AbstractKihlmanite-(Ce),
Ce2TiO2[SiO4](HCO3)2(H2O),
is a new rare-earth titanosilicate carbonate, closely related to
tundrite-(Ce). It is triclinic, P, a = 4.994(2), b
= 7.54(2), c = 15.48(4) Å, α = 103.5(4), β = 90.7(2), γ =
109.2(2)o , V = 533(1) Å3,
Z = 2 (from powder diffraction data) or
a = 5.009(5), b = 7.533(5),
c = 15.407(5) Å, α = 103.061(5), β = 91.006(5), γ =
109.285(5)°, V = 531.8(7) Å3, Z
= 2 (from single-crystal X-ray diffraction data). The mineral was found in
the arfvedsonite-aegirine-microcline vein in fenitized metavolcanic rock at
the foot of the Mt Kihlman (Chil’man), near the western contact of the
Devonian Khibiny alkaline massif and the Proterozoic Imandra-Varzuga
greenstone belt. It forms brown spherulites (up to 2 cm diameter) and
sheaf-like aggregates of prismatic crystals, flattened on {010} and up to
0.5 mm diameter. Both spherulites and aggregates occur in interstices in
arfvedsonite and microcline, in intimate association with golden-green
tundrite-(Ce). Kihlmanite-(Ce) is brown, with a vitreous lustre and a pale
yellowish-brown streak. The cleavage is perfect on {010}, parting is
perpendicular to c and the fracture is stepped. Mohs hardness is ∼3. In
transmitted light, the mineral is yellowish brown; pleochroism and
dispersion were not observed. Kihlmanite-(Ce) is biaxial (+), α = 1.708(5),
β = 1.76(1), γ = 1.82(1) (589 nm), 2Vcalc = 89°. The optical orientation is Y ^
c = 5°, other details are unclear. The calculated and
measured densities are 3.694 and 3.66(2) g cm−3, respectively.
The mean chemical composition, determined by electron microprobe, is:
Na2O 0.13, Al2O3 0.24, SiO2
9.91, CaO 1.50, TiO2 11.04, MnO 0.26, Fe2O3
0.05, Nb2O5 2.79, La2O3 12.95,
Ce2O3 27.33, Pr2O3 2.45,
Nd2O3 8.12, Sm2O3 1.67,
Gd2O3 0.49 wt.%, with CO2 15.0 and
H2O 6.0 wt.% (determined by wet chemical and Penfield methods,
respectively), giving a total of 99.93 wt.%. The empirical formula
calculated on the basis of Si + Al = 1 atom per formula unit is
(Ca0.16Na0.11Mn0.02)∑0.29[(Ce0.98La0.47Pr0.09Nd0.29Sm0.06Gd0.02)∑1.91(Ti0.82Nb0.12)∑0.94O2
(Si0.97Al0.03)∑1O4.02(HCO3)2.01](H2O)0.96.
The simplified formula is Ce2TiO2(SiO4)(HCO3)2·H2O. The mineral reacts
slowly in cold 10% HCl with weak effervescence and fragmentation into
separate plates. The strongest X-ray powder-diffraction lines [listed as
d in Å(I) (hkl)] are
as follows: 15.11(100)(00),
7.508(20)(00),
6.912(12)(01),
4.993(14)(00),
3.563(15)(01),
2.896(15)(1). The crystal structure of kihlmanite-(Ce) was
refined to R1 = 0.069 on the basis of 2441 unique observed reflections
(MoKα, 293 K). It is closely related to the crystal
structure of tundrite-(Ce) and is based upon
[Ce2TiO2(SiO4)(HCO3)2]
layers parallel to (001). Kihlmanite-(Ce) can be considered as a
cationdeficient analogue of tundrite-(Ce). The mineral is named in honour of
Alfred Oswald Kihlman (1858–1938), a remarkable Finnish geographer and
botanist who participated in the Wilhelm Ramsay expeditions to the Khibiny
Mountains in 1891–1892. The mineral name also reflects its occurrence at the
Kihlman (Chil’man) Mountain.
X-Ray Powder Diffraction of Einsteinium Compounds
