[Shale, clay, natural deposit or product lease; conditions;

improvement mortgages void.] The commissioner may also execute leases for the mining, extraction or disposition of shale, clay or other natural deposits in or upon, or products of, state lands, not otherwise provided for in this chapter, upon such terms and conditions as he may deem for the best interests of the state, not repugnant to law. Any mortgage upon improvements on any such lands so leased shall be void.