Sr2B5OH center dot H2O, a synthetic borate related to hilgardite

The crystal structure of a new strontium borate, Sr2B5O9OH .H2O, space grou p C2, a 10.2571(6). b 8.0487(2), c 6.4043(4) Angstrom, p 127.860(2)degrees, Z= 2, has been determined by X-ray diffraction using a single crystal grow n under hydrothermal conditions. The structure is closely related to that o f the zeolite-like hilgardite minerals, Ca2B5O9Cl .H2O, in particular that of triclinic hilgardite-1A. The main structural differences are a more symm etrical conformation of the [B5O12](9-) pentaborate chains and a random dis tribution of OH- and H2O in Sr2B5O9OH .H2O, that result in the monoclinic s ymmetry. The role of the Sr cations in determining the conformation of the pentaborate chain is discussed by comparison with hilgardite-1A.