Synthetic Ca-exchanged birnessite (CaBi) was studied by X-ray and selected-area electron diffraction (XRD, SAED). The substructure of CaBi may be described with a four-layer monoclinic subcell with a = 5.150, b = 2.844, c = 4c' = 28.16 Aa, and beta = 90.3. Two different superstructures of CaBi were distinguished. CaBi type I has cell parameters A = 3a = 15.45, B = 3b = 8.472 Aa. The stacking sequence in this unit cell may be described as defect-free OSOS, where successive layers are shifted relative to their predecessors by 0 (O) or b/2 (S) along the b axis. The complete description of stacking involves the structure of the layer itself, the structure of the interlayer, and the shift from this layer to the next one. CaBi type II can be described as a regular interstratification of A P = 3/2(a-b), B P = 4b, gamma = 118.9 degrees and A P = 3/2(a+b), B P = -4b, gamma = 118.9 degrees supercells that are connected by a mirror plane in projection on the a-b plane. Its stacking sequence is a random interstratification of OSOS (90%) and OOOS (10%) structural fragments. Most CaBi crystals appeared to consist of intergrown type I and type II sub-crystals. As in Na-rich birnessite, the A = 3a superstructure arises from the ordered distribution of Mn (super 3+) -rich rows parallel to  and separated from each other along  by two Mn (super 4+) rows. In Mn (super 3+) -rich rows heterovalent Mn cations are regularly distributed according to Mn (super 3+) Mn (super 3+) Mn (super 4+) (CaBi type I, B = 3b) and Mn (super 3+) Mn (super 3+) Mn (super 4+) Mn (super 4+) (CaBi type II, B = 4b) sequences. Super-periodicities along the b axis are induced by these regular distributions of heterovalent Mn atoms in Mn (super 3+) -rich rows and of associated interlayer Ca. No significant amount of layer vacancies was detected. Idealized structural formulae for CaBi type I and II are Ca(Mn (super 3+) 2 Mn (super 4+) 7 )O 18 and Ca(Mn (super 3+) 2 Mn (super 4+) 10 )O 24 , respectively.