PURPOSE: To obtain a material for casting dies having superior gas transmittance than casting sand by heating and drying a sepiolite block to make powder, subjecting this powder to further heat treatment to remove water of crystallization, pulverizing, and effecting the reaction with a monohalide silane compd. to change the surface of the sepiolite powder to a hydrophobic surface.

CONSTITUTION: Sepiolite blocks expressed by the formula Si12Mg8O30(OH)4(H2 O)4.(6-8)H2O are dried at 100-300°C to make powder. This sepiolite powder is heated at 450-550°C to remove all of the water of crystallization and to change the powder into a state expressed by Si12Mg8O30(OH)4. Then, this pulverized sepiolite powder is allowed to react with a monohalide silane compd. expressed by the formula in an inert gas atmosphere such as gaseous N2. Thus, one or more parts of OH groups on the surface of the sepiolite after calcination are changed into O-Si(R)3 groups. Thereby, the desired sepiolite having hydrophobic surface can be obtained. The monohalide silane compd. above-mentioned is, for example, phenyldimethylchlorosilane.