FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hello,
A collection of minerals can become a repository: we look at the specimens without really seeing them.
But if they are occasionally photographed, questions may arise.
In a corner of a showcase, I saw asleep since 1985, this specimen of smoky quartz with “parallel growth”.
Photographing it was the source of my questions:
1) Its origin is unclear: Minas Gerais, Brazil.
2) I don't understand its growth.

The ternary axes are quite parallel.
A very slight rotation of the two crystals exists, as in the Alpine Gwindel quartz.
The r and z faces are not parallel. The position of the edges obviously confirms my problem.
I want help to understand. THANKS.

Note: the quartz structure is nagging to me. During its growth, lattice changes cause many internal twins. Its piezoelectricity is often disturbed.

[Eck Noch]

I don’t think that there is much going on here besides the obvious apparent left handed Dauphiné twin. All other faces are just interpenetrated crystals that eventually absorbed by a faster growing crystal. This would explain why the r and z faces are rotated 90°. Unfortunately, something that would have been really cool to see, would have been if the overtaken crystals had been rusty and left handed Dauphiné twins.

[Bob Morgan]

On the prism faces I think I can see some 'mosaic' splitting, where imperfections in the crystal structure build up and there is slight splitting and subsequent growth of two parts slightly misaligned from each other. A continuation of that process can result in split terminations that are slightly off. It's also not unusual for Dauphine twinning to run through quartz crystals. Finding one where two terminations are in a Dauphine relation is fairly rare.
It's both interesting and lovely!

[Roger Warin]

Thank you all for your responses.
Yet Eck's analysis was striking like a good return from a tennis ball! Thanks Eck. Accustomed to the discreet twins of the Dauphinois Alps, I had never seen this didactic treasure sleeping at the showcase.
This Brazilian quartz comes from an old German collection. His dealer hadn't seen the twin either.
Bob insists on the subtle variations of the "siloxane" bonds of this tectosilicate. For nothing, the initial network is disoriented by a random deposit of the material of the twin.

Quartz is really a problem and I believe that an ideal (piezoelectric) crystal is very rare. It is really paradoxical compared to the definition of the notion of crystal required by the IMA.
This seems to me to be due to the simultaneous appearance of α-quartz and moganite (and not beryl morganite) recently discovered in chalcedonies and cherts.

“This SiO2 modification is not uncommon as a minor component of finely crystalline chalcedony (as well as chert or flint), although some samples may contain up to about 50 vol.% of moganite”. (Mindat).
But this moganite is unstable and quickly transforms into α-quartz.
Thank you all for opening my eyes.
Soon I will ask you another question about an ideal twin from Japan, as drawn in Dana's handbook.
Note: it seems useful to me to test the presence of moganite in Libyan Desert glass.