The structure of a new mineral jadarite, LiNaSiB(3)O(7)(OH) (IMA mineral 2006-36), has been determined by simulated annealing and Rietveld refinement of laboratory X-ray powder diffraction data. The structure contains a layer of corner-sharing, tetrahedrally coordinated Li, Si and B forming an unbranched vierer single layer, which is decorated with triangular BO(3) groups. The Na ion is situated between the tetrahedral layers in a distorted octahedral site. As the very high boron content in this mineral makes obtaining neutron diffraction data very problematic, ab initio optimization using VASP was used to validate the structure and to better localize the H atom. The H atom is located on the apex of the triangular BO(3) group and is involved in a weak intralayer hydrogen bond. The final Rietveld refinement agrees with the ab initio optimization with regard to a hydrogen bond between the H atom and one of the tetrahedral corner O atoms. The refined structure seems to be of a remarkably high quality given the complexity of the structure, the high proportion of very light elements and the fact that it was determined from relatively low-resolution laboratory data over a limited 2theta range (10-90 degrees 2theta).