2024 年 58 巻 3 号 p. 45-56
REE abundance patterns, tetrad effect, and Jørgensen’s theory have been considered in relation to REE Geochemistry. The present author proposed a theoretical equation for thermochemical data for ligand-exchange reactions between a pair of isomorphous Ln (III) compound series: ΔYobs=q(aq+b)(q+25)+c+(9/13)n(S) C1(q+25)+m(L) C3(q+25), where q denotes the number of 4f electrons with Ln3+=(4f)q, n(S) and m(L) are coefficients for 4f interelectron repulsion energy given by total spin and total orbital angular momentum quantum numbers (S and L) for Ln3+, and a, b, c, C1, and C3 are constants given by the least-squares regression. We call it Jørgensen–Kawabe equation, which is applicable to series changes of ΔHr, ΔSr, and ΔGr for ligand-exchange reactions noted above. If a polymorphic change occurs in Ln (III) compound series, a suitable correction for ΔYobs is necessary to satisfy the isomorphous series condition. The series change of ΔGr for LnO1.5(cub)+(3/2) F2(g)=LnF3(rhm)+(3/4) O2(g) has been considered by using thermochemical data for LnO1.5 (Robie et al., 1979) and for LnF3 (Chervonnyi, 2012),and the tetrad effect of ΔGr and its temperature change have been examined. From 298 to 1200 K, the series changes of ΔGr can be regressed successfully by Jørgensen–Kawabe equation, in which convex upward tetrad effects are always observed. But the magnitude of convex tetrad effect begins to diminish at 900 K. Its extrapolation to above 1200 K suggests that the convex tetrad effect may diminish totally at about 1400 K. This is due to the fact that ΔHr and ΔSr for the reactions have similar tetrad effects. ΔGr’s at higher temperatures above 1400 K are expected to show a concave upward tetrad effect controlled by (-TΔSr) because of ΔGr=ΔHr-TΔSr. In REE minerals of lanthanite and kimuraite, each Ln (III) ion is in a state of isomorphous series. This satisfies the condition of Jørgensen–Kawabe equation, and it is interesting to regress the REE patterns for lanthanite and kimuraite by Jørgensen–Kawabe equation. The following three types of REE patterns have been examined; (a) a pattern of logarithmic REE concentration ratios between a coexistent pair of lanthanite and kimuraite from Japan, (b) a pattern of logarithmic REE concentration ratios between a pair of lanthanite samples from Japan and New Zealand, and (c) logarithmic chondrite-normalized REE patterns for lanthanite and kimuraite. The three REE patterns are all reproduced satisfactorily by Jørgensen–Kawabe equation. The REE pattern of (a) can be understood in the similar way like the series variation of ΔGr for the reactions between LnF3(rhm) and LnO1.5(cub) discussed here, but (b) and (c) cannot correspond to ΔGr for such single-step reactions, because (b) and (c) are not dealing with the equilibrium pairs. However, the chondritic REE must be the common REE source for terrestrial samples. If reaction steps of geochemical evolution from the chondritic material to the particular natural material are combined, this may correspond to the REE pattern of (c).Since the REE pattern of (b) is the difference of patterns of (c),the REE pattern of (b) may also be compatible with Jørgensen–Kawabe equation.
