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Although near-surface seawater is supersaturated with CaCO₃, only a minor part of it is abiogenic (e.g., carbonate cements). The possible reason for such a phenomenon has attracted much attention in the past decades. Substrate effects on the heterogeneous nucleation and growth of CaCO₃ at various Mg²⁺/Ca²⁺ ratios may contribute to the understanding of the origin of abiogenic CaCO₃ cements. Here, we used in situ atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy to study the heterogeneous nucleation and growth of CaCO₃ on both calcite (104) and aragonite (110) surfaces. The results show that (1) calcite spiral growth occurs on calcite (104) surfaces by monomer-by-monomer addition; (2) the aggregative growth of aragonite appears on aragonite (110) surfaces through a substrate-controlled oriented attachment (OA) along the [001] direction, followed by the formation of elongated columnar aragonite; and (3) Mg²⁺ inhibits the crystallization of both calcite and aragonite without impacting on crystallization pathways. These findings disclose that calcite and aragonite substrates determine the crystallization pathways, while the Mg²⁺/Ca²⁺ ratios control the growth rate of CaCO₃, indicating that both types of CaCO₃ substrate in shallow sediments and aqueous Mg²⁺/Ca²⁺ ratios constrain the deposition of abiogenic CaCO₃ cements in the ocean.