Vigezzite
General
Category	Oxide mineral
Formula (repeating unit)	(Ca,Ce),(Nb,Ta,Ti)2O6
IMA symbol	Vgz[1]
Crystal system	Orthorhombic
Crystal class	Dipyramidal (same H-M symbol)
Space group	Pnmb
Unit cell	a = 11.065 Å, b = 7.527 Å c = 5.343 Å; α=90.00° β=90.00°, γ=90.00°; Z=4
Identification
Color	Orange-yellow
Crystal habit	Flat Prismatic crystals
Cleavage	Along {100}
Fracture	conchoidal
Mohs scale hardness	4.5-5
Luster	Sub Metallic
Diaphaneity	Subtranslucent to Opaque
Density	5.54 g/cm3
Optical properties	biaxial positive
Refractive index	nα= 2.140 nγ= 2.315
Birefringence	δ=0.175
Dispersion	extreme
References	[2][3][4]

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 (Ca,Ce),(Nb,Ta,Ti)₂O₆

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 feldspars. 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/cm³.

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)_Σ2.02O₆,.

Chemical composition

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

• List of minerals

References