Lomonosovite is a phosphate–silicate mineral with the idealized formula Na10Ti4(Si2O7)2(PO4)2O4 [2] early Na5Ti2(Si2O7)(PO4)O2 or Na2Ti2Si2O9*Na3PO4.[3]
| Lomonosovite | |
|---|---|
| General | |
| Category | Mineral |
| Formula (repeating unit) | Na10Ti4(Si2O7)2(PO4)2O4 |
| IMA symbol | Lom[1] |
| Crystal system | Triclinic |
| Unit cell | a = 5.49 Å, b = 7.11 Å, c = 14.5 Å α = 101°, β = 96°, γ = 90° |
| Identification | |
| Cleavage | Perfect |
| Fracture | Irregular |
| Tenacity | Brittle |
| Mohs scale hardness | 3–4 |
| Density | 3.12 – 3.15 |
| Optical properties | Biaxial(−) |
| Refractive index | nα = 1.670 nβ = 1.750 nγ = 1.778 |
| Birefringence | 0.108 |
| 2V angle | 56° |
The main admixtures are niobium (up to 11.8% Nb2O5), manganese (up to 4.5 %MnO) and iron (up to 2.8%).[4]
Discovery and name
The mineral was discovered by V.I. Gerasimovskii [5] in Lovozersky agpaitic massif. Named for Mikhail Lomonosov – famous Russian poet, chemist and philosopher, but the earlier – mining engineer.[6]
Crystal structure
According to X-ray data, lomonosovite structure was determined is triclinic unit cell with parameters: a = 5.44 Å, b = 7.163 Å, c = 14.83 Å, α = 99°, β = 106°, and γ = 90°, usually centrosymmetric (sp. gr. P-1),[7] but acentric varieties (polytype) are also reported.[4]
The crystal structure of lomonosovite is based on three-layer HOH packets consisting of a central octahedral O layer and two outer heteropolyhedral H layers. Ti- and Na centered octahedra are distinguished in the O layer, whereas the H layers are composed of Ti-centered octahedra and Si2O7 diorthogroups, (like in other heterophyllosilicates, for example lamprophyllite). The interpacket space includes Na+ cations and PO43- anions.[7]
Properties
Lomonosovite forms lamellar and tabular crystals with perfect cleavage. It is macroscopically brown, from cinnamon-brown to black. It is transparent in thin plates. The luster vitreous to adamantine.
Its pleochroism is strong from colorless to brown. The refractive index is 
Hardness 3–4 Density 3.12 – 3.15.[3]
Origin
Accessory mineral of peralkaline agpaitic nepheline syenites (like Khibina and Lovozero massif, Russia, Ilimaussaq intrusion, Greenland) important mineral of agpaitic pegmatites and peralkaline fenites.[3]