An extended Hiickel calculation was employed to calculate the total electronic energy and the electron density during the breakage of the Si-O networks of a (F3SiO)2FSiOSiF(OSiF3)2 cluster modelled on the a-cristobalite structure. The Si-O networks are opened by the attack of F- ions on the silicon atoms, and the reaction is exthothermic by 2.7 eV through the attack of H+ ions on the oxygen atoms. Although the hydrogen termination of the oxygen atoms is an early reaction, the fluorine termination of the silicon atoms is a late reaction. The atomic bond population on the Si-O bonds decreases to zero by opening the Si-O networks. We conclude that the a-cristobalite and a-quartz SiO2 are dissolved in HF solutions since the Si-O networks are easily opened by the attack of F- ions. Our conclusion indicates that both cosite and stishovite SiO2, which are not dissolved in the HF solutions, are composed of Si-O networks that can be hardly opened by the attack of F- ions. Moreover, we propose the continual breakage of Si-O networks without the desorption of H2O molecules as an etching mechanism of SiO2 films.