Merrillite
{{Short description|Phosphate mineral}}
{{Infobox mineral
| name = Merrillite
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| category = Phosphate mineral
| formula = Ca9NaMg(PO4)7
| molweight =
| strunz = 8.AC.45
| dana =
| system = Trigonal
| class = Ditrigonal pyramidal (3m)
(same H-M symbol)
| symmetry = R3c
| unit cell = a = 10.362 Å,
c = 37.106 Å; Z = 6
| color = Colorless to white
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| habit = Occurs as anhedral grains
| twinning =
| cleavage = Poor - indistinct
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| tenacity = Brittle
| mohs =
| luster = Vitreous
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| gravity = 3.1 (measured)
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| opticalprop = Uniaxial (-)
| refractive = nε=1.62, nω=1.623
| birefringence = 0.0030
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| references = [https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Merrillite Mineralienatlas][http://www.webmineral.com/data/Merrillite.shtml Merrillite data on Webmineral]
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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.{{cite web|title=Merrillite|url=http://www.mindat.org/min-6577.html|publisher=Mindat|accessdate=6 January 2013}}{{cite web|title=Merrillite|url=https://www.mindat.org/min-55746.html|publisher=Mindat|accessdate=13 September 2022}}
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.{{cite web|title=Cerite Supergroup|url=https://www.mindat.org/min-55118.html|publisher=Mindat|accessdate=13 September 2022}}
- Merrillite Group
- Merrillite Sub-group
- Ferromerrillite
- Keplerite
- Matyhite
- Merrillite
- Whitlockite Sub-group
- Hedegaardite
- Strontiowhitlockite
- Whitlockite
- Wopmayite
In September 2022 the discovery of another merrillite group mineral, changesite–(Y), was announced,{{cite web|title=New mineral found by Chinese scientists|url=https://www.chinadaily.com.cn/a/202209/10/WS631bd991a310fd2b29e76f79.html|publisher=China Daily|accessdate=13 September 2022}} but, {{As of|2022|09|lc=y}}, 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. 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.
Occurrence
Merrillite is a very important constituent of extraterrestrial rocks.{{citation needed|date=September 2022}} It occurs in lunar rocks and in meteorites (for example, pallasites and martian meteorites).{{cite journal|last=Jolliff|first=Bradley L.|author2=John M. Hughes |author3=John J. Freeman |author4=Ryan A. Zeigler |name-list-style=amp |title=Crystal chemistry of lunar merrillite and comparison to other meteoritic and planetary suites of whitlockite and merrillite|journal=American Mineralogist|date=2006|volume=91|issue=10|pages=1583–1595|doi=10.2138/am.2006.2185|bibcode=2006AmMin..91.1583J|s2cid=140580667}}
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.{{cite journal |last1=Kaminsky |first1=Felix V. |last2=Zedgenizov |first2=Dmitry A. |title=First find of merrillite, Ca3(PO4)2, in a terrestrial environment as an inclusion in lower-mantle diamond |journal=American Mineralogist |date=2022 |volume=107 |issue=8 |pages=1652–1655 |doi=10.2138/am-2022-8175|s2cid=251071674 }}
References
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