Nucleogenic neon isotope production resulting from α decay of naturally occurring U and Th was calculated for typical compositions of the mantle and crust using a simple non-Monte Carlo code for neutron transport. The (α,n), (α,p), and (n,α) reactions, leading to the production of neon isotopes in the outgoing reaction channel, are taken into account in the calculations. In the first stage of calculation, neon production from (α,n) and (α,p) reactions on O17, O18, and F19 is considered, and the neutron energy spectrum from (α,n) 18O, and 19F is considered, and the neutron energy spectrum from (α,n) reactions on the major elements is derived. This spectrum is further used for calculation of the neon isotope yield from the reactions Mg24(n,α)Ne21, Mg25(n,α)Ne22, and Na23(n,α)Ne20. The calculated production of nucleogenic neon is dominated by (α,n) reactions with oxygen in the Earth's mantle and crust. The present calculations give Ne/21He4 production ratios that are approximately half those of the previous estimates by Rison <1980> and Kyser and Rison <1982>. The present study also has shown that the nucleogenic Ne21 production from the Mg24(n,α) reaction is only about one-fifth the previous estimate. The contribution of nucleogenic Ne21 from the Mg24(n,α) reaction is insignificant compared with nucleogenic Ne21 production from O18(α,n). Our estimate of the nucleogenic Ne/radiogenic21 He4 production ratio both in the mantle and in the crust is 4.5¿10-8. This production ratio is quite consistent with nucleogenic Ne/radiogenic21 He4 ratios actually observed in crustal fluid samples ((4.6¿0.8)¿10-8). The error in calculation of the nucleogenic neon production, resulting from a random statistical variation of the entry neutron yields and cross sections, is estimated to be less than 5%.¿ 1997 American