Solubility and local structure of transmission elements in calcium silicate compounds has not been well understood. We investigate the local structure of chromium ions dissolved in merwinite (3CaOMgO2SiO(2)) of a monoclinic crystal structure. The acceptance of doping elements into merwinite has not been reported before. We found that chromium ions are soluble in merwinite in air and that chemical valence of the dissolved Cr ions varies with annealing temperature. The absorption edge in the x-ray absorption near edge structure (XANES) of Cr-doped merwinite indicated that octahedrally coordinated Cr3+ ions were mainly formed when annealed at 1673K in air. A pre-edge peak was also detected, indicating the existence of tetrahedrally coordinated high-valence Cr ions. Conversely, through annealing of merwinite at 1123K in air, tetrahedrally coordinated Cr6+ ions were found to be the main form of chromium. XANES spectra simulated by first-principle calculations were used to explain the structural features in the observed spectra. We propose the coexistence of Cr3+ ions in octahedral Mg2+ sites and high-valence Cr ions in tetrahedral Si4+ sites. In addition, a change in the chromium ion oxidation state in tetrahedral coordination sites was suggested by XANES spectroscopy of Cr-doped merwinite synthesized at 1673K and reannealed at 1123K.