ABSTRACT Gyrolite (Ca 4 Si 6 O 15 (OH) 2 ·4H 2 O) is a crystalline calcium silicate hydrate which forms hydrothermally and is primarily used as an adsorbent for wastewater treatment. The stability of gyrolite is poorly understood, as well as the potential impact of Fe, which can be present as an impurity in the raw materials used. This study investigates the effect of Fe(III) on the formation of gyrolite at 200°C. The presence of Fe(III) during synthesis leads to the formation of hydroandradite, amorphous silica, a poorly crystalline calcium silicate hydrate with Ca/Si = 0.4, and less gyrolite. No or very small uptake of Fe(III) in the gyrolite structure was observed. The solubility of gyrolite in the absence and presence of Fe(III) was determined through dissolution experiments at 20°C to 80°C. The results show that gyrolite has a higher stability than previously reported, suggesting it may be a long‐term stable phase together with xonotlite at molar Ca/Si ratios < 1.0.