Ga(4)B(2)O(9), an aluminium-free mullite-type compound, was prepared by a boric-acid flux method and its structure was determined using powder X-ray diffraction techniques, in combination with transmission electron microscopy, solid-state (11)B MAS-NMR and IR spectroscopies. GaO(6) octahedra share edges in a trans-manner forming one-dimensional chains along the b direction, and the chains are further cross-linked by GaO(5), BO(3) and BO(4) groups into a three-dimensional mullite-type structure. The disorder of the inter-chain groups results in a small unit cell for Ga(4)B(2)O(9) compared with that for Al(4)B(2)O(9), an ordered compound with a superstructure. By deconstructing the structure of Ga(4)B(2)O(9), we were able to identify the fundamental building units and their linking rules which can be used to reconstruct the ordered and disordered structures. For Ga(4)B(2)O(9), we found that the structure is intrinsically disordered within the ac plane, but ordered along the b axis. The three-dimensional structure can then be constructed by stacking the disordered ac sheets along the b axis ((1/2)b) with a (1/2)a shift. The fundamental building units and exclusivity rules identified in this gallium borate mullite may also be useful for understanding other related mullite phases. The structure analysis applying the proposed method is used to recognize the structural features of Al(4)B(2)O(9) and Al(18)B(4)O(33).