Lomonosovite

General
Category	Mineral
Formula; (repeating unit)	Na10Ti4(Si2O7)2(PO4)2O4
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°

Lomonosovite is a phosphate–silicate mineral with the idealized formula Na₁₀Ti₄(Si₂O₇)₂(PO₄)₂O₄ [1] early Na₅Ti₂(Si₂O₇)(PO₄)O₂ or Na₂Ti₂Si₂O₉*Na₃PO₄ [2].

The main admixtures are niobium (up to 11.8% Nb2O5), manganese (up to 4.5 %MnO) and iron (up to 2.8%)[3].

Discovery and name

The mineral was discovered by V.I. Gerasimovskii [4] in Lovozersky agpaitic massif. Named for Mikhail Lomonosov - famous Russian poet, chemist and philosopher, but the earlier - mining engineer[5].

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)[6] , but acentric varieties (polytype) are also reported[3].

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 [6].

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 [2].

Hardness 3-4 Density 3.12 - 3.15 [2].

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[2].

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References

"COMMISSION ON NEW MINERALS".
"Handbook of mineralogy" (PDF).
Rastsvetaeva, Zaitsev, Pekov (2020). "Crystal Structure of Niobium-Rich Lomonosovite with Symmetry P1 from the Khibiny Massif (Kola Peninsula)". Crystallography Reports. 65, No. 3: 22–427.CS1 maint: multiple names: authors list (link)
Gerasimovskii, V. I. "Lomonosovite, a new mineral". Dokl. Akad. Nauk SSSR. 70.
"Maburg. Song of Yu.Lores".
Rastsvetaeva, Simonov, Belov (1971). "Crystalline structure of lomonosovite, Na5Ti2[Si2O7][PO4]O2". Dokl. Akad. Nauk SSSR. 197 (1): 81–84.CS1 maint: multiple names: authors list (link)

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[1] "COMMISSION ON NEW MINERALS".

[:0-2] "Handbook of mineralogy" (PDF).

[:1-3] Rastsvetaeva, Zaitsev, Pekov (2020). "Crystal Structure of Niobium-Rich Lomonosovite with Symmetry P1 from the Khibiny Massif (Kola Peninsula)". Crystallography Reports. 65, No. 3: 22–427.CS1 maint: multiple names: authors list (link)

[4] Gerasimovskii, V. I. "Lomonosovite, a new mineral". Dokl. Akad. Nauk SSSR. 70.

[5] "Maburg. Song of Yu.Lores".

[:2-6] Rastsvetaeva, Simonov, Belov (1971). "Crystalline structure of lomonosovite, Na5Ti2[Si2O7][PO4]O2". Dokl. Akad. Nauk SSSR. 197 (1): 81–84.CS1 maint: multiple names: authors list (link)