Crystal chemistry and polytypism of tyrolite

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) [Angstrom], [beta] = 98.074(11)[degrees], V = 1590.3(3) [[Angstrom].sup.3] has been refined to [R.sub.1] = 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) [Angstrom] [beta] = 96.432(9)[degrees], V = 3154.4(6) [[Angstrom].sup.3]] has been refined to [R.sub.1] = 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 As[O.sub.4] tetrahedra. Two adjacent A sublayers are linked by the octahedral chains of the B sublayer resulting in formation of the 18 [Angstrom] thick ABA slab. The ABA slab is sandwiched between sublayers of [Ca.sup.2+] cations and [H.sub.2]O molecules. Adjacent nanolayers are connected by hydrogen bonds to the interlayer species (carbonate anions and [H.sub.2]O 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 [Angstrom] in comparison to the relative position of the nanolayers in tyrolite-1M. The structural formula of tyrolite can be written as [[Ca.sub.2][Cu.sub.9] [(As[O.sub.4]).sub.4][(OH).sub.8](C[O.sub.3])[([H.sub.2]O).sub.11]] [([H.sub.2]O).sub.x] where x = 0-1. Keywords: Tyrolite, 'clinotyrolite,' crystal structure, copper arsenate, nanolayers, polytypes