FMF - Friends of Minerals Forum, discussion and message board

[bob kerr]

There's a few double terminated schorls from Erongo that have been around for a while that have two unique terminations. One side is smooth and typical - and the other side can best be described as "rough" or hundred of small termination. See photos.

Is there a name for this type of double terminations?

thanks,
bob

[Matt_Zukowski]

Maybe it is just preferential dissolution of the basal pinacoid.

[Philippe Durand]

Interesting subject
I have a french schorl with the same configuration
and I always ask myself if it is dissolution or primary termination

I remember seeing beryls with this type of termination too

[Herman van Dennebroek]

[lluis]

I think that the "moor heads" tourmaline from Elba (and probably from more places....) have the same shape. As far as I remember and have read, the black caps are not elbaite, but povondraite. And I have also read and told to me personally Dr. Joan Viñals that many black tourmalines named Schorl could very well be povondraites...

If that serves for anything....

With best wishes

Lluís

[Roger Warin]

Hello,
In Schorl, there is no pinacoid (2 parallel faces) but 2 pedions (single face) because there is no center of symmetry in the crystal.
The lower base is called the analogous pole, the upper top is the antilogous pole.

Photos are difficult to interpret. My opinion is that the analogous pole corresponds to the 1st photo, while the antilogous pole would be the second photo. This would be the termination with a parallel growth of fine crystals.
The analogous pole is generally poorer in facets than the antilogous pole.
The analogous pole is generally less steep.
Here is an example described by Lacroix on a tourmaline from Betroka, Madagascar.

[Bob Morgan]

This crystal appears to have developed in both directions at the same time due to its attachment on one side.
Another possible explanation is that an impurity was deposited selectively on the growing face of one end that resulted in splitting of the crystal.

[bob kerr]

My apologies for the poor photo quality. Here's a photo that I think better shows the two terminations. The "smooth" side to the right in the photo is very typical for tourmalines and the opposite termination (rough) to the left in the photo may indeed be many, many smaller terminations. Some thoughts:
- Note about 1/3 of the way from the left of the crystal the sides transition to smooth
- the crystals are typically all the same size and kind of stubby
- assuming the crystal started growing surrounded by the same fluids, something occurred that changed the characteristics of the termination - ??
- is there a technical term for this termination characteristic?

thanks for the thoughts so far,
bob

[Roger Warin]

Yes, I agree.
But we must take into account the kinetics of growth.
The smallest Miller index faces are those with the highest reticular densities (highest atom densities) and are therefore those that grow the slowest (given the need to assemble many atoms to finalize the growing side).
It is therefore quite natural that these faces predominate in crystalline structures.
So, as the analogous pole carries the truncations (Lacroix)
Analog pole: p (10.1) b1 (01.2)
it grows less quickly than the antilogous pole bearing steeper faces with larger indices:
e1 (02.1) p (10.1) e3 (40.3) d2 (21.1) d3/2 (32.1)

In the growth of the crystal, 2 factors intervene,
1) the atomic (reticular) density of the face.
2) the accessibility of the mother solution to the crystallization site, which directly depends on the speed of deposition: rapid growth gives skeletal crystals (eg. snow crystals) favoring the tops and edges at the expense of the core of the face.
It's a logistical problem like in horrible war.
Bob, can you say what is the termination at the top of the little crystals?

Very nice photos of a case that I have never observed.

[bob kerr]

I do not have a real good camera that is capable of close-focus - here's about the best I can do right now.

But - looking at these "rough" terminations through a 10x loop it is clear that all the small terminations are indeed "rough" - either never terminated cleanly to begin with or damaged in shipment or damaged while collecting in the mine.

But it seems a few of the terminations in more protected vugs should be clean if they were clean to begin with - however this is not the case.

bob

[Bob Morgan]

Looking through the photos in mindat, I believe this is a product of skeletal growth in tourmaline.
Schorls from, Namibia show a variety of growth features. Some are a product of very fast spherocytic growth. Others show splitting on one end - photos 536005, 20690, and328789.
Then there are those that have split ends without the ends diverging outward like the specimen you have.
An earlier stage of skeletal growth can be viewed in photos 928103 and854368.
More developed such growth is shown in photos 388512,2378982-4, and 338444.
Then there some resembling your specimen in photos 143213 and 332040.
I suspect there is some etching of the split ends. It wouldn't have to be extensive to render the small terminations as indefinite. I have a similar specimen and the sides of the small crystals are still shiny. Also, light shines moreso off the plane roughly perpendicular to the c-axis.
A more extreme skeletal crystal is in photo 391851.
One indicator that the basic shape is not due to extensive etching is that in photos 431915 and 41146 two crystals side by side show the splitting in opposite directions and matrix nearby seem unaffected.
This sems to be the tourmaline development similar to skeletal quartz with threeway ridges at the termination.

I hope this helps. You have quite a lovely demonstration of this habit.

[Roger Warin]

Hello,
This comment helps me too.
Because we know (although it still surprises me) that the crystallization of pegmatite minerals is very fast.
The unit of the abscissa axis giving the growth rate is of the order of a week and even less. A 1 m long quartz crystal can form in 1 day!
These conditions are favorable for skeletal growth.
The unit on the abscissa varies from milliseconds to 1 million years.
The growth of garnets is slow.

[bob kerr]

I was chatting with Rocko Rosenblatt about these, and he says they're foitite. I looked in mindat and found it described as:

"Rod-like crystals with the common prisms and pyramids of the tourmaline group."

but it doesn't mention these different terminations.

I'm not sure if these specimens form in one direction as schorl and the other as foitite??

Anyway, pretty unique material.

bob

[Bob Morgan]

Several photos of foitite from Namibia show the foitite end as light to white. Also, the site says it takes a chemical confirmation for foitite.
If yours is foitite on that end, the change from shorl would probably be subtle given the relative same composition of the two minerals.
Finally, there are some photos of less hairlike single crystals of foitite and none that have them bunched as on the termination of the shorl without the shorl connection.
A chemical analysis would be the only way to confirm or rule out that option.
Getting that would probably inform a number of collectors who have similar specimens. I hope you will get that.

[lluis]

I correct myself...
When I said povondraite,I wish to tell "foitite"

With best wishes

Lluís