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Fianelite

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Fianelite
General
CategoryVanadate
Formula
(repeating unit)
Mn2V(V,As)O7 · 2H2O
IMA symbolFnl[1]
Strunz classification08.FC.05
Crystal systemMonoclinic
Crystal class2/m
Space groupP21/n
Unit cella = 7.809;Å, b = 14.554;Å
c = 6.705 Å, β = 93.27(3)◦
Identification
ColorOrange-Red
Crystal habitPlaty
TwinningNone observed
CleavageGood on {001} and {100}
Mohs scale hardness3
LusterVitreous
DiaphaneityTransparent
Specific gravity3.22 gm/cc
Density3.21 g/cm3
Refractive index1.85±0.02
Birefringenceδ=1.820
DispersionGood
References[2][3][4]

Fianelite is a mineral belonging to the manganese vanadate category, found in iron-manganese ores. Named after the place where it was found, Fianel, a mine located in Val Ferrera (Ferrera valley), in the canton of Graubünden, Swiss. This mineral is found in small amounts in metamorphosed iron-manganese ores. This is the last crystallization of vanadate at the site since medaite was the last vanadate mineral being crystallized, but because of retrograde metamorphism, occurring at the place, vanadium moved into medaite veinlets, forming cross-cutting fianelite on medaite.[2]

Occurrence

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This mineral is found in the Suretta nappe which is composed of three main units going from a quartzite base to dolomitic and calcitic marble, to a calcschist top. Fianelite is only found at the Triassic dolomitic and calcitic marbles unit which contains metamorphosed iron and manganese ores. Fianelite is found as small fractures of less than 1 mm in size in association with quartz, aegirine, and iron hydroxide-oxides like limonite. These fractures are found cross-cutting veinlets, which are made out of quartz, rhodonite, kutnohorite, aegirine, parsettensite, and different vanadium minerals, that are at the same time cross-cutting lenses of medaite-rich rocks which are found in the iron and manganese ores.[5][2]

Physical properties

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Fianelite's crystals grow in polycrystalline crusts and rosettes that can measure up to 2.5 mm in diameter, and when strangely found as single crystals, they can measure up to 0.2 mm in diameter. It presents good parallel cleavage on {001} and {100}. The single crystals resent a platy habit and striations parallel to the a-axis. VHN values were measured and used to determine a Mohs hardness of approximately 3.[2]

Optical properties

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Fianelite is an orange-red mineral with a vitreous luster and orange streak. Its crystals are transparent with striations parallel to the a-axis. The crystals are biaxial, so they have two optic axes meaning that they will split light rays into two. The angle between the two optic axes is less than 10°. It shows a strong pleochroism on (010) displaying different colors from yellow to red.[2]

Chemical properties

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The empirical formula for fianelite is Mn1.98(V1.57As0.44Si0.01)O7·2H2O. Manganese dioxide (MnO6) and manganese trioxide (MnO2O6) are found in chains of octahedrons that are connected by two tetrahedrons of orthovanadate (VO4), from which one of them bonds with arsenic.[2]

Chemical composition

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Oxide Wt%
V2O5 38.23
As2O5 13.57
SiO2 0.12
MnO 37.49
Total 89.41
H2O 9.64
Total 99.05

[2]

X-ray crystallography

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Fianelite belongs to the monoclinic crystal system with a space group of P21/n. The unit cell dimensions are a = 7.809(2) b = 14.554(4) c = 6.705(4) β = 93.27(3)◦ Z=4

d-spacing Intensity hkl
3.039 100 022
5.32 80 120
2.721 60 231
1.593 60 163
3.436 50 220
3.259 50 012
2.573 50 212
2.444 50 320

[2]

See also

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References

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  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c d e f g h Joel, Brugger, Peter, Berlepsch; Description and crystal structure of fianelite, Mn2V(V,As)O7.2H2O, a new mineral from Fianel, Val Ferrera, Graubunden, Switzerland. American Mineralogist 1996; 81 (9-10): 1270–1276. doi: https://doi.org/10.2138/am-1996-9-1025.
  3. ^ "Fianelite". Mindat.org.
  4. ^ "Fianelite". webmineral.com.
  5. ^ Stampfli, Gérard & Mosar, Jon & Marquer, Didier & Marchant, Robin & Baudin, Thierry & Borel, G.. (1998). Subduction and obduction processes in the Swiss Alps. Tectonophysics. 296. 10.1016/S0040-1951(98)00142-5.