FMF - Friends of Minerals Forum, discussion and message board

[Josele]

Iceland spar and its amazing birefringence have been the beginning of many mineral vocations, mine among them. Children perceive it as something magic, intriguing enough to wake up interest in crystals.
Aside birefringence, this sample also has mystery for more aged people:

Oriented dislocation, low angle twinning or just almost parallel growth?

Comments are welcome!

[Bob Morgan]

Hi Josele,
Yet another interesting crystal! Thanks.

This resembles blocky or mosaic structure, although I've never noticed such in calcite.
The source of blocky structure is an accumulation of imperfections that build up tension in the growing crystal until it splits for further growth. The two blocks are only slightly misaligned but can grow into a few degrees of misalignment, while having an overall appearance of a single crystal.
The boundaries between blocks are rarely crystallographic like these seem to be, although I'm open calcite doing this.
Unfortunately these are cleaved surfaces, or there might be some growth features that would confirm blocky structure.
I don't see twinning to be involved. The morphology is too consistently a single rhomb across the boundary. But perhaps Pete or some of our other calcite enthusiasts can set me straight on this.

[Riccardo Modanesi]

Hi to everybody!
The evident double refraction of calcite is a physical property of this mineral, that's why you see a double line if you put a crystal on a paper with a drawn line on. Another mineral where you can see a strange optical phenomenon is ulexite, the so-called "television rock". In the latter case you see the drawn line getting just under the top part of the stone.
Greetings from Italy by Riccardo.

[Sante Celiberti]

[Josele]

Thanks for the answers. As Sante said, my doubts concern to the dislocation, discontinuity or whatever it is the "anomaly" that crosses the sample.
If it were in a position not related to crystallographic elements it would be clear, just an indiscriminate dislocation. But its alignment with calcite rhombohedron edges makes me think in a more intimate relation with crystallography.

[Bob Morgan]

Another possible explanation is mechanical twinning. Pete Richards should recognize it if it is that.
I looked on Mindat and one photo of such twinning appears to have the same orientation as your dislocation. It is photo 7LQ-RX7: https://www.mindat.org/photo-97424.html

[Josele]

Bob, thanks for notice this, I was not aware of this kind of calcite twinning.

In my sample the only one discontinuity runs parallel to the bisector of the rhomb obtuse angle while in your example multiple mechanical twinning is parallel to the bisector of the acute angle of rhomb, maybe the same but inverted phenomenon. In this Mindat photo can not see its orientation in lateral faces but I suppose also runs parallel to a rhomb edge.

[Josele]

From what I have been able to understand reading related articles, calcite mechanical twinning occurs repeatedly creating multiple lamellar structure. I don't know if this phenomenon has something to do with my sample of Iceland spar.

Well, I will stay with this doubt. My understanding doesn't go any further.

Thanks for your attention.

[Bob Morgan]

Yes, that is the case. Perhaps there is a very small pair of such making the disparity.

In Rocks and Minerals Vol.74 No. 5, Sept-Oct. 1999 Pete Richards has a drawing of a twin on p.311 with "mechanical twin lamelllae" in each individual of the twin. I can't tell if they have the same orientation of your disparity.
Pete should weigh in on this.