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Bracewellite

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Formula:
CrO(OH)
System:
Orthorhombic
Colour:
Deep red to black
Hardness:
5½ - 6½
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.

Classification of Bracewellite

Approved
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
6.1.1.5

6 : HYDROXIDES AND OXIDES CONTAINING HYDROXYL
1 : XO(OH)
7.14.2

7 : Oxides and Hydroxides
14 : Oxides of Cr
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First Recorded Occurrence of Bracewellite

Place of Conservation of First Recorded Material:
The Natural History Museum, London, England (No. 1979,136)
Year of Discovery:
1967
Geological Setting of First Recorded Material:
In an alluvial placer deposit, in fine-grained intergrowth with other chromium oxide-hydroxide minerals
Associated Minerals at First Recorded Locality:

Physical Properties of Bracewellite

Sub-Vitreous, Sub-Metallic
Diaphaneity (Transparency):
Translucent, Opaque
Colour:
Deep red to black
Streak:
Dark brown
Hardness (Mohs):
5½ - 6½
Tenacity:
Brittle
Cleavage:
None Observed
Fracture:
Irregular/Uneven
Density:
4.45 - 4.48 g/cm3 (Measured)    4.28 g/cm3 (Calculated)

Crystallography of Bracewellite

Crystal System:
Orthorhombic
Class (H-M):
mmm (2/m 2/m 2/m) - Dipyramidal
Cell Parameters:
a = 4.492Å, b = 9.860Å, c = 2.974Å
Ratio:
a:b:c = 0.456 : 1 : 0.302
Unit Cell Volume:
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 Data:
d-spacingIntensity
4.901 (vvw)
4.065 (vvs)
3.314 (vw)
2.648 (vs)
2.546 (vw)
2.548 (w)
2.404 (vs)
2.458 (w)
2.209 (vw)
2.149 (w)
1.678 (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.

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.

Chemical Properties of Bracewellite

Formula:
CrO(OH)
Essential elements:
All elements listed in formula:
Analytical Data:
(1) Chemical analysis of type material, estimated to contain about 30 wt.-% of eskolaite
(2) Theoretical values
Electron microprobe analysis (Milton, 1967) indicated that bracewellite varied from 1-12% in Fe2O3 and 2-5% Al2O3.
         (1)      (2)

TiO2    (0.17)
Al2O3   (6.2)
Fe2O3  (15.1)
Cr2O3  (70.6)   (89.40)
V2O3    (0.64)
H2O     (6.9)   (10.60)

sum     99.6    100.00 wt.-%

Relationship of Bracewellite to other Species

Member of:
Other Members of Group:
DiasporeAlO(OH)
GroutiteMn3+O(OH)
Montroseite(V3+,Fe3+)O(OH)
TsumgalliteGaO(OH)
4.FD.05SpertiniiteCu(OH)2
4.FD.10DiasporeAlO(OH)
4.FD.10GroutiteMn3+O(OH)
4.FD.10GuyanaiteCrO(OH)
4.FD.10Montroseite(V3+,Fe3+)O(OH)
4.FD.10TsumgalliteGaO(OH)
4.FD.15ManganiteMn3+O(OH)
4.FD.20Yttrotungstite-(Y)YW2O6(OH)3
4.FD.20Yttrotungstite-(Ce)(Ce,Nd,Y)W2O6(OH)3
4.FD.20Unnamed (Nd-analogue of Yttrotungstite-(Ce))(Nd,Ce,La)W2O6(OH)3
4.FD.25FrankhawthorneiteCu2Te6+O4(OH)2
4.FD.30KhinitePb2+Cu32+[Te6+O6](OH)2
4.FD.30Khinite-3TPb2+Cu32+Te6+O6(OH)2
7.14.1EskolaiteCr2O3
7.14.3GrimaldiiteCrO(OH)
7.14.4GuyanaiteCrO(OH)
7.14.5YimengiteK(Cr,Ti,Fe,Mg)12O19
7.14.6Mathiasite(K, Ba, Sr)(Zr, Fe)(Mg,Fe)2 (Ti,Cr,Fe)18O38
7.14.7McconnelliteCuCrO2
7.14.8RedledgeiteBa(Ti64+Cr23+)O16
7.14.9HawthorneiteBa(MgTi3Cr4Fe4)O19
7.14.10ZincochromiteZnCr2O4
7.14.11Wattersite(Hg2+)2Hg2+(CrO4)O2
7.14.12ChromiteFe2+Cr23+O4
7.14.13MagnesiochromiteMg(Cr,Al,Fe)2O4

Other Names for Bracewellite

Name in Other Languages:

Other Information

Not fluorescent in UV
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Bracewellite

Reference List:
Milton, C., Appleman, D. E., Appleman, M. H., Chao, E. C. T., Cuttitta, F., Dinnin, J. I., Dwornik, E. J., Ingram, B. L. and Rose, H. J., Jr. (1976): Merumite, a complex assemblage of chromium minerals from Guyana. U.S. Geol. Surv. Prof. Paper 887, 1-29. - American Mineralogist (1977), 62, 593 (abstract).

Anthony, J. W. et al. (1997): Handbook of Mineralogy, Vol. 3, 75.

Internet Links for Bracewellite

Localities for Bracewellite

map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
(TL) indicates type locality for a valid mineral species. (FRL) indicates first recorded locality for everything else. ? indicates mineral may be doubtful at this locality. All other localities listed without reference should be considered as uncertain and unproven until references can be found.
Australia
 
  • South Australia
    • Eyre Peninsula
      • Middleback Range
        • Iron Knob
Minerals of Iron Monarch, Glyn L. Francis
Guyana
 
  • Mazaruni District
    • Kamakusa
Geol.Soc.Amer.,Progr.Ann. Meeting (1967) p. 151; MINERALOGICAL MAGAZINE, DECEMBER 1984, VOL. 48, PP. 560 2
Mineral and/or Locality  
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