Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Jolliff, B. L. Articles by Zeigler, R. A. Search for Related Content GeoRef GeoRef Citation

Crystal chemistry of lunar merrillite and comparison to other meteoritic and planetary suites of whitlockite and merrillite

¹ Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, U.S.A.
² Office of the Provost, 348B Waterman Building, University of Vermont, Burlington, Vermont 05405, U.S.A.

Correspondence: ^* E-mail: blj{at}levee.wustl.edu

Merrillite, also known as "whitlockite," is one of the main phosphate minerals, along with apatite, that occur in lunar rocks, martian meteorites, and in many other groups of meteorites. Significant structural differences between terrestrial whitlockite and lunar (and meteoritic) varieties warrant the use of "merrillite" for the H-free extraterrestrial material, and the systematic enrichment of REE in lunar merrillite warrants the use of "RE-merrillite." Laser Raman spectroscopy of extraterrestrial merrillite and terrestrial whitlockite confirms the absence of H in the former and presence of H in the latter. Lunar merrillite, ideally (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 3 atoms per 56 O atoms, or up to ~18 wt% as (Y,RE)₂O₃. In the absence of extensive SiP substitution, the "availability" of the Na site limits Y+REE substitution to ~2 atoms per 56 O atoms. Higher concentrations of Y+REE, with coupled substitution of Si for P to balance charge are possible, but rare in lunar material. Intrinsically low Na concentrations in lunar rocks, combined with the typical formation of merrillite in late-stage basaltic mesostasis or residual, intercumulus melt pockets, produce these high REE concentrations. Lunar merrillite typically contains 0.1–0.4 Na atoms per 56 O atoms. For comparison, martian merrillite contains significantly higher Na concentrations (up to 1.7 Na atoms per 56 O atoms) and much lower REE concentrations. Meteoritic merrillite has relatively low REE contents, but exists in both Ca-rich and Na-rich varieties. Concentrations of Fe and Mg in all varieties sum to near 2 atoms per 56 O atoms. Merrillite in lunar crustal lithologies typically has Mg >> Fe; however, Fe-rich mare basalts contain up to 1.8 Fe²⁺ per 56 O. The structure of merrillite accommodates a variety of substitutions, and the compositional characteristics reflect conditions and processes specific to the parent planet.

Key Words: Merrillite • whitlockite • Moon • Mars • electron microprobe • ion microprobe • Raman spectroscopy • rare earth elements

This article has been cited by other articles:

				J. Konzett and D. J. Frost The High P-T Stability of Hydroxyl-apatite in Natural and Simplified MORB--an Experimental Study to 15 GPa with Implications for Transport and Storage of Phosphorus and Halogens in Subduction Zones J. Petrology, November 1, 2009; 50(11): 2043 - 2062. [Abstract] [Full Text] [PDF]

				J. J. Freeman, A. Wang, K. E. Kuebler, B. L. Jolliff, and L. A. Haskin CHARACTERIZATION OF NATURAL FELDSPARS BY RAMAN SPECTROSCOPY FOR FUTURE PLANETARY EXPLORATION Can Mineral, December 1, 2008; 46(6): 1477 - 1500. [Abstract] [Full Text] [PDF]

				J. M. Hughes, B. L. Jolliff, and J. Rakovan The crystal chemistry of whitlockite and merrillite and the dehydrogenation of whitlockite to merrillite American Mineralogist, August 1, 2008; 93(8-9): 1300 - 1305. [Abstract] [Full Text] [PDF]