Francisite - Occurrence, Properties, and Distribution

Topics Covered

Introduction to Francisite
Properties of Francisite
How to Identify Francisite
Global Distribution
Occurrence of Francisite and Useful Mineral Association
References

Introduction to Francisite

Chemical Formula – Cu3Bi(SeO3)2O2Cl

Francisite is an orthorhombic-dipyramidal light green mineral, containing selenium, oxygen, copper, chlorine, and bismuth.

It was named in honor of Glyn Francis, the Quality Control Officer at the Iron Monarch quarry, Australia, who first collected the mineral from the quarry in 1990.

Properties of Francisite

The following are the key properties of Francisite:

  • Cell Data
    • Space Group: Pmmn
    • a = 6.354(1)
    • b = 9.630(1)
    • c = 7.220(2)
    • Z = 2

 

  • Crystal Data
    • Orthorhombic
    • Point Group: 2/m 2/m 2/m
    • As bladed crystals, elongated along [010], with dominant forms {100}, {011}, and {101}, to 0.25mm; in divergent sprays
    • Twinning: Contact twins on {100}
    • X-ray Powder Pattern: 3.39 (100), 2.866 (80), 2.652 (70), 5.31 (60), 2.491 (60), 1.588 (60), 1.695 (35)

 

  • Chemical Composition
    Elements Content 1 Content 2
    Bi2O3 35.75 32.31
    CuO 32.62 33.10
    SeO2 27.23 30.78
    Cl 4.11 4.92
    -O= Cl2 0.93 1.11
    Total 98.78 100.00
  • Optical Properties
    • Optical Class: [Biaxial]
    • Pleochroism: Distinct, from pale green to bright green || [010]
    • Orientation: Length-slow
    • α = > 1.79
    • β = > 1.79
    • γ = > 1.79
    • 2V(meas.) = n.d

 

  • Estimated Properties
    Electron density Bulk density (electron density) = 4.80 g/cm3
    note: Specific gravity of Francisite = 5.42 g/cm3
    Photoelectric PEFrancisite = 557.48 barns/electron
    U = PEFrancisite x ρElectron density = 2673.78 barns/cm3
    Fermion index Fermion index = 0.01
    Boson index = 0.99
    Radioactivity
    Francisite is not radioactive

How to Identify Francisite

Francisite can be found in a light green color, with adamantine luster, and light green streak. It has a transparent appearance.

The hardness of francisite ranges from 3 to 4.

Global Distribution

Francisite is distributed in the following places:

  • Iron Monarch quarry, Iron Knob, South Australia
  • Baccu Locci mine, near Villaputzu, Sarrabus district, Sardinia, Italy

Occurrence of Francisite and Useful Mineral Association

Francisite occurs as a rare secondary mineral in an oxidized barite lens in Pre-cambrian sedimentary iron formation.

It is closely associated with minerals such as barite, djurleite, bismuth, Cu–Bi–Ag selenides, naumannite, muscovite, and chorargyrite.

References

 

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