Talc

 

Talc is one of the softest minerals—its hardness defines a ‘1’ on Moh’s scale of hardness. I just learned its structure at the molecular level, and I can’t resist showing it to you.

Talc is layers of octahedra sandwiched in tetrahedra!

Since these sandwiches form separate parallel sheets, I bet they can easily slide past each other. That must be why talc is so soft.

The octahedra are magnesium oxide and the tetrahedra are silicon oxide… with some hydroxyl groups attached to liven things up. The overall formula is Mg3Si4O10(OH)2. It’s called ‘hydrated magnesium silicate’.

The image here was created by Materialscientist and placed on Wikicommons under a Creative Commons Attribution-Share Alike 3.0 Unported license.

9 Responses to Talc

  1. Stasheff, James says:

    octahedra sandwiched in between layers of tetrahedra!

  2. It looks like a disc sandwiched between two bearing mechanisms ! In normal bearings the spherical balls have one point of contact and this looks like the same has been achieved with tetrahedra. No wonder talc is so soft. A very evocative image.

  3. fredkintanar says:

    This image reminds me of phospholipid bilayers, as in the self-assembling membranes of cells, and extended surfaces like the connectome of an organism’s nervous system. Perhaps talc could provide a medium for higher level regularities as well.

    • John Baez says:

      Talc is one of the minerals that form clay. It turns out that in general, clay minerals have a sheet-like structure and are composed of tetrahedral silicate groups and octahedral metal oxide groups. The chemist and molecular biologist Graham Cairns-Smith hypothesized that life originated from clay by exploiting this sheet-like structure:

      • Wikipedia, Graham Cairns-Smith: Clay hypothesis.

  4. Presumably the graphic is meant to show that the tips of the tetrahedra are bound to the surfaces of the octahedra by a potential. Locallythis could be very deep and steep to present the layer of tetrahedra from sliding over the layer of octahedra. What about the layer which is formed faces of the tetrahedra? How are they connected to the next layer? Is there a simple reason why the potential there is not very strong?

    • John Baez says:

      The graphic shows an actual space between layers. I’m not sure how realistic the distance is here. Anyway, I guess the layers can slide easily over each other, but I don’t know why.

      I hope someone who knows more mineralogy comments on this!

  5. Wyrd Smythe says:

    Does it work like graphite then?

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