[Rockhounds] Re: How to make Fluorite octahedra?

[Kreigh Tomaszewski]

Johan,

You may be able to cut or grind sugar cubes, but you won't be able to 
cleave them. Sugar cubes don't have a regular form to their grains.

Cleavage is determined by the atomic structure, and is a breaking along 
a smooth face at the macro and molecular level.

Let me propose an experiment for you that would work with younger kids 
to explain the concept of cleavage that they could do hands on, and be 
safer than giving them knives and fluorite clubes.

Find a small, clear (to transluscent), square box. Get a bunch of 
marbles or ball bearings. You want to fill the box up, making nice 
straight rows of the balls, that are even in the x, y, and z 
directions. The balls and the box need to be just the right right size 
so the rows are almost tight against each edge of the box. Each ball 
will represent one molecule of CaF2. You will be simulating a fluorite 
cube.

Looking at one face of the box you should be able to slip a piece of 
cardboard (the knife) into the crystal lattice to cleave off an 
octahedron

  . . . . ./. . . . . .
  . . . ./. . . . . . .
  . . ./. . . . . . . .
  . ./. . . . . . . . .
  ./. . . . . . . . . .
/. . . . . . . . . . .
  . . . . . . . . . . .
  . . . . . . . . . .
  . . . . . . . . . .
  . . . . . . . . . .
  . . . . . . . . . .
  . . . . . . . . . .

Repeat for the other three corners from that side. Turn it 90 degrees, 
lop off four corners, and your cube has turned into an octahedron.

When using ball bearings you can attach a super magnet to make the ball 
bearings 'stick' together and carefully remove the section cleaved by 
the cardboard 'knife' to show the cleaved face is as smooth and as 
regular as one of the sides.

Now that I think of it, the risk of having ball bearings all over a 
classroom floor is a lot like handing kids knives. Maybe you should use 
a big plastic box and ping pong balls, and just do a demonstration.

Hope this helps.

Kreigh





On Sunday, Jun 22, 2008, at 17:55 America/Detroit, Johan Maertens wrote:

> I love Dave Barry. His contributions appeared in my Michigan
> newspaper until he stopped. We considered this a great loss.
> When we got covered by what looked like glacier size Lake Effect
> quantities of snow, reading a Dave Barry book gave us sufficient
> courage to run out naked and shovel all the snow of our driveway and
> of all our neighbors (I am making this up, we were actually wearing
> thongs).
> Dave came up with great names for music bands.
> I got only as far as being a member of the "Snow Drifters".
>
>
> Back on topic:
> Required are fluorite and pocket knives en children to make
> octahedra.
> Somehow these three together do not make me comfortable.
>
> I will have them practice first on sugar cubes with plastic table
> knives.
> Note: Axel, there are no Tierlemont-cubes in the USA, mail me some
> "klontjes" will ya..
>
>
> --
>
> Johan Maertens
>
>
>
> -- 
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