Seller retaining mineral rights

[Joe P.]

I have an opportunity to purchase a 10 acre vacant land parcel at a low price and the seller says he wants to retain 10% of the mineral rights for 29 years.  

I have not yet come across a situation such as this.  Why would he only want to retain 10% and why for 29 years?  Anyone with more experience see any red flags with this situation?  Should I have a lawyer draw up a clause for this to put into my sale agreement contract?