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PhotosDislocation, twinning or...?

20th Nov 2020 18:16 UTCJosé Zendrera 🌟 Manager

Maybe you can help me to understand this crystallographically oriented discontinuity:

Two photos of same face of cleaved Iceland spar showing a re-entrant angle with a well defined and straight "negative edge".

Here discontinuity runs parallel to the bisector of the obtuse angle of rhombohedral face and in other faces also fits with crystallographic elements as seen in attached pics.

I don't know what kind of discontinuity it is:

Dislocation?

Twinning?

Edges are 7, 4.5 and 4 cm in side.

20th Nov 2020 18:18 UTCJosé Zendrera 🌟 Manager

At the backside the edge is a broken line with a small portion running parallel to long rhombohedral edge.

Crossing edge forms an angle of 50.5º (maybe some tenths up/down) with the rhombohedral edge.

Faces at one and another side of crossing edge form a 5.5º angle (maybe a few tenths up/down).

20th Nov 2020 18:19 UTCJosé Zendrera 🌟 Manager

Discontinuity crosses lateral faces running parallel to short edge.

Here angle between faces at one and another side of the edge is smaller, about 1.5º (+/- 0.2º)

In accord with position of discontinuity edge relative to rhombohedron {101} faces, discontinuity seems occur in a plane close to (110).

20th Nov 2020 20:42 UTCErik Vercammen Expert

José Zendrera

20th Nov 2020 20:43 UTCErik Vercammen Expert

Calcite under high pressure can develop twinning, with the twin plane being the "short" diagonal of the romb face.

20th Nov 2020 21:25 UTCJosé Zendrera 🌟 Manager

That's exactly the case, Erik, thanks for your help. I've read a little about mechanical twinning in calcite but I not understand how it works. In my understanding "normal" twinning occurs in the very beginning of crystal growth, in nucleation. Mechanical twinning seems to occur once crystal is already formed, is this correct?

20th Nov 2020 21:46 UTCMark Holtkamp

See http://www.minsocam.org/ammin/AM14/AM14_245.pdf ,

According to fig. 2 in this article, with deformation twinning, the twin plane should be the long diagonal of the rhomb face. Also see

https://www.eng-atoms.msm.cam.ac.uk/VideoFolder/Calcite

If it is a single twin one would expect a much larger re-entrant angle?

21st Nov 2020 00:47 UTCKeith Compton 🌟 Manager

Erik

That was certainly something I didn't know. I'll have to go and look that up.

Ah .. deformation twin (now I remember) ... gets harder to recall/find things in my mind library - the librarian is a little slower with age.

22nd Nov 2020 12:42 UTCJosé Zendrera 🌟 Manager

Calcite

MacDonald Mine, Monteagle Township, Hastings Highlands Municipality, Hastings County, Ontario, Canada

This photo (kindly found by Bob Morgan) shows lamellar structure due to strain twinning. Here it is parallel to the long diagonal but in accord with first paper cited by Mark ( http://www.minsocam.org/ammin/AM14/AM14_245.pdf ) strain twinning can occur also parallel to the short rhombohedral face diagonal.

But this mechanical twinning is polysynthetic, occurs in multiple layers which thickness depends of temperature (please see https://se.copernicus.org/articles/10/307/2019/se-10-307-2019.pdf ).

Although discontinuity in my sample have same crystallographic orientation than twin lamellae I could not find any paper related to only one plane of discontinuity as occurs in my sample.

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