From the worksheet for insulators and conductors (http://www.allaboutcircuits.com/worksheets/candi.html):

"Q How much voltage can a 50 mm thick piece of mica withstand before breaking down?

A Approximately 1000 volts, or 1 kV. "



According to http://www.princeton.edu/~chaol/Rsch/Mica.html and http://www.sciencemadness.org/talk/files.php?pid=190313&aid=11801 the dielectric strength of Mica is much, much greater, around the order of 100kV per mm.
https://www.google.com/#

 

I have some mica caps rated at 1000 volts and they arent 50 mm thick including the outer insulation. please chech your math on the "withstand voltage".

 

The textbooks I have all give different values for mica's breakdown voltage.

The values range from 500-3000 RMS A/C volts per .001", . Different types of mica have different insulating values based on the variations of metallic compounds in the mica. To insure safety and freedom from equipment failure all the texts recommend a large safety factor of 5-10 in design applications. The clear/lightly yellow variety is slightly better in all the tables of material breakdown voltage points.

If the mica is located where oils and greases can affect it, the safety factor should be increased.

Air under a pressure of 5-20 atmospheres and dessicated is the best insulator in the lists.

 

I think the OP's post was about the error in the link.

Off by a factor of 10,000 or so?

 

I have some mica caps rated at 1000 volts and they arent 50 mm thick including the outer insulation. please chech your math on the "withstand voltage".

You also have to distinguish between DC voltage and R.F. voltage. Peak R.F. voltages need to be derated by at least 2.8:1 over DC...sometimes more.