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Hematite twins (article in Rocks & Minerals Jan/Feb 2018)
  
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Duncan Miller




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PostPosted: May 06, 2018 01:11    Post subject: Hematite twins (article in Rocks & Minerals Jan/Feb 2018)  

This is a question for Peter Richards. I have been reading your beautifully illustrated article with Mirjan Žorž (in Rocks & Minerals Jan/Feb 2018) about hematite twins from the Kalahari Manganese Field, and am a bit baffled. The penetration twin on {001} I understand. The contact twin on {001} you rightly say is more readily visualised as twinning on {010}. What I don't understand is its equivalence with twinning on {001}. Please could I ask you to help with some additional explanation.

Edit: Actually I think I have got it now. With reference to the drawing in Figure 3, one half of the crystal is reflected in (001), which is equivalent to a rotation of 180 degrees around [010]. Is that correct?
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Pete Richards
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PostPosted: May 13, 2018 19:28    Post subject: Re: Hematite twins (article in Rocks & Minerals Jan/Feb 2018)  

Hi Duncan,

Sorry to be so slow in answering your question. But my "patience" allowed you to figure it out yourself, for the most part!

The article is probably guilty of not providing enough detail to make this easily understood. I'm glad that someone tried to figure it out.

Step one is to observe that, in terms of the structural relationships among the twins, a twin by reflection on {001} is equivalent to a rotation of one portion through 180° about [001], the c-axis. In the case of these diamond-shaped twins, one half is related by that 180° rotation to the other half, so it is the same twin law.

The overall morphology, however, suggests twinning by reflection across {010}, Here the article is careless, in saying that this is the same thing as twinning on {001}. IT IS NOT!

At the level of overall morphology it is, but in detail it is not. Later in the article it is pointed out that the triangular decorations (manifestations of the three-fold symmetry of the underlying crystal structure) point in opposite directions on each half of the twin. This is consistent with twinning by rotation through 180° about [001], but not with reflection across {010}, under which operation the triangles should point in the same direction on each half.

I hope this resolves your sense of confusion. You were right to be confused!

Thanks for catching this!

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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Duncan Miller




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PostPosted: May 14, 2018 04:15    Post subject: Re: Hematite twins (article in Rocks & Minerals Jan/Feb 2018)  

Hello Pete

Thanks for responding and your clarification. The inverted triangles are what got me thinking of rotation about [010] = mirror on {001} (which at first I could not visualise at all), but rotation about [001] as you describe has the same effect. It is clear they are not the result of reflection on {010} or they would point in the same direction.

Now, am I correct in thinking the sides of the diamond-shaped contact twins are crystallographically equivalent to second order prism faces (in order for the composition plane to be (010))?

Duncan
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