Formula: Ca9NaMg(PO4)7
Anhydrous normal phosphate, merrillite group
Crystal System: Trigonal
Specific gravity: 3.1 measured, 2.88 calculated

Colour: Colourless, white

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).


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.

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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