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[Elise]

Hi Pete, Duncan, John and all -

This morning I was reading an entry regarding emeralds in the Abstracts section of the most recent Gems & Gemology, Winter 2012, p. S6 : "Pressure-temperature-fluid constraints for the Emmaville-Torrington emerald deposit, New South Wales,Australia: Fluid inclusion and stable isotope studies." The G&G abstract stated: "The process of emerald precipitation, dissolution, and subsequent re-precipitation was evident in the disrupted, irregular zones shown by backscatter electron and CL images. These images also revealed Brazil-law twinning. "

....and my jaw dropped.

Thinkng that must be a typo, I went to the original paper by the author (see full paper reference below) - on page 292 it states " Brazilian twinning is also observed in the emerald crystal (Figure 7) showing intersectorial zoning, which could also be due to chemical starvation during quick growth of the crystal." The caption of Figure 7 on the same page is: "Photomicrographs of backscatter electron image versus
cathode luminescence image of the emerald crystal. Good correlation between light coloured (CL and BSE) and coloured beryl/emerald zones. Central zoning in centre
of image indicates emerald precipitation, dissolution and subsequent reprecipitation of the more parallel outer growth zones. The intersectorial zoning is also referred to
as Brazilian twinning."

The authors give a reference for "Brazilian twinning", but I did not chase it down (see below).

Is this paper suggesting that there is actual twinning in this beryl and that this "Brazilian twinning" is the same or similar to Brazil-law twinning in quartz? I haven't heard of the term "Brazilian twinning" (to me, this appears to be only color zoning due to composition changes) and if I'm correct that this a twist on the accepted term "Brazil-law", then I hope not to perpetuate a twisted term, but rather help halt its further usage.

I am fascinated by Brazil-law twinning and by quartz, but my understanding of how this very complex phenomenon is triggered and its formation doesn't jive with its possibility in beryl. ( some references on the bottom of my essay here http:(slash, slash)www(dot)nordskip(dot)com(slash)ametrine(dot)html) Is there any other mineral known to exhibit Brazil-law twinning?

Any insights welcome!
Elise


Pressure-temperature-fluid constraints for the Emmaville-Torrington emerald deposit, New South Wales,Australia: Fluid inclusion and stable isotope studies.L. Loughrey, D. Marshall, P. Jones, P. Millsteed,and A. Main, Central European Journal of Geo -sciences, Vol. 4, No. 2, pp. 287–299.

Author's reference for "Brazilian twinning" Ramseyer, K. and Müllis, J., Geologic application of cathodoluminescence of silicates. In M. Pagel, V. Barbin, P. Blanc, and D. Ohnenstetter,Eds., Cathodoluminescence in Geosciences, Springer-Verlag, Berlin, 2000, 177–191.

[Pete Richards]

Elise,

I have no idea what "Brazilian twinning" is, and I am unable to access the journal that apparently contains the definition. I don't find anything on the web.

In any case, Brazil-law twinning in quartz is twinning by reflection across {11-20}. In beryl, {11-20} is a mirror plane in the beryl structure, so it cannot function as a twin plane. Reflection across {11-20} merely reproduces the structure - it does not produce anything new.

So if that's what Brazilian twinning in beryl is supposed to be, it's bogus.

In response to your other question, Brazil-law twinning could occur in other minerals with sufficiently low symmetry. However, such names usually just apply to given species, so even if another mineral had twinning by the same geometric relationship, it probably would not be called Brazil-law twinning.

[Roger Warin]

About Brazil Quartz twin.
I think there is a big difference in crystal structure between quartz and beryl.
Quartz is built on a three-dimensional framework (SiO2)n. For silica, variations in temperature and pressure easily lead to different crystalline phases, but they are still silica. Quartz is the low temperature phase of crystalline silica.
Twins often appear in the upper part of a domain of a phase stability. Disruption of the lattice may occur.
The Brazil twin is also called the Chiral Law. Therefore, in the structure, only the quartz chirality changes.
Starter is accustomed to seeing such twins formed from joining or penetration of two or more individuals. The Brazil twin is very common, but as internal twinning. In this case, individuals are not associated, but the inner zones of the crystal have different optical orientations. Composition planes of the Brazil twin separating the various fields become curved surfaces.
Roger.

[Duncan Miller]

Is this a morphological Brazil-law twin? It appears to have x-faces in both upper corners of the prism face.

[Pete Richards]

The morphology is consistent with Brazil=-law twinning, but is not compelling evidence for it. The crystal should be examined in light reflected from the various faces to look for evidence of differences within a face in its luster, which is indicative of a twin boundary. Dauphiné twin boundaries tend to be curvy, while Brazil twin boundaries tend to be marked by straight boundaries.

There are several problems with identifying this as a twin just based on the two little faces. One is the question of whether they are really x faces or whether they are s faces; this could only be resolved by goniometric measurement, since only one (x or s) is present. If they are s faces rather than x, they are symmetric across the prism edge, so their presence on two successive edges is indicative of twinning, but not diagnostic for which kind, Dauphiné or Brazil. To make matters worse, these faces could be present even in the absence of twinning - such faces are reported in the literature but are much less common than the true s and x faces.

So the frustrating reality is that this could be a Brazil-law twin, but the evidence of the two little faces is only suggestive, and confirmatory evidence is probably not available.

[Duncan Miller]

Hello Pete - Thanks for your very prompt response. The crystal does have natural etching. Here is a photograph showing the opposite prism face. Curvy or straight, or should one use magnification?

[Bob Morgan]

I doubt very much that this is a Brazil law twin. The two faces you propose as x faces are actually s faces. (See Drawing) They look like x faces because they are adjacent to large steep rhombohedral faces (the in between green faces).
With s faces on adjacent corners this is certainly a twin. As Pete said it could be either. Since clear evidence of Brazil Law twinning is lacking, it's more likely a Dauphene Law twin.

I'm sorry I can't get the drawing in this post. FOM doesn't allow the file.
Maybe Pete could tell me how to post a drawing.

[Pete Richards]

I thought maybe I saw that in the first image you posted, one the left side....

Those look like Dauphiné twin boundaries to me, sorry to say. Dauphiné-law twins are much more common, or much more commonly recognizable, than Brazil-law twins.

[Bob Morgan]

Duncan,
I just saw your second photo. That's definitely the sign of Dauphene Law twinning.

[Duncan Miller]

[Kevin Conroy]

Hi Pete,

It's been a LONG time since I sat in a mineralogy class, but for the life of me I can't see a difference between the Dauphiné and Brazil twin drawings. Could you please point out what I should be looking for?

Cheers,
Kevin

[Bob Morgan]

Kevin,
You got the point. With only s faces they wouldn't look different. They could be either.

Duncan,
As regards to Dauphene twinning revealed by light etching - it's interesting that when a twin boundary goes around an edge from r to z the etch pattern switches. That's because on one side of the edge it's an r face and on the other a z face.

[Kevin Conroy]

Thanks Bob!

[Duncan Miller]