Monohydrocalcite (MHC) is hydrated calcium carbonate, which plays an active role in many geological processes, carbonate biomineralization and can be used for fundamental science (as a paleoenvironmental indicator) and industry (for removal of hazardous anions). Despite a great number of works, the conditions preferable for MHC formation / stabilisation and MHC crystal chemical patterns in relation to Mg and H2O are not clarified yet. In the course of current work, we conducted 38 syntheses to obtain information on MHC formation at different Mg/Ca ratios (0–12), pH (~9–12) and temperature (23 and 3 °C). Newly-formed carbonate precipitates were studied by means of X-ray powder diffraction, optical and scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman and Fourier-transform infrared (FTIR) spectroscopies. Phase diagram for MHC, calcite, aragonite and dypingite as a function of pH and Mg concentration in solution at T= 23 °C and Ca/CO3=0.5 was obtained. We demonstrated that MHC could be stable in dry conditions for up to two years and that the time of crystallisation is important for ACC transformation to MHC. Our results on synthetic MHC stability show that the widespread idea that MHC is a short-lived intermediate phase is wrong. For the first time, on the basis of a regular change in the unit cell parameters the possibility of significant incorporation of magnesium in MHC has been proven. According to FTIR data, it is shown to be accompanied by an increase in the water content, which leads to multidirectional change in a and c MHC parameters.