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Mineralogical ClassificationCrystalchemistry of merrillite

10th Sep 2006 10:43 UTCMarco E. Ciriotti Manager

A revised formula is presented for the merrillite:
Na2–x(Mg,Fe2+,Mn2+)2<(P,Si)O4>14



Reference:

• Hughes, J.M., Jolliff, B.L., Gunter, M.E. (2006): The atomic arrangement of merrillite from the Fra Mauro Formation, Apollo 14 lunar mission: The first structure of merrillite from the Moon. American Mineralogist, 91, 1547-1552.


Abstract:

The atomic arrangement of lunar merrillite has been refined to R = 0.0452 in R3c using X-ray diffraction data recorded on a CCD detector; previous attempts at structure solution using a point detector were not successful because of the poorly crystallized nature of the lunar material. The atomic arrangement of merrillite has a structural unit of <(Mg,Fe)(PO4)6>2 16- that forms a bracelet-and-pinwheel unit that is common in hexagonal-closest-packed layers. The individual structural units are not polymerized and exist in layers at z = 1/6, 1/3, 1/2, 2/3, and 5/6. In lunar merrillite, the <(Mg,Fe)(PO4)6>2 16- structural units are linked by a <(Ca,REE)18Na2(PO4)2>32+ interstitial complex, formed of Ca1O8, Ca2O8, Ca3O8, NaO6, and P1O4 polyhedra.

There has long been speculation regarding the relationship between merrillite and terrestrial whitlockite, and the solution of the Fra Mauro merrillite atomic arrangement allows the characterization of the lunar phase. Lunar merrillite and terrestrial whitlockite have largely similar atomic arrangements, but the phases differ due to the presence or absence of hydrogen. In whitlockite, H is an essential element and allows the charge balance. Hydrogen is incorporated into the whitlockite atomic arrangement by disordering one of the phosphate tetrahedra and forming a PO3(OH) group. Lunar merrillite is devoid of hydrogen, and thus no disordered tetrahedral groups exist. Charge balance for substituents Y and REE (for Ca) is maintained by Si ↔ P tetrahedral substitution and ■ ↔ Na at the Na site. The structure solution demonstrates the effectiveness of the CCD detector in unraveling previously intractable diffraction data and urges that previously analyzed lunar material be reexamined using this instrumentation.


Reference:

• Jolliff, B.L., Hughes, J.M., Freeman, J.J., Zeigler, R.A. (2006): Crystal chemistry of lunar merrillite and comparison to other meteoritic and planetary suites of whitlockite and merrillite. American Mineralogist, 91, 1583-1595.


Abstract:

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 conÞ rms the absence of H in the former and presence of H in the latter. Lunar merrillite, ideally (Mg, Fe2+, Mn2+)2
(Na2–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 Si↔P 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 signiÞ cantly 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.

10th Sep 2006 10:44 UTCMarco E. Ciriotti Manager

The new cell parameters after structure refinement are: (space group R3c) 10.2909 10.2909 36.8746

10th Sep 2006 22:43 UTCJim Ferraiolo

These two papers give the structure for what essentially is Dowty's (1977) "RE-merrillite" . I have modified the merrillite formula so it shows the end-member composition approved in 1976, and omitting the rare earths.


It is very probable that this will actually be a new merrillite species, but additional work and submission to the IMA would have to be done.
 
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