + Quartz
 Locality:
Paprok, Kamdesh District, Nuristan Province, Afghanistan
 Dimensions: FOV: 2.6 mm
 Description:
Botched photomicrograph with a phone microscope camera, FOV: 2.6 mm, one shot, no stacking.

Image of a prism face.

Not all crystals seem be in contact with each other.
 Viewed:  1251 Time(s)

IMG20220423222233.jpg



IMG20220423222325.jpg
 Mineral: Tourmaline + Quartz
 Locality:
Paprok, Kamdesh District, Nuristan Province, Afghanistan
 Dimensions: FOV: 2.6 mm
 Description:
Botched photomicrograph with a phone microscope camera, FOV: 2.6 mm, one shot, no stacking.

Another image of the same prism face.
 Viewed:  1238 Time(s)

IMG20220423222325.jpg


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Pete Richards
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PostPosted: Apr 23, 2022 19:53    Post subject: Re: Tourmaline - Quartz epitaxy?  

Thanks for the close-up pictures. I think they confirm that the quartz is not continuous throughout, which probably means that the quartz grew as several to many crystals oriented mostly the same direction.

In years past I assembled a database of more than 600 species pairs reported in epatictic association. Tourmaline and quartz is not one of those pairs. I am sure it is not comprehensive, but it summarizes what I could find in the literature. I agree - one would think someone would have found it before.

The way to evaluate this question further would be to compare the structure of the silicate framework along that tourmaline edge with that on the prism face of quartz.

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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Josele




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PostPosted: Apr 24, 2022 09:01    Post subject: Re: Tourmaline - Quartz epitaxy?  

Well, then there are very few possibilities of epitaxy in my sample, if any. After all this parallel growth will remain unexplained at the moment.

Thanks for your help.

Pete, I sent you a PM.
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