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Theoparacelsite is a recently discovered orthorhombic copper arsenate hydroxide mineral with the ideal chemical formula Cu 3 As 2 O 7 (OH) 2 , but because of disorder only two smaller formula units CuAs 2/3 H 2/3 O 3  are established in the orthorhombic unit cell with lattice parameters  a  = 4.6644(5) Å,  b  = 8.3212(8) Å and  c  = 2.9377(3) Å, and space group  Pbmm . We show that this rare mineral is iso-typic to the unique inorganic spin-Peierls phase CuGeO 3  ( T SP   = 14.3 K) and to the recently discovered copper polysilicate, CuSiO 3 , which is also a quasi-one-dimensional  S  = 1/2 anti-ferromagnetic chain compound ( T N   = 7.9 K). We discuss in detail the previously reported crystallographic data of the phases at room temperature in view of its ability to attain similar low-dimensional physical properties and conclude that the arsenate with its  S  = 1/2 spin chains of edge-sharing CuO 4+2  octahedra may be a further inorganic compound that could show the spin-Peierls effect. Although the Cu-O(2)-Cu angle, which is considered to be important for the magnetic interaction and ground state properties, has been found for the arsenate (98.8°) almost identical to that of the germanate (99.1°), the modified inter-chain coupling of the arsenate as a defect variant of the CuGeO 3  structure type may suppress the spin-Peierls effect and favour other states of magnetic order (chiral spin chains, AF order). With respect to the rarity of the mineral some remarks on synthesis routes are made.

Isotypism and low-dimensional physical properties of CuAs 2/3 H 2/3 O 3 (theoparacelsite), the spin-Peierls phase CuGeO 3 and CuSiO 3