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Formula: Ca9NaMg(PO4)7
Anhydrous normal phosphate, merrillite group
Crystal System: Trigonal
Specific gravity: 3.1 measured, 2.88 calculated
Colour: Colourless, white
Environments
Lower mantle
Stony meteorites
Moon and Mars
Merrillite, sometimes also called whitlockite, is one of the main
phosphate minerals, along
with apatite, that occur in lunar rocks, martian
meteorites, and in many other groups of
meteorites.
Significant structural differences between terrestrial whitlockite and lunar and meteoritic varieties
warrant the use of "merrillite" for the H-free extraterrestrial material, whereas terrestrial
whitlockite contains essential H
(AM 91.1583-1595).
Localities
At the Sorriso river, Juína, Mato Grosso, Brazil, merrillite was identified in 2022 as an inclusion in lower-mantle
diamonds. It was associated with former bridgmanite, CaSi-
and CaTi- perovskites and ferropericlase. This is the
first report of merrillite in a terrestrial environment; previously, it was known only in
meteorites and lunar rocks. The find of
merrillite suggests a larger variety of mineral species in the lower mantle than was previously assumed
(AM 107.1652-1655).
The Suizhou meteorite, Xihe, Zengdu District, Suizhou, Hubei, China.
Merrillite is an important accessory phosphate mineral in many different groups of
meteorites, including martian
meteorites, and a major
carrier of rare earth elements in lunar rocks. The Suizhou meteorite is a
shock-metamorphosed L6-chondrite. The Suizhou merrillite is found to transform to tuite at high
pressures, pointing to the likelihood of finding rare-earth-element bearing tuite on the Moon as a result
of shock events on rare-earth-element merrillite
(AM 100: 2753-2756).
The Shergotty Martian meteorite, Gaya District, Bihar, India.
Whitlockite and merrillite are two calcium-phosphate minerals found in terrestrial and
planetary igneous rocks, sometimes coexisting with apatite.
Whitlockite has essential structural hydrogen, and merrillite is devoid of hydrogen.
Whitlockite components have yet to be discovered in samples of extraterrestrial merrillite,
despite evidence for whitlockite-merrillite solid solution in terrestrial systems. However,
the Shergotty martian meteorite has been reported to contain both apatite and merrillite.
It was determined that the merrillite in Shergotty was properly identified (ie no whitlockite
component), and it coexists with OH-rich apatite. The absence of a
whitlockite component in Shergotty merrillite and other planetary merrillites may be a
consequence of the limited thermal stability of H in whitlockite (stable only at temperature less than
1050oC), which would prohibit merrillite-whitlockite solid-solution at high
temperatures. In fact, if a whitlockite component in extraterrestrial merrillite is discovered,
it may indicate formation by or equilibration with hydrothermal or aqueous fluids
(AM 99.1347-1354).
The type locality is considered to be the Alfianello meteorite, Alfianello, Brescia Province, Lombardy, Italy.
Mars.
Merrillite occurs as a dominant primary calcium-phosphate
mineral in Martian
meteorites and therefore presumably also on Mars. The mineral
is an important phase in
exploring differences in geologic processes between Earth and Mars, and also has astrobiological implications due to
its potential role as a
significant source of the bio-essential nutrient phosphate. Until recently, merrillite was not thought to occur
terrestrially as a discrete
mineral phase. It can, however, be synthesised from the similar terrestrial mineral,
whitlockite,
through dehydrogenation, creating coarse crystalline merrillite for use in Mars-relevant studies
(AM 99: 1221-1232).
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