Bracewellite
A valid IMA mineral species
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About Bracewellite
Formula:
CrO(OH)
Colour:
Deep red to black
Lustre:
Sub-Vitreous, Sub-Metallic
Hardness:
5½ - 6½
Specific Gravity:
4.45 - 4.48
Crystal System:
Orthorhombic
Member of:
Name:
Named in 1967 by Charles Milton, Daniel E. Appleman, Edward Ching-Te Chao, Frank Cuttita, Joseph I. Dinnin, Edward J. Dwornik, Margaret Hall, Blanche L. Ingram, and Harry J. Rose, Jr. in honor of Smith Bracewell [active as a geological author at least in the time period 1927-1962], formerly Director, British Guyana Geological Survey and professor at the University of the West Indies, who first noted the occurrence of merumite, which proved to be eskolaite with a mixture of chromium minerals, including the mineral subsequently named for him.
Polymorph of:
Isostructural with:
Minute prismatic crystals in vugs or intergrown in the groundmass of eskolaite masses. As with other Cr mineral species from the Merume River locality, such as grimaldiite, guyanaite, and mcconnellite, this species is not universally present in every specimen and verification of each specimen is required.
Diaspore Group.
Diaspore Group.
Unique Identifiers
Mindat ID:
748
Long-form identifier:
mindat:1:1:748:9
IMA Classification of Bracewellite
Approved
IMA Formula:
Cr3+O(OH)
Approval year:
1967
First published:
1976
Classification of Bracewellite
4.FD.10
4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
F : Hydroxides (without V or U)
D : Hydroxides with OH, without H2O; chains of edge-sharing octahedra
4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
F : Hydroxides (without V or U)
D : Hydroxides with OH, without H2O; chains of edge-sharing octahedra
6.1.1.5
6 : HYDROXIDES AND OXIDES CONTAINING HYDROXYL
1 : XO(OH)
6 : HYDROXIDES AND OXIDES CONTAINING HYDROXYL
1 : XO(OH)
7.14.2
7 : Oxides and Hydroxides
14 : Oxides of Cr
7 : Oxides and Hydroxides
14 : Oxides of Cr
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 |
|---|---|---|
| Bwe | 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 Bracewellite
Sub-Vitreous, Sub-Metallic
Transparency:
Translucent, Opaque
Colour:
Deep red to black
Streak:
Dark brown
Hardness:
5½ - 6½ on Mohs scale
Tenacity:
Brittle
Cleavage:
None Observed
Fracture:
Irregular/Uneven
Density:
4.45 - 4.48 g/cm3 (Measured) 4.28 g/cm3 (Calculated)
Optical Data of Bracewellite
Type:
Biaxial
Comments:
RI is not reported for this species as the refractive indices are probably very high, and more impostantly, the mineral is very dark, approaching opaque. No reflected light studies are known.
Chemistry of Bracewellite
Mindat Formula:
CrO(OH)
Elements listed:
Crystallography of Bracewellite
Crystal System:
Orthorhombic
Class (H-M):
mmm(2/m2/m2/m) - Dipyramidal
Cell Parameters:
a = 4.492 Å, b = 9.860 Å, c = 2.974 Å
Ratio:
a:b:c = 0.456 : 1 : 0.302
Unit Cell V:
131.72 ų (Calculated from Unit Cell)
Z:
4
Morphology:
Very rarely as "minute deep red to black prismatic crystals [60-200 microns in size] of bracewellite line microscopic fissures and vugs in merumite". bracewellite is not present in all alluvial grains of merumite and, when present, has been found to constitute negligible amounts and in rare cases, up to a dominant part, of a merumite grain.
Comment:
Space Group originally given is Pbnm
X-Ray Powder Diffraction
Powder Diffraction Data:
| d-spacing | Intensity |
|---|---|
| 4.901 Å | (vvw) |
| 4.065 Å | (vvs) |
| 3.314 Å | (vw) |
| 2.648 Å | (vs) |
| 2.546 Å | (vw) |
| 2.548 Å | (w) |
| 2.404 Å | (vs) |
| 2.458 Å | (w) |
| 2.209 Å | (vw) |
| 2.149 Å | (w) |
| 1.678 Å | (wb) |
Comments:
Recorded on type material. Eskolaite was prominent in the purest sample that could be found and lines attributable to impurities were subtracted from the data set.
Geological Environment
Paragenetic Mode(s):
| Paragenetic Mode | Earliest Age (Ga) |
|---|---|
| Stage 7: Great Oxidation Event | <2.4 |
| 47a : [Near-surface hydration of prior minerals] | |
| 47h : [Near-surface oxidized, dehydrated minerals] |
Type Occurrence of Bracewellite
Place of Conservation of Type Material:
The Natural History Museum, London, England (No. 1979,136)
Geological Setting of Type Material:
In an alluvial placer deposit, in fine-grained intergrowth with other chromium oxide-hydroxide minerals
Associated Minerals at Type Locality:
Synonyms of Bracewellite
Other Language Names for Bracewellite
Relationship of Bracewellite to other Species
Member of:
Other Members of Diaspore Group:
| Diaspore | AlO(OH) | Orth. mmm(2/m2/m2/m) |
| Goethite | Fe3+O(OH) | Orth. mmm(2/m2/m2/m) : Pnma |
| Groutite | Mn3+O(OH) | Orth. mmm(2/m2/m2/m) : Pnma |
| Montroseite | (V3+,Fe3+)O(OH) | Orth. mmm(2/m2/m2/m) |
| Tsumgallite | GaO(OH) | Orth. mmm(2/m2/m2/m) |
Common Associates
Associations Based on Photo Data:
| 1 photo of Bracewellite associated with Grimaldiite | CrO(OH) |
| 1 photo of Bracewellite associated with Guyanaite | CrO(OH) |
Related Minerals - Strunz-mindat Grouping
| 4.FD.05 | Spertiniite | Cu(OH)2 |
| 4.FD.10 | Diaspore | AlO(OH) |
| 4.FD.10 | Goethite | Fe3+O(OH) |
| 4.FD.10 | Montroseite | (V3+,Fe3+)O(OH) |
| 4.FD.10 | Tsumgallite | GaO(OH) |
| 4.FD.10 | Groutite | Mn3+O(OH) |
| 4.FD.10 | Guyanaite | CrO(OH) |
| 4.FD.15 | Manganite | Mn3+O(OH) |
| 4.FD.20 | Yttrotungstite-(Y) | YW2O7(OH) · H2O |
| 4.FD.20 | Yttrotungstite-(Ce) | CeW2O7(OH) · H2O |
| 4.FD.25 | Frankhawthorneite | Cu2Te6+O4(OH)2 |
| 4.FD.30 | 'Khinite-3T' | Pb2+Cu2+3Te6+O6(OH)2 |
| 4.FD.30 | Housleyite | Pb6CuTe6+4O18(OH)2 |
| 4.FD.30 | Khinite | Pb2+Cu2+3[Te6+O6](OH)2 |
Fluorescence of Bracewellite
Not fluorescent in UV
Other Information
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Internet Links for Bracewellite
mindat.org URL:
https://www.mindat.org/min-748.html
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References for Bracewellite
Localities for Bracewellite
Showing 2 localities.
symbol to view information about a locality.
The Locality List
- This locality has map coordinates listed.
- This locality has estimated coordinates.
ⓘ - Click for references and further information on this occurrence.
? - Indicates mineral may be doubtful at this locality.
- Good crystals or important locality for species.
- World class for species or very significant.
(TL) - Type Locality for a valid mineral species.
(FRL) - First Recorded Locality for everything else (eg varieties).
All localities listed without proper references should be considered as questionable.
Australia | |
| Francis (2010) |
Guyana (TL) | |
| Geol.Soc.Amer. (1967) +1 other reference |
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Merume River, Rest of Region 8, Potaro-Siparuni Region, Guyana