The (M \mathrm{CuSi}{4} \mathrm{O}{10}(M=\mathrm{Ca}, \mathrm{Sr}, \mathrm{Ba})) compounds possess the gillespite (\left(\mathrm{BaFeSi}{4} \mathrm{O}{10}\right)) structure, which is tetragonal, (P 4 / n c c, Z=4). Data sets of 2365 step intensities were recorded for the title compounds with (\lambda=1.400 \AA) at (T=295 \mathrm{~K}). For (M=\mathrm{Ca}, a=7.3017) (3) (\AA, c=15.1303) (6) (\AA, V_{\text {cell }}=806.66) (9) (\AA^{3}, R_{\text {exp }}=0.0453, R_{\text {up }}=0.0515), for 487 reflections. For (M=\mathrm{Sr}, a=7.3707) (2) (\AA, c=15.5904) (6) (\AA, V_{\text {cell }}=846.9) (1) (\AA^{3}, R_{\text {exp }}=0.0433 . R_{\text {up }}=0.0489), for 521 reflections. For (M=\mathrm{Ba}, a=7.4409) (3) (\AA, c=16.1367) (8) (\AA, V_{\text {ccll }}=893.4) (1) (\AA^{3}, R_{\text {exp }}=0.0466, R_{w p}=0.0522), for 546 reflections. The gillespite structure contains an unbranched single silicate layer, which can be viewed as a tesselation of corner-linked (\mathrm{Si}{4} \mathrm{O}{10}) rings. The (\mathrm{Cu}) in (M \mathrm{CuSi}{4} \mathrm{O}{10}) is in square-planar coordination with the nonbridging oxygens of the corrugations of the silicate layer. The alkaline-earth atom, in distorted cubic coordination, joins together adjacent (\left[\mathrm{CuSi}{4} \mathrm{O}{10}\right]^{2-}) slabs. Increasing the size of the alkaline-earth atom increases the cell dimensions anisotropically (i.e.. (\Delta c>\Delta a) ) due to the inflexibility of the silicate anion. Roompressure experiments show no evidence for oxygen intercalation or polytypism. The lattice mismatch between the (\left[\mathrm{CuSi}{4} \mathrm{O}{10}\right]^{2-}) slabs of the gillespite structure type and the square-planar (\mathrm{CuO}{2}) layers of the superconducting cuprates is small. which suggests possible applications of these materials as exotic thin-film substrates for the high (T{\mathrm{c}}) cuprates or other novel structures having cuprate and silicate layers. 1993 Academic Press. Inc.