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Segnitite
A valid IMA mineral species
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About Segnitite
Edgar Ralph Segnit
Formula:
PbFe3+3AsO4(AsO3OH)(OH)6
May contain, as other Fe(III) members, minor Sb, Cu and Zn substituting for Fe.
Colour:
Greenish to yellowish-brown
Lustre:
Adamantine, Vitreous
Hardness:
4
Specific Gravity:
4.2
Crystal System:
Trigonal
Member of:
Name:
Named in honor of Edgar Ralph Segnit (5 September 1923, Adelaide, South Australia, Australia - 13 July 1999, Melbourne, Australia), geologist and gemmologist at CSIRO Division of Mineral Chemistry in Melbourne, for his contributions to Australian mineralogy.
The arsenate analogue of kintoreite.
Difficult to distinguish from beudantite, with which it forms a series.
Related to kolitschite.
Difficult to distinguish from beudantite, with which it forms a series.
Related to kolitschite.
Unique Identifiers
Mindat ID:
3606
Long-form identifier:
mindat:1:1:3606:7
IMA Classification of Segnitite
Approved
IMA Formula:
Pb2+Fe3+3(As5+O4)(As5+O3OH)(OH)6
Approval year:
1991
First published:
1992
Classification of Segnitite
8.BL.10
8 : PHOSPHATES, ARSENATES, VANADATES
B : Phosphates, etc., with additional anions, without H2O
L : With medium-sized and large cations, (OH, etc.):RO4 = 3:1
8 : PHOSPHATES, ARSENATES, VANADATES
B : Phosphates, etc., with additional anions, without H2O
L : With medium-sized and large cations, (OH, etc.):RO4 = 3:1
42.7.4.4
42 : HYDRATED PHOSPHATES, ETC.CONTAINING HYDROXYL OR HALOGEN
7 : (AB)2(XO4)Zq·xH2O
42 : HYDRATED PHOSPHATES, ETC.CONTAINING HYDROXYL OR HALOGEN
7 : (AB)2(XO4)Zq·xH2O
20.5.13
20 : Arsenates (also arsenates with phosphate, but without other anions)
5 : Arsenates of Ti and Pb
20 : Arsenates (also arsenates with phosphate, but without other anions)
5 : Arsenates of Ti and Pb
Mineral Symbols
As of 2021 there are now IMA–CNMNC approved mineral symbols (abbreviations) for each mineral species, useful for tables and diagrams.
| Symbol | Source | Reference for Standard |
|---|---|---|
| Sgt | IMA–CNMNC | Warr, L.N. (2021). IMA–CNMNC approved mineral symbols. Mineralogical Magazine, 85(3), 291-320. doi:10.1180/mgm.2021.43 |
Physical Properties of Segnitite
Adamantine, Vitreous
Transparency:
Transparent, Translucent
Colour:
Greenish to yellowish-brown
Streak:
Pale yellow
Hardness:
4 on Mohs scale
Tenacity:
Brittle
Cleavage:
Distinct/Good
Good on {0001}
Good on {0001}
Fracture:
Irregular/Uneven
Density:
4.2 g/cm3 (Measured) 4.77 g/cm3 (Calculated)
Optical Data of Segnitite
Type:
Uniaxial (-)
RI values:
nω = 1.975 nε = 1.955
Max. Birefringence:
δ = 0.020
Based on recorded range of RI values above.
Based on recorded range of RI values above.
Interference Colours:
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.
Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.
Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.
Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.
Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.
Surface Relief:
Very High (positive)
Relative to Canada balsam mounting medium (n ≈ 1.537).
Relative to Canada balsam mounting medium (n ≈ 1.537).
This shows the grain boundary and Becke line effect under plane-polarised
light, based on the contrast between this mineral's average refractive
index and the mounting medium. It does not take into account mineral
colouration.
In focus
Interference Figure:
This shows the idealized uniaxial interference figure - the conoscopic
(convergent-light, Bertrand-lens-in) view, for a grain cut with the optic axis
centred and vertical. The coloured rings are isochromatics, computed with the
same physics as the Michel-Lévy bar above; the dark cross is the isogyre.
For a genuinely uniaxial mineral viewed this way, that cross stays perfectly stationary if you rotate the stage - unlike a biaxial mineral, where it splits apart on rotation. That invariance is itself the standard diagnostic test for telling uniaxial and biaxial minerals apart at the microscope.
For a genuinely uniaxial mineral viewed this way, that cross stays perfectly stationary if you rotate the stage - unlike a biaxial mineral, where it splits apart on rotation. That invariance is itself the standard diagnostic test for telling uniaxial and biaxial minerals apart at the microscope.
Pleochroism:
Weak
Comments:
Pale to medium yellow.
Chemistry of Segnitite
Mindat Formula:
PbFe3+3AsO4(AsO3OH)(OH)6
May contain, as other Fe(III) members, minor Sb, Cu and Zn substituting for Fe.
May contain, as other Fe(III) members, minor Sb, Cu and Zn substituting for Fe.
Element Weights:
Chemical Analysis
Oxide wt%:
| 1 | |
|---|---|
| PbO | 28.13 % |
| ZnO | 0.21 % |
| Fe2O3 | 32.04 % |
| As2O5 | 28.83 % |
| SO3 | 0.86 % |
| H2O calc | 7.94 % |
| Total: | 98.01 % |
Sample references:
Crystallography of Segnitite
Crystal System:
Trigonal
Class (H-M):
3m(32/m) - Hexagonal Scalenohedral
Space Group:
R3m
Cell Parameters:
a = 7.359(3) Å, c = 17.113(8) Å
Ratio:
a:c = 1 : 2.325
Unit Cell V:
802.59 ų (Calculated from Unit Cell)
Z:
3
Morphology:
Rhombohedral (mainly acute rhombohedra), pseudo-octahedral, pseudo-cubic or tabular crystals; very rarely also acicular.
The principal forms present in the type material are {112} and {001}.
The principal forms present in the type material are {112} and {001}.
Twinning:
None observed
X-Ray Powder Diffraction
Powder Diffraction Data:
| d-spacing | Intensity |
|---|---|
| 3.092 Å | (100) |
| 5.966 Å | (50) |
| 3.678 Å | (40) |
| 2.283 Å | (30) |
| 1.992 Å | (30) |
| 1.840 Å | (25) |
| 2.849 Å | (20) |
Geological Environment
Paragenetic Mode(s):
| Paragenetic Mode | Earliest Age (Ga) |
|---|---|
| Stage 7: Great Oxidation Event | <2.4 |
| 47c : [Carbonates, phosphates, borates, nitrates] | |
| 47d : [Arsenates, antimonates, selenates, bismuthinates] |
Geological Setting:
Rare mineral found in the oxidation zone of hydrothermal ore deposits.
Type Occurrence of Segnitite
General Appearance of Type Material:
Greenish brown to yellowish brown, pseudo-octahedral crystals up to 1 mm across, and as aggregates, crusts and clusters of yellowish brown rhombohedral crystals up to 5 mm high.
Place of Conservation of Type Material:
Museum Victoria, Melbourne, Australia, M30044.
Geological Setting of Type Material:
Oxidation of primary ore rich in galena and arsenopyrite-loellingite.
Associated Minerals at Type Locality:
Alto das Quelhas do Gestoso Mine, Gestoso, Manhouce, São Pedro do Sul, Viseu, Portugal