Calcium carbonate hydroxide hydrate or basic calcium carbonate (BCC) is a metastable, relevant phase occurring in various industrial carbonation processes and biologically induced calcium-carbonate precipitation. The best conditions for the long-term storage of BCC are in a dry, non-heated environment. This is confirmed by powder X-ray analysis. Under humid conditions sample material transforms into vaterite-type calcium carbonate, while under a dry heating process it dehydrates and dehydroxylates to calcite-type calcium carbonate. Under ambient humidity and temperature conditions the dehydrated modification does not convert back to BCC or calcite-type calcium carbonate, but also transforms to the vaterite-type calcium carbonate. High-quality scanning electron microscopy images of a rapidly converted vaterite-type calcium carbonate reveal imperfect pseudomorphs of vaterite after the platy BCC. Literature data reported the chemical formula of BCC as a sesquihydrate. The observed and calculated weight losses are close to the calculated value for a monohydrate, which leads to the corrected chemical formula of BCC as Ca3(CO3)2(OH)2·H2O or 2CaCO3·Ca(OH)2·H2O. Structure solution and Rietveld refinement (Rwp = 11.2%) based on powder X-ray patterns reveal the hitherto unknown atomic arrangement. It crystallizes to a monoclinic lattice with space group Pc (no. 7); the cell parameters of the pseudo-orthorhombic cell are a = 8.6552(2), b = 6.5529(2), c = 7.0801(2) Å, β = 90.059(4)°, V = 401.56(2) Å3, Z = 2 and ρcalc = 2.417 g∙cm−3. The structure model of BCC consists of three edge-sharing Ca-polyhedra interconnected by two CO3 groups. Based on bond-valence calculation two oxygen positions can be attributed to hydroxyl groups, one oxygen position represents a water molecule. The main structural motif is a chain of edge-sharing Ca–O2(OH)4 octahedra running parallel to the c-axis. The structure exhibits similarities to those of vaterite, portlandite and hydromagnesite.