Alternating layers of calcite and aragonite, precipitated from mineral spring in fracture zones of serpentine mass, occur at Kashio, Oosika mura, Nagano Prefecture, Japan. Element mapping by means of Micro-area X-ray Fluorescence (MXRF) conclusively demonstrated that strontium concentrates more in aragonite layer, and not in calcite layer. Other elements including magnesium show no positive correlation with aragonite precipitation. MXRF analyses of aragonite samples from other localities and origins also indicated concentration of Sr as major impurity component, demonstrating that Sr plays essential role in metastable nucleation of aragonite in the precipitation of CaCO₃ polymorphs from aqueous solution. This can be understood on the basis of modifications of surface energy term in CaCO₃ nucleation in the presence of Sr. Temperature changes trigger to increase the Sr concentration in mineral spring, leading to metastable nucleation of aragonite.