Flower-like Si nanostructures are formed in high yield, by heating an SiO2 plate at ca. 1600 degrees C (Ta heater) under Ar (100 Torr). The product consists of metal-free cubic phase Si nanowires surmounted by bulbous Si tips. HRTEM observations show that the nanowires contain kink and twinning defects, whereas the tips are generally well-crystallized and covered with a thin layer of amorphous SiOx (x = 1-2). A growth model is proposed to account for these observations.