Using hydrothermal processing it is possible to prepare crystalline powders with a controlled particle size, controlled stoichiometry, and in some cases controlled particle shape [1]. The authors have used hydrothermal processing to prepare well-crystallized fine particles of ceramics with a uniform shape and mono-distributed size without compositional fluctuation [2–4]. The preparation of fine crystals with controlled morphology is quite important for designing the physicochemical properties of the material. Gyrolite [Ca8(Si12O30)(OH)4 · 7H2O] is one type of calcium silicate hydrates with a layered structure [5]. Gyrolite is an attractive material as an ion exchanger, because Ca ions in the interlayer sheet can exchange for other cations such as the heavy metallic ions Cd2+, Pb2+, Zn2+ etc. [6]. In a past report, it seemed that gyrolite was first synthesized by Flint et al. in 1938, but later work indicated that the products which they obtained was certainly truscottite, not gyrolite [7]. Using CaO and silicic acid as starting materials, gyrolite has been synthesized hydrothermally at temperatures from 180 to 250 ◦C under the saturated vapor pressure [8, 9], but the products must have not only gyrolite but also some impurities. When the Ca/Si molar ratio of the mixture of CaO and silicic acid was lower than that found in gyrolite, the gyrolite was easily synthesized, but it was not pure because excess Si should be present as amorphous silica or should be taken into the structure of the gyrolite. On the other hand, when the Ca/Si molar ratio of the mixture of CaO and silicic acid was equal to that found in gyrolite, that is, the Ca/Si molar ratio was 2/3, the only phase of gyrolite was quite difficult to obtain because of heterogeneity of starting materials and narrow range of synthesis condition. In this study, using calcium silicate glass, we synthesized pure gyrolite fine particles by a hydrothermal method. The product is expected to have homogeneous composition, because the glass has a homogeneous composition for atomic order. A calcium silicate glass was made from CaCO3 and SiO2 (both are special grade reagent, Wako Pure Chemical Industries, Ltd., Japan). CaCO3 and SiO2 were mixed at a molar ratio Ca/Si= 2/3, which was the same Ca/Si ratio as that found in gyrolite. The mixture was melted at 1600 ◦C for 5 h in air and the melt was quenched by dropping into water. It was confirmed by X-ray diffraction (XRD) that the quenched substance was a glassy phase (CS-glass) without any crystals. Some pieces of glass were ground into a powder with a size less than 38 μm (passing through a 390 mesh sieve). Using the CS-glass, gyrolite was synthesized hydrothermally with stirring at 200 ◦C for 3, 9 and 27 h under the saturated vapor pressure of NH3aq. (pH 11– 12 at R.T.). The XRD patterns of samples prepared from CSglass under the hydrothermal conditions are shown in Fig. 1. The specimens treated hydrothermally at 200 ◦C for 9 or 27 h consisted of gyrolite without any other crystalline phases. It was found that the gyrolite treated