Merrillite: Properties and Occurrences

Merrillite is a calcium phosphate mineral with the chemical formula Ca9NaMg(PO4)7. It is a calcium phosphate mineral found in some meteorites. It is an anhydrous, sodic member of the whitlockite group. It is the anhydrous end-member of the merrillite–whitlockite solid solution series, with whitlockite (Ca9[Fe, Mg][HPO4][PO4]6) being the hydrogenated end member.

Merrillite was revalidated as a valid species in 1976. Merrill (1915) described the mineral from (at least) 4 meteorites, which may be considered type localities: Alfianello, Dhurmsala, Pultusk, Rich Mountain.

Discovery and naming

The mineral is named after George P. Merrill (1854–1929) of the Smithsonian Institution. 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.

General Information

  • Category: Phosphate mineral
  • Formula: (repeating unit) Ca9NaMg(PO4)7
  • Crystal system: Trigonal
  • Crystal class: Ditrigonal pyramidal (3m) (same H-M symbol)

Properties

  • 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 (-)
  • Density: 3.1 g/cm3 (Measured) 2.88 g/cm3 (Calculated)

Occurrence

Merrillite is a very important constituent of extraterrestrial rocks. Natural merrillite is generally igneous, although it can occur as a metamorphic product in chondritic meteorites. It occurs in pallasites, lunar rocks, martian meteorites, and many other meteorite groups.