Lomonosovite

Lomonosovite is a phosphate–silicate mineral with the idealized formula Na₁₀Ti₄(Si₂O₇)₂(PO₄)₂O₄ early Na₅Ti₂(Si₂O₇)(PO₄)O₂ or Na₂Ti₂Si₂O₉*Na₃PO₄.

The main admixtures are niobium (up to 11.8% Nb2O5), manganese (up to 4.5 %MnO) and iron (up to 2.8%).

Discovery and name

The mineral was discovered by V.I. Gerasimovskii in Lovozersky agpaitic massif. Named for Mikhail Lomonosov - famous Russian poet, chemist and philosopher, but the earlier - mining engineer.

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), but acentric varieties (polytype) are also reported.

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 PO₄^3- anions.

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 $\alpha$= 1.654-1.670 $\beta$ = 1.736 - 1.750 $\gamma$=1.764-1.778 2V=56-69.

Hardness 3-4 Density 3.12 - 3.15.

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.

References

Crystals
Phosphate minerals
Silicate minerals

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