Vigezzite is a variant of the mineral aeschynite containing calcium, cerium, niobium, tantalum, and titanium. It was first discovered near Orcesco,

Valle Vigezzo Valle Vigezzo (Vigezzo Valley) is a valley in the Lepontine Alps in northwestern Italy on the border with Ticino, Canton Ticino of Switzerland. It forms the eastern branch of the Ossola valley. It is also called the Valle dei Pittori (Valley of ...

, 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)₂ O₆

Occurrence

Vigezzite is found in association with albitic rock in

miarolitic cavities Miarolitic cavities (or ''miarolitic texture'') are typically crystal-lined irregular cavities or vugs most commonly found in granitic pegmatites, and also in a variety of igneous rocks. The central portions of pegmatites are often miarolitic as ...

. Albitic rock is rock which contains the mineral

albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. I ...

and is closely associated with

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

s. When found with these minerals, the vigezzite crystals are implanted on the surface of the other minerals and rock. It occurs in

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...

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 Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...

, 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.

References

{{Authority control Niobium minerals Tantalum minerals Orthorhombic minerals Cerium minerals Minerals described in 1979