Ga1-xMnxN (0 <= x <= 0.18) films grown onto amorphous silica substrate by reactive sputtering are characterised by high resolution transmission electron microscopy, energy dispersive spectroscopy, and energy filtered transmission electron microscopy. The electron transmission images and the electron diffraction patterns evidence the presence, at the substrate-film interface, of a few tens of nm thick intermediate layer with a high density of non-oriented nanocrystals (NCs). This intermediate layer represents the nucleation site for the subsequent growth of a compact Ga1-xMnxN columnar nanostructure, whose thickness (600-900 nm) is only limited by the deposition time. The columnar region shows a fibre texture with the c axis of the wurtzite nanocrystals corresponding to the column axis, both disposed perpendicular to the film surface. The thickness of the initial NC-rich layer and the coalescence of the nanocolumns are found to have a systematic dependence on the Mn concentration. No evidence of Mn segregation or of Mn rich phases is observed even for the samples with the highest Mn concentration. The correlation between the observed film microstructure and the reactive sputtering deposition parameters is discussed. (C) 2011 Elsevier B.V. All rights reserved.