How to make blank is a default value for decimal data type in XML schema?

One attribute of my schema is of type decimal. Initially it should be blank(" "). So it throws validation exception. I need to get rid of that. So how to make blank is a default value for decimal data type.
regards,
Ranjan
[235 byte] By [ranjanbaisak] at [2007-11-19 2:43:32]