COMPOSITIONAL VARIATIONS IN LUNAR MERRILLITE

In a companion abstract, Hughes and Jolliff report the first structural refinement of lunar merrillite (aka “lunar whitlockite”). Here, we discuss compositional variations in merrillite in lunar and planetary materials in the context of the new structural refinement. Significant structural differences between terrestrial whitlockite and lunar (and meteoritic) merrillite warrant the use of “merrillite” for the H-free extraterrestrial varieties, and the systematic enrichment of REE in lunar merrillite warrants the use of “RE-merrillite.”  The H-free extraterrestrial varieties have a variably occupied Na site (equivalent to Ca(IIA) of Calvo and Gopal, 1975, Am. Min. 60, p.120); vacancies on this site provide a favorable charge-balance for Y+REE↔Ca substitution. Lunar merrillite, approximately (Mg,Fe²⁺,Mn²⁺)₂[Ca_18-x (Y,REE)_x](Na_2-x)(P,Si)₁₄O₅₆, contains high concentrations of Y+REE, reaching just over 2 atoms per 56 O, or up to ~14 wt.% as (Y,RE)₂O₃.  In the absence of extensive Si↔P substitution, the “availability” of the Na site limits Y+REE substitution to ~2 atoms per formula unit. Intrinsically low Na concentrations in lunar rocks, combined with the typical formation of merrillite in late-stage mesostasis or residual, intercumulus melt pockets, conspire to produce these high REE concentrations. Lunar merrillite typically contains 0.1-0.4 Na atoms per 56 O.  For comparison, martian merrillites trend to higher Na concentrations (up to 1 Na per 56 O) and lower REE concentrations (Lundberg et al., 1988, GCA 52, p.2147), and meteoritic merrillites have relatively low Na and REE contents (but higher Ca).  Experimental results (Colson et al., 1993, LPS 24, p.323) indicate that extensive substitution of Si for P can also balance charges, leading to Y+REE well in excess of 2 per 56 O; however, this substitution is minor in lunar RE-merrillite.  Fe and Mg concentrations sum to near 2 atoms per 56 O, and lunar varieties typically contain Mg>>Fe. Merrillite in Fe-rich Apollo 15 mare basalt 15475, however, contains ~1.8 Fe²⁺ per 56 O. Mg/Fe values are higher in merrillite than in coexisting Fe-Mg silicates and apatite. In summary, the structure of merrillite accommodates a variety of substitutions, and the compositional characteristics reflect conditions and processes specific to the parent planet.