Vigezzite is a variant of the mineral aeschynite containing calcium, cerium, niobium, tantalum, and titanium. It was first discovered near Orcesco, Valle Vigezzo, Provo Novara, Northern Italy, in cavities of an albitic rock. The crystals of Vigezzite are flat prismatic crystals up to 2-3 mm length of an orange-yellow color.The name Vigezzite was chosen to draw attention to the locality that has produced the first occurrence of a Ca-Nb-Ta-mineral with Nb dominance over Ta, crystallizing with the aeschynite structure. The ideal chemical formula for vigezzite is (calcium,Cerium),(Niobium,tantalum,titanium)2oxygen6
Occurrence
Vigezzite is found in association with albitic rock in miarolitic cavities. Albitic rock is rock which contains the mineral
albite and is closely associated with
. When found with these minerals, the vigezzite crystals are implanted on the surface of the other minerals and rock. It occurs in
pegmatite dikes that contain potassium feldspar converted to
sodic feldspar by an
albitization process. This dike formation is a direct relation to the metamorphism that occurs in the area and is the reason for the formation of vigezzite.
Physical properties
Vigezzite is an orange-yellow colored transparent mineral forming thin brittle crystals with a sub metallic luster. It exhibits a hardness of 4.5-5 on the Mohs hardness scale. Vigezzite forms long prismatic crystals up to 2-3 mm in length, elongated along their a-axis and flattened on (010). The crystals are strongly striated on (010) parallel to prism axis. The prisms are terminated by the pinacoids (100) only. The measured density is 5.54. g/cm
3.
Optical properties
Vigezzite is biaxial positive, which means it will refract light differently along two axes. It exhibits an extreme dispersion, and displays no
pleochroism.
Chemical properties
The empirical chemical formula for vigezzite is (Ca
0.82,Ce
0.24)
Σ1.06(Nb
0.9,Ta
0.62,Ti
0.5)
Σ2.02O
6,.
Chemical composition
|
|
| CaO | 12.0 |
| Ce2O3 | 10.5 |
| TiO2 | 10.5 |
| Nb2O5 | 31.0 |
| Ta2O5 | 36.0 |
| Total | 100.0 |
X-ray crystallography
Vigezzite is in the orthorhombic crystal system, with
space group Pnmb. The
unit cell dimensions are a=7.55 Å, b=11.03 Å, c=5.36 Å, α=90.00°, β=90.00°, γ=90.00°. Because the mineral occurs in small but well-shaped single crystals, X-ray photographs are of excellent quality. For vigezzite, the morphological prism axis parallels the crystallographic a-axis.
See also
