Found the answer to my question.
This document does a good job of describing Mifare .
and this post:
Currently using an external writer I am able to write NDEF records to tags that were previously written to by the Nokia 6131.
I am also able to use an external writer to write to a new tag that has its key-a and key-b set to ff ff ff ff ff ff. The problem is the 6131 refuses to read the newly written tag unless its key-a is set to d3 f7 d3 f7 d3 f7 .
So my question is can anybody point me to a Mifare document that explains how to write a new 'key-a' / 'key-b' and what the ACL's are for the keys .