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EducationPOTD: Cinnabar crystal shape

10th Oct 2017 12:18 UTCHerwig Pelckmans

Dear all,

Looking at the POTD,

https://www.mindat.org/photo-816500.html

I have a hard time figuring out how this can be a trigonal crystal ... sure looks more like orthorhombic to me!

But then looks can be deceiving ...

Cheers, Herwig

MKA (Belgium)

10th Oct 2017 12:46 UTCAlfredo Petrov Manager

I have a flattened "pseudo-cubic" quartz crystal that looks exactly that same shape, so if it's possible for quartz, it must be possible for cinnabar too. But yes, you're right, it is unusual.

10th Oct 2017 15:04 UTCHarold Moritz 🌟 Expert

Keep in mind that while there is only one cube form (with Miller indices {100} and exactly 90-degree angles between faces), there are many rhombohedral forms possible with different Miller indices and interfacial angles (just peruse the crystal drawings of calcite, for example). In theory a form just skewed off of a cube form by only a degree is a rhombohedron but to the casual eye it will look like a cube, as some calcites do (see drawing no. 8 https://www.mindat.org/min-859.html). Then throw in natural distortion and photo perspective and you can get the result in the POTD. But measure the interfacial angles and they will always be correct for the form. Nature is both messy and reliable, but therein lies the fun.

10th Oct 2017 16:20 UTCSteve Hardinger 🌟 Expert

It's about time that gypsum gets the respect it deserves with a POTD. Oh and the cinnabar is nice too.

12th Oct 2017 11:40 UTCHerwig Pelckmans

Dear Harold,

I agree, you are absolutely right about there being many different rhombohedral forms.

I have no problem "seeing a rhombohedron instead of a cube in the photo", so to speak.

What bothers me is the 2 small crystal faces that modify that rhombohedron.

They are oposite of each other and "at the same side of the upper rhombohedral plane".

That does not indicate a trigonal symmetry ...

Cheers, Herwig

MKA (Belgium)

12th Oct 2017 12:38 UTCErik Vercammen Expert

The "upper" small face may be a pinacoid, and the large triangular face may be part of a prism. In that case, there should (in case all faces are equally developed, what is rare) triangular faces on the far left and the far right corners of the crystal.

12th Oct 2017 14:46 UTCJohan Kjellman Expert

Erik may have a point, alternatively the large triangular face is the pinacoid (perspective distorts then the symmetry of the triangular axis) and the surrounding faces comprise the three upper rhombohedra, two of them being not fully develloped, inhibited by the substrate mineral.

cheers

12th Oct 2017 14:59 UTCHarold Moritz 🌟 Expert

Hello Herwig:

Yes, I did notice the triangular faces cutting off the two corners and thought about them cuz they are apparently "unsymettrical". Erik beat me to it! And Johan snuck in a good comment, too, while I was writing this one.

If one of the triangular faces is a pinacoid, then it should be repeated only on the opposite side diagonally through the crystal (the bottom side attached to the gypsum), rather than on the same side where the other triangular face is. Calcite drawing 16 shows this combination of forms. So I dont think that is it, unless this crystal is twinned, but I dont see other evidence of that. I dont think cinnabar follows the same twin laws as calcite.

Calcite drawing 59 shows a hexagonal prism intersecting the rhomb, in which case there are 3 triangular faces rather than 2 on the same side of the rhomb. In line with Erik, I think this particular crystal is just showing unequal development of the faces of a prism form, considering that two of the faces have different sizes and so the third face is just missing (messy Nature again). To know for sure, of course, measurements are needed, otherwise we can speculate forever.

Endless fun with crystal forms!

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