The ammonia diffusion method (ADM) is one of the most widely used strategies for the bioinspired synthesis of minerals. Herein, we present investigations of the mineralization mechanism using an advanced ADM to solve the limitations of the conventional ADM. This allows us to confirm the presence of liquid calcium carbonate precursor species in additive-free and polymer-stabilized gas diffusion systems, indicating that liquid-liquid phase separated species exhibit sufficient kinetic stability to be detected. Time-dependent experiments reveal the role of these precursor phases during the early stages of the crystallization process, demonstrating that liquid calcium carbonate mineral precursors play an important role in the precipitation pathway and must be generally considered for the interpretation of gas diffusion experiments, with and without additives. This discovery poses an important step in the understanding of how minerals are formed, highlighting that nonclassical mineralization processes must be considered for material synthesis. Last but not least, the advanced ADM may be useful for the exploration of the formation mechanism of other minerals than calcium carbonate, which are also of broad interest in the materials chemistry community.