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Category Phosphate mineral
(repeating unit)
Strunz classification 08.AC.45
Crystal symmetry Trigonal ditrigonal pyramidal
H-M symbol: (3m)
Space group: R 3c
Unit cell a = 10.362 Å, c = 37.106 Å; Z=6
Color Colorless to white
Crystal habit Occurs as anhedral grains
Crystal system Trigonal
Cleavage Poor - indistinct
Tenacity Brittle
Luster Vitreous
Specific gravity 3.1 (measured)
Optical properties Uniaxial (-)
Refractive index nε=1.62, nω=1.623
Birefringence 0.0030
References [1][2]

Merrillite is a calcium phosphate mineral with the chemical formula Ca9NaMg(PO4)7. It is an anhydrous, sodic member of the whitlockite group.[2]

Discovery and naming[edit]

The mineral is named after George P. Merrill (1854-1929) of the Smithsonian Institution.[2] Merrill had described the mineral from four meteorites in 1915: the Alfianello, Dhurmsala, Pultusk and Rich Mountain meteorites. However, the mineral was not recognized as distinct from whitlockite by the IMA until 1975.[2]


Merrillite is a very important constituent of extraterrestrial rocks. It occurs in pallasites, lunar rocks, martian meteorites and many other meteorite groups.[3]


  1. ^ Merrillite data on Webmineral
  2. ^ a b c d "Merrillite". Mindat. Retrieved 6 January 2013. 
  3. ^ Jolliff, Bradley L.; John M. Hughes, John J. Freeman and Ryan A. Zeigler (2006). "Crystal chemistry of lunar merrillite and comparison to other meteoritic and planetary suites of whitlockite and merrillite". American Mineralogist 91: 1583–1595. doi:10.2183/am.2006.2185.