Very basic xtallography questions

Bev Vincent MaxDevore at hotmail.com
Mon Jan 10 09:59:13 EST 2000


<gmol at my-deja.com> wrote in message news:8582ug$v6i$1 at nnrp1.deja.com...

> I am a humble programmer with a chemistry background who has gotten
> stuck into a crystallography project (something I know very little
> about).  I have found a couple of really good on-line sites like Bob's
> Rock Shop that explains basics very well, but I am having trouble with a
> couple of concepts (I am going to mix them in with my questions, which I
> know may be have catagoreical mistakes)
>
> 1) To artifically "build" a crystal, all you need is the unit cell and
> space group (right?). Is there anyway you can determine the crystal
> faces (that is what atoms sit in a face) given the unit cell and
> symmetry group?

To build a crystal in theory, you need the unit cell dimensions, space group
and the contents of the asymmetric unit of the unit cell (i.e. tell where
all the atoms are). However, there is no way to determine the crystal faces
from this information. A crystal's habit (description of the crystal shape)
is as much a characteristic of the substance as its color or unit cell,
though the habit may be altered by changing the crystallization method
(different solvent, for example). The only way to determine the faces is
through experimental methods. Typically, modern crystallographers use the
X-ray diffractometer, which provides the cell dimensions and the relative
orientation of the unit cell on the device. The user can then position faces
in the diffracting condition and determine their indices. From this info, it
is possible to determine what atoms sit on external faces. It is also
possible to do this using an optical goniometer, though this method is not
well known these days and may be becoming a dying art.







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