Face-to-face wafers were used to observe anomalous tungsten deposition in the gap-edge between wafers. In the WF6-H-2 atmosphere, three regions are identified : (i) an open-deposition region (region A), (ii) a half-sealed deposition region (region B), and (iii) an etching or tunnel region (region C). In the WF6-Ar atmosphere, there are only two regions : (i) an open deposition region (region A’), and (ii) a half-sealed deposition region (region B’). The third region disappears because HF does not form in the absence of H-2. Different chemical reactions are expected in different regions, dictated by the local gas composition. A half-sealed structure model proposed here is supported by thermodynamic calculations, and applied to explain encroachment, wormholes, and other well-known effects during the chemical vapor deposition of tungsten from tungsten hexafluoride.