Hello knaga18,
There are many different types of clay such as kaolinite, montmorillonite, smectite, illite, chlorite, etc. Each type of clay has its own density. As a really rough guess, use the value 2.6 kg/litre. The porosity of a clay particle greatly depends on the age, pressure, and type of clay. The size of the particle is quite small, and can usually be between 0.1-200 microns.
Could I ask for the specific reason for this question? This is just to help me provide a more meaningful answer.
Thanks,
Eugene