Muonium was found to be abundant in solid Ne as a long-lived species, amounting to 80% of the total asymmetry. In liquid Ne muonium accounts for about 5% of the asymmetry. The complete absence of muonium in Ne gas was once considered a key argument that the crucial factor to muonium formation was the relative size of the ionization potential of the stopping medium compared to that of muonium. . Clearly, this is not the complete picture, at least for solid rare gases. Ne has a vacuum ionization potential of 22 eV, well above that of muonium, so that once the muon kinetic energy has fallen below about 8 eV, it is impossible for muonium to be formed by a hot muon colliding with a Ne atom. Likewise, once the muon has reached thermal equilibrium with the lattice, muonium cannot be formed by simply abstracting an electron from a nearby atom. Formation of muonium while the muon is still energetic ought to be unaffected by the phase of the material, at least in the condensed phases. Since muonium is abundant only in solid Ne, it appears to be a result of properties unique to the solid phase. Free electrons in solid Ne are known to have an extremely high mobility, on the order of 2000 cm/V-s, about the same as in typical semiconductors, and 6 orders of magnitude less in the liquid phase where they form voids in the liquid due to their zero-point motion pushing Ne atoms apart. Taken together the evidence indicates that it is the electron transport properties of the crystalline state that enable formation of some of the muonium in solid N2 and almost all the muonium in solid Ne.