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Vigezzite, (Ca, Ce) (Nb, Ta,Ti)2O6, a new aeschynite-type mineral from the Alps

Published online by Cambridge University Press:  05 July 2018

S. Graeser ,
H. Schwander ,
H. Hänni  and
V. Mattioli
S. Graeser
Affiliation:
Mineralogical Institute, University, CH-4056 Basle
H. Schwander
Affiliation:
Mineralogical Institute, University, CH-4056 Basle
H. Hänni
Affiliation:
Mineralogical Institute, University, CH-4056 Basle
V. Mattioli
Affiliation:
Via Keplero 5, I-20124 Milano, Italy
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Summary

A new mineral species, vigezzite (Ca,Ce) (Nb,Ta,Ti)2O6, was found near Orcesco, Valle Vigezzo, Prov. Novara, Northern Italy. It forms fiat prismatic crystals up to 2–3 mm length of an orange-yellow colour. The crystals are elongated along the a-axis and flattened on (010). Cleavage at right angles to the prism axis is distinct, fracture conchoidal. Vickers hardness VHN = 396 kp/mm2, corresponding to in the Mohs scale. Refraction indices are high: α = 2.14, γ = 2.315 (NaD, 589 µm), birefringence is 0.175. Optic angle 2Vγ is large. Though intensively yellow coloured, the mineral shows no pleochroism. α = c, β = b, γ = a; optic axial plane is parallel to (010).

Vigezzite is orthorhombie with space group Pmnb or P21nb; a = 7.559±0.01, b = 11.028±0.01, c = 5.360± 0.004 Å, Z = 4. The six strongest lines of the X-ray powder pattern are 4.821 (9) (011), 3.784 (8) (200), 3.036 (10) (031), 2.974 (10) (211), 1.7129 (6) (242), 1.6018 (7) (431).

Electron microprobe analyses lead to the empirical formula (Ca0.82Ce0.24)Σ1.06(Nb0.9Ta0.62Ti0.5)Σ2.02O6. The mineral is not completely homogenous, the ratio Nb/(Nb+Ta) varies from 0.55–0.62. Rynersonite Ca (Ta,Nb)2O6, may represent the Ta-dominant equivalent of vigezzite. Vigezzite occurs together with several other Nb-minerals such as pyrochlore, columbite and fersmite, whose crystallographical and chemical data are reported, in an albitic rock of presumably pegmatitic origin.

Type
Research Article
Information
Mineralogical Magazine , Volume 43 , Issue 328 , December 1979 , pp. 459 - 462
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1979

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References

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