| Merrillite | |
| Category: | Phosphate mineral |
| Formula: | Ca9NaMg(PO4)7 |
| Imasymbol: | Mer[1] |
| Strunz: | 8.AC.45 |
| 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 |
| Habit: | Occurs as anhedral grains |
| Cleavage: | Poor - indistinct |
| Tenacity: | Brittle |
| Luster: | Vitreous |
| Gravity: | 3.1 (measured) |
| Opticalprop: | Uniaxial (-) |
| Refractive: | nε=1.62, nω=1.623 |
| Birefringence: | 0.0030 |
| References: | [2] [3] |
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 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,, it is not yet clear where this new mineral sits in the merrillite group hierarchy.
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.
Merrillite is a very important constituent of extraterrestrial rocks. 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]