Tex. Nat. Res. Code § 53.154

The board shall set the royalty rate on production of sulphur, coal, lignite, salt, and potash from land leased under this subchapter. The royalty rate set must be at least one-eighth of the gross production or the market value of the sulphur produced and at least one-sixteenth of the gross production or the market value of the coal, lignite, salt, and potash produced.