Trimethylsilyl derivatives were prepared from hemimorphite by the reaction between the mineral and the trimethylsilylating reagent which was the reaction mixture of hexamethyldisiloxane, chlorotrimethylsilane and an organic solvent (the direct method of trimethylsilylation of silicates). The silylated derivatives were analyzed by gas chromatography and the gas chromatographic patterns varied with the kind of organic solvents used in the trimethylsilylating reagent. The fully silylated derivative of disilicic acid, [(CH3)3Si]6Si2O7, was obtained in a high selectivity when acetone or tetrahydrofuran was used, whereas incompletely trimethylsilylated products occurred in a high yield when an alcohol was used as the silylating reagent. A combined gas chromatography-mass spectrometric analysis of the incompletely silylated derivatives indicated that unsilylated silanol groups were esterified with an alcohol. The trimethylsilylated products obtained from hemimorphite by the Lentz method, whose trimethylsilylating reagent was the reaction mixture of hexamethyldisiloxane, hydrochloric acid, water and an organic solvent, contained incompletely trimethylsilylated derivatives even in the reaction system containing acetone or tetrahydrofuran. These incompletely silylated derivatives were considered to be the silanol-containing partial trimethylsilyl derivatives of disilicic acid on the basis of the GC-MS data.