FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hello,
I acquired this beautiful Brazilian smoky quartz crystal in Germany 40 years ago (!).
I don't know where it came from. Can you help me?
It is translucent and obviously belongs in a display case.
Dimensions: 16.5 cm high. Weight: 2425 g.
This large monocrystal is terminated by two other small quartz crystals growing parallel to the c-axes.
What I find interesting is that the last crystal, the smallest one, is twinned “morphologically” according to the Dauphine law. Such a twin is very frequent by interpenetration, but it is very rare when the morphology of the group highlights it, like the Japanese twin. I don't know of any photos (but of course Dana's drawings).
From a geometric point of view, the Dauphiné twin results from a 180° rotation of the c axis. All the individuals are from the same hand (electrical twinning).
The individuals in this group are identified by the relative position of the positive and negative rhombohedra r and z.
Can we say that each individual in the group is not of the interpenetrating type (polygon mosaic)?
After this initial description, I believe that the growth of the group occurred in two stages:
1) Formation of the main crystal and its lateral outgrowth.
2) Subsequent formation of the last small crystal, much more colorless, still with parallel c-axes but the introduction of a Dauphiné twin on the initial lattice.
I would like to receive Pete's opinion. Thank you.

[Pete Richards]

An interesting post, Roger, and two very nice specimens. Yes, Dauphiné twins. Probably.

As it happens, Carl Francis and I have just published an article in Rocks & Minerals on Brazil-law twinning, and have a companion article on Dauphiné twinning in press. In both of these we often struggled to decide whether a particular specimen was twinned or not.

Dauphiné twinning can be a complex mosaic of small inter-woven segments or can relate major parts of crystals to each other, but most of the criteria we use to recognize the twinning are themselves conventions that are hard to prove in a particular case. r faces are presumed to be the larger faces on the terminations, but plenty of examples can be found where this pattern does not hold true even in the six faces on a single termination. We assume the x face tells us the hand of a quartz crystal, and that repeated x faces show twinning, but there are three other forms of quartz described in the literature that are in complementary geometric positions, and are considered rarer than the x face.

We are stuck in a situation where we common collectors have to make our interpretations without the possibility of certainty in most cases. I finally became comfortable with the fact that such interpretations ultimately are probability statements, and will sometimes be wrong (but no one will probably ever find out).

Roger's photos show two kinds of Dauphiné twins. The first is a contact twin in which the twinning is revealed by the opposite placement of large and small - i.e. r and z (we assume) - faces on the parallel terminations. The pattern is sharp and consistent, and I do not have too much trouble accepting this as a Dauphinê twin. When during growth the twinning occurred is not clear to me, though it might have occurred when the termination split.

The other crystal shows two large well-developed x faces, on adjacent prism faces. This is an indication of penetration Dauphiné twinning. It is not consistent with an untwinned crystal - unless one of the x faces is not really an x face but one of the equivalent rarely-seen faces. The similarity of surface texture on the two faces argues that they are the same crystallographically and therefore this is a twin. One would be more comfortable with this interpretation if indications of the twin boundary could be found, perhaps in the form of delicate lines interrupting striations on the prism faces, or other subtle difference in surface luster. On a crystal like this, at least some of the rhombohedral faces are likely to be partly r and partly z, with twin boundaries along the edges or somewhere within the faces.

There is strong evidence from many studies related to industrial uses of quartz that most quartz crystals are Dauphiné twins or Brazil twins or both. Untwinned crystals appear to be rare. One reason that the ideal pattern of big r faces and small z faces is often not well seen could be that the faces have both r and z components, and therefore all faces tend to grow at about the same rate.

Much more could be said; I hope others will chime in!

[Bob Carnein]

Pete, Can you possibly send a reference (for those of us who are twin fans but not experts) to a Dauphine contact twin? If there are two different "Dauphine" twin laws (contact and penetration), why are they both given the same name? This is brand new to me and adds a layer of confusion to what I thought I knew..

[Pete Richards]

Frondel, in Dana's System of Mineralogy 7th edition vol. 3 pag3 81 figures 4 and 5 illustrates Dauphiné contact twins. In these, there are two crystals in contact along their lengths, with their c-axes parallel. A true parallel growth would show the terminal faces of r and z in parallel orientation. In a Dauphiné twin, one individual is related to the other as via a rotation of 180° about the long axis. In this situation, r faces on one individual are parallel to z faces on the other individual, and vise versa.

[Bob Carnein]

Thank you. I'll check that out.

[Bob Morgan]

Amir Akhavan wrote a well-illustrated article that reveals how to identify r from z faces using the features on rhombohedral faces as well as how to distinguish right-handed from left-handed. It's in Mineralien Welt 6/2021. It also determines Brazil twinning in some cases.
Not all crystals show the necessary features, but those that do are interesting to 'read'.