Merrillite
General
CategoryPhosphate mineral
Formula
(repeating unit)		Ca9NaMg(PO4)7
IMA symbolMer[1]
Strunz classification8.AC.45
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
(same H-M symbol)
Space groupR3c
Unit cella = 10.362 Å,
c = 37.106 Å; Z = 6
Identification
ColorColorless to white
Crystal habitOccurs as anhedral grains
CleavagePoor - indistinct
TenacityBrittle
LusterVitreous
Specific gravity3.1 (measured)
Optical propertiesUniaxial (-)
Refractive indexnε=1.62, nω=1.623
Birefringence0.0030
References[2][3][4]

Merrillite is a calcium phosphate mineral with the chemical formula Ca9NaMg(PO4)7. It is an anhydrous, sodium-rich member of the merrillite group of minerals.[4][5]

Mineral species, sub-group and group

Merrillite is a distinct mineral species but it also gives its name to a set of similar minerals, which together form the merrillite sub-group of minerals. The merrillite sub-group and the whitlockite sub-group together form the merrillite group of minerals.[6]

In September 2022 the discovery of another merrillite group mineral, changesite–(Y), was announced,[7] but, as of September 2022, it is not yet clear where this new mineral sits in the merrillite group hierarchy.

Discovery and naming

Merrillite is named after George P. Merrill (1854–1929) of the Smithsonian Institution.[4] In 1915, Merrill had described the mineral from four meteorites: Alfianello, Dhurmsala, Pultusk and Rich Mountain. However, it was not until 1975 that it was recognized as distinct from whitlockite by the International Mineralogical Association.[4]

Occurrence

Merrillite is a very important constituent of extraterrestrial rocks.[citation needed] It occurs in lunar rocks and in meteorites (for example, pallasites and martian meteorites).[8]

In 2022, for the first time, merrillite was found in a terrestrial environment, as an inclusion in lower-mantle diamonds from Sorriso River, Juína, Brazil.[9]

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Mineralienatlas
  3. ^ Merrillite data on Webmineral
  4. ^ a b c d "Merrillite". Mindat. Retrieved 6 January 2013.
  5. ^ "Merrillite". Mindat. Retrieved 13 September 2022.
  6. ^ a b "Cerite Supergroup". Mindat. Retrieved 13 September 2022.
  7. ^ "New mineral found by Chinese scientists". China Daily. Retrieved 13 September 2022.
  8. ^ 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 (10): 1583–1595. Bibcode:2006AmMin..91.1583J. doi:10.2138/am.2006.2185. S2CID 140580667.
  9. ^ Kaminsky, Felix V.; Zedgenizov, Dmitry A. (2022). "First find of merrillite, Ca3(PO4)2, in a terrestrial environment as an inclusion in lower-mantle diamond". American Mineralogist. 107 (8): 1652–1655. doi:10.2138/am-2022-8175. S2CID 251071674.


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