Lomonosovite

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].

General
CategoryMineral
Formula
(repeating unit)		Na10Ti4(Si2O7)2(PO4)2O4
Crystal systemTriclinic
Unit cella = 5.49 Å, b = 7.11 Å, c = 14.5 Å α = 101°, β = 96°, γ = 90°
Identification
CleavagePerfect
FractureIrregular
TenacityBrittle
Mohs scale hardness3-4
Density3.12 - 3.15
Optical propertiesBiaxial(-)
Refractive indexnα = 1.670 nβ = 1.750 nγ = 1.778
Birefringence0.108
2V angle56°

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 PO43- 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 = 1.654-1.670 = 1.736 - 1.750 =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].

References
  1. "COMMISSION ON NEW MINERALS".
  2. "Handbook of mineralogy" (PDF).
  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".
  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)
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