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]

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]

Mineral species, sub-group and group

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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

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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

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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

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  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.