Hi,
There are some points I really want to clear up in this topic....I promise to finish my chain of doubts as quicly as possible!

I'll put in my first questions....

1. Rotoinversion is a combination of inversion and rotation-- often it ends up as having the same effect on the crystal as reflection or just inversion--why,then is it recognized as a separate symmetry element? I could also put my question as 'what is the 'physical significance of rotoinversion,' since I read that all the symmetry elements have important roles in determining the chemical or physical properties of a crystal'-what is it in case of rotoinversion?

2. Inversion through a centre is also a symmetry element -- it is said that an object having one centre of inversion has one 'roto-inversion axis'--what is the relation between the centre of symmetry and 'rotoinversion'--why are they defined in terms of eachother?

Can an object have more than one centre of symmetry?

3. Why is it not possible for a crystal to have a 5- or 8- fold rotational symmetry axis?

4. Again,translational symmetry elements must have some significance in determining the properties of crystals--firstly,why are they of only two types,screw and glide axes--couldn't we produce any other kind-like rotation followed by inversion?

(I read that "A dislocation may glide only in the plane which contains both its line and its Burgers vector" - Is this the significance of 'glide' in crystal dislocations?)

Please help!

Can an object have more than one centre of symmetry?

No. However, centre will change if object extends into hidden dimensionality.

Thanks, for the answer.

Please try to help me in regard to the other questions also.

Wouldn't any atom, and some more places between, be a centre of symmetry in a crystal?