The crystal structures of the 1M and 2M polytypes of tyrolite have been solved from single-crystal X-ray diffraction data. The structure of tyrolite-1M [monoclinic, P2/c, a = 27.562(3), b = 5.5682(7), c = 10.4662(15) Å, β = 98.074(11)°, V = 1590.3(3) Å3] has been refined to R1 = 0.086 on the basis of 2522 unique observed reflections collected using synchrotron radiation at the Swiss-Norwegian beamline BM01 of the European Synchrotron Research Facility (SNBL at the ESRF). The structure of tyrolite-2M [monoclinic, C2/c, a = 54.520(6), b = 5.5638(6), c = 10.4647(10) Å, β = 96.432(9)°, V = 3154.4(6) Å3] has been refined to R1 = 0.144 on the basis of 2666 unique observed reflections obtained from a non-merohedrally twinned crystal using in-house X-ray radiation and a STOE IPDS II image-plate diffractometer. The structures are based upon complex nanolayers consisting of Cu, As, and Ca coordination polyhedra. The core of the nanolayer is a copper arsenate substructure consisting of A and B sublayers. The B sublayer consists of chains of edge-sharing Cu octahedra running along the b axis. The A sublayer contains trimeric units of Cu octahedra sharing corners with AsO4 tetrahedra. Two adjacent A sublayers are linked by the octahedral chains of the B sublayer resulting in formation of the 18 Å thick ABA slab. The ABA slab is sandwiched between sublayers of Ca2+ cations and H2O molecules. Adjacent nanolayers are connected by hydrogen bonds to the interlayer species (carbonate anions and H2O molecules). The structures of tyrolite-1M and tyrolite-2M differ by the stacking sequence of the nanolayers only. The adjacent nanolayers in tyrolite-2M are shifted by b/2 = 2.8 Å in comparison to the relative position of the nanolayers in tyrolite-1M. The structural formula of tyrolite can be written as [Ca2Cu9(AsO4)4(OH)8(CO3)(H2O)11](H2O)x where x = 0–1.

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