Please explain in detail. I\'m studying for my mineralogy exam and will have sho
ID: 283808 • Letter: P
Question
Please explain in detail. I'm studying for my mineralogy exam and will have short essays over these questions.
1.Mineral descriptions sometimes describe cleavages using parentheses, (hkl) or braces, {hkl}. E.g., the cleavage for chrysoberyl is described as “good [011], poor (010)”. Why do we use two different notations.
2.We use unit cell parameters to describe the shape of a unit cell. What are these parameters. Why do we need fewer unit cell parameters for some crystal systems than for others?
3.When using indices to describe lines within a unit cell we divide through to get the lowest index (e.g., direction [120] = direction [240]) why do we do this? But for miller indices describing families of planes within unit cells, we don’t divide through (e.g., plane families (120) vs (240)) Why not?
Explanation / Answer
1) ( ) is used when Miller indices are used as face symbols .
{ } are used for crystal form (a face + symmetric equivalents )
[ ] represents a zone axis in the in the crystal..
The Miller indices when used as face symbols are enclosed in parentheses, as the (111) face for example.
Form symbols are enclosed in braces, as the {111} form for example.
Zone symbols are enclosed in brackets, [111] for example and denote a zone axis in the crystal.