Calcite growing in biomimetic hydrogel environments incorporates the gel during its growth. The amount of occluded gel within the composite is mainly determined by the interaction between gel strength and crystallization pressure, with the latter being directly related to supersaturation and growth rate. In previous work we established a direct correlation between increased amounts of occluded gel with misorientations in the growing calcite crystals or aggregates. The presence of Mg2+ in the growth environment adds complexity to the internal structuring of the mineral.In this contribution we examine the effects of Mg2+ on the mechanical parameters of gelatin hydrogel and silica hydrogel by mechanical shear stress tests, we determine characteristics of the gel fabric occluded in the calcite using selective etching techniques and high-resolution field emission scanning electron microscope (FE-SEM) imaging, and we use electron backscatter diffraction (EBSD) to study co-orientation or misorientation in the calcite crystals or aggregates.We show that two independent mechanisms are responsible for the complex impact of Mg2+ in the growth medium on the calcite/gel composites. First, addition of 0.1 M Mg2+ reduces the yield-strength of the gels by about 50%. While gelatin gel shows continuous strain hardening in a similar way for Mg-bearing and Mg-free systems, the silica-gel weakens after reaching an ultimate shear strength, where the strain associated with the maximum in strength shifts by 350% to higher values. The decreased gel strength in the Mg-bearing systems leads to decreased amounts of occluded gel. Second, incorporation of Mg2+ in the growing calcite (i) increases its solubility and thus decreases crystallization pressure, and (ii) introduces small angle grain boundaries due to misfit strains which lead to “split growth”, i.e. misoriented subunits of the calcite or – ultimately – spherulitic growth. Our study further clearly shows that Mg not only influences the organization of the mineral component within the aggregate but also the fabric of the occluded gel matrix. The fabrics of the occluded gel change from compact gel membranes to finely dispersed networks with increasing Mg and, correspondingly, decreased crystallization pressure via increasing solubility as more Mg incorporates into calcite structure. This circumstance initiates the large variety of calcite crystal co- and misorientation patterns and hierarchical assemblies that we find in the investigated composites.