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Category Phosphate mineral
(repeating unit)
Strunz classification 8.AC.45
Crystal system Trigonal
Crystal class Ditrigonal pyramidal (3m)
(same H-M symbol)
Space group R3c
Unit cell a = 10.362 Å,
c = 37.106 Å; Z = 6
Color Colorless to white
Crystal habit Occurs as anhedral grains
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][3]

Merrillite is a calcium phosphate mineral with the chemical formula Ca9NaMg(PO4)7. It is an anhydrous, sodic member of the whitlockite group.[3]

Discovery and naming[edit]

The mineral is named after George P. Merrill (1854–1929) of the Smithsonian Institution.[3] Merrill had described the mineral from four meteorites in 1915: the Alfianello, Dhurmsala, Pultusk, and Rich Mountain meteorites. The mineral was not recognized as distinct from whitlockite, however, by the IMA until 1975.[3]


Merrillite is a very important constituent of extraterrestrial rocks. It occurs in pallasites, lunar rocks, martian meteorites, and many other meteorite groups.[4]


  1. ^ Mineralienatlas
  2. ^ Merrillite data on Webmineral
  3. ^ a b c d "Merrillite". Mindat. Retrieved 6 January 2013. 
  4. ^ Jolliff, Bradley L.; John M. Hughes; John J. Freeman & 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.