help with understanding these mineral terms?

Question

1) crystal face
2) crystal form
3) the differences between 1) & 2)
4) twin and twinning. Please include some examples and how they relate to 1) and 2)

Answer 1

1) A crystal face is pretty self explanatory. Say you have a cube. The 6 sides of that cube are also its faces. So a cubic crystal has 6 faces.

2) Crystal form is the same thing as its habit. The geometrical way the atoms arrange its self. For instance, in halite, the sodium and the chlorine arrange themselves in a cubic form or habit always.

3) Already explained.

4) This occurs when two crystals share some of the same crystal lattice points in a symmetrical way. The Wiki article sites some good examples http://en.wikipedia.org/wiki/Crystal_twinning

Now that you have some information to go by, you can start making some inferences about relationships between them. This is upper level stuff here, and if you are a geology major you need to start figuring this out on your own or you'll never make it. Mineralogy is a tough course. Its a weed out course. it requires a lot of effort.

Answer 2

Hi Gideon!

Just happened to be browsing the earth science questions tonight. Geomatic had some pretty good answers... I'll weigh in as well:

1) A crystal face is any well-defined plane that terminates a crystal. Basically, a flat side of a crystal.

2) The term "crystal form" is used colloquially to refer to the geometric shape of a crystal. In crystallography, though, it has a very specific meaning. It's a set of crystal faces that are related to each other by the symmetry operations of a point group. A crystal form can consist of anywhere from 1 to 24 faces, depending on the crystallographic orientation of the faces and the point group you're in. A crystal form can be either open (does NOT enclose a finite region of space) or closed (DOES enclose a finite region of space). Here are some examples of crystal forms so you can get an idea what I mean:

cube (closed)
tetragonal pyramid (open... has no bottom)
hexagonal prism (open... picture a straw with six sides)
tetrahedron (closed)

3) A face is just one plane. A form is a SET of planes that are all related to each other by symmetry.

4) Twinning is essentially when two crystals of different orientations are physically attached, and are related by some symmetry operation. This operation is often either a mirror plane, or a rotation axis. A good example of a mirror twin is in rutile, the mineral I study. A gorgeous example of two rutile crystals joined by a mirror plane is here: http://metafysica.nl/kourimsky_189.jpg

An example of a twin formed by a rotation axis are the famous "carlsbad" twins in orthoclase. The twin axis is the c axis:
http://www.gc.maricopa.edu/earthsci/imagearchive/orthoclase.htm

Twin planes can be described by a specific crystallographic orientation (called the "twin law") by Miller indices, the same as a crystal face can, but besides this, there is no real connection.