Jun 3 2014
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