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.
Rare-earth mineral deposits of the Mountain Pass District, San Bernardino County, California