Diamonds from Juina province, Brazil, and some others localities reveal the existence of a deep, Ca-rich carbonate-silicate source different from ultramafic and eclogite compositions. In this study, we describe the first observation of merwinite (Ca2.85Mg0.96Fe0.11Si2.04O8) in a diamond; it occurs as an inclusion in the central growth domain of a diamond from the São Luiz river alluvial deposits (Juina, Brazil). In addition, the diamond contains inclusions of walstromite-structured CaSiO3 in the core and (Mg0.86Fe0.14)2SiO4 olivine in the rim. According to available experimental data, under mantle conditions, merwinite can only be formed in a specific Ca-rich and Mg- and Si-depleted enviroment that differs from any known mantle lithology (peridotitic or eclogitic). We suggest that such chemical conditions can occur during the interaction of subduction-derived calcium carbonatite melt with peridotitic mantle. The partial reduction of the melt could cause the simultaneous crystallization of Ca-rich silicates (CaSiO3 and merwinite) and diamond at an early stage, and (Mg0.86Fe0.14)2SiO4 olivine and diamond at a later stage, after the Ca-Mg exchange between carbonatite melt and peridotite has ceased. This scenario is supported by the presence of calcite microinclusions within merwinite.