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Mottramite

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Mottramite
Mottramite (yellow) with fluorite and quartz. From Lower Silesia, Poland.
General
CategoryVanadate minerals
Formula
(repeating unit)		PbCu(VO4)(OH)
Strunz classification08.BH.40
Dana classification41.5.2.2
Crystal systemOrthorhombic
Space groupmmm (2/m 2/m 2/m) - Dipyramidal
Unit cella = 7.68Å, b = 9.27Å, c = 6.03Å
Identification
Formula mass402.69 g [1]
ColorGrass-green, olive-green, yellow-green, siskin-green, blackish brown, nearly black
Crystal habitEncrustations, aggregates of plume-like forms and radial crystals[1]
CleavageNone observed
FractureIrregular/uneven, sub-conchoidal
TenacityBrittle
Mohs scale hardness3 - 3½
LusterGreasy
StreakYellowish green
DiaphaneityTransparent, opaque
Specific gravity5.9
Optical propertiesUsually biaxial (-) rarely biaxial (+)
Refractive indexnα = 2.170(2) nβ = 2.260(2) nγ = 2.320(2)
Birefringenceδ = 0.150
PleochroismVisible X=Y= canary yellow to greenish yellow, Z= brownish yellow
2V angleMeasured: 73° , calculated: 46°
DispersionStrong r > v rarely r < v
SolubilityReadily soluble in acids[2]
Common impuritiesZinc
References[3] [2] [1] [4]

Mottramite is an orthorhombic anhydrous vanadate hydroxide mineral, PbCu(VO4)(OH), at the copper end of the descloizite subgroup. It was formerly called cuprodescloizite or psittacinite (this mineral characterized in 1868 by Frederick Augustus Genth). Duhamelite is a calcium- and bismuth-bearing variety of mottramite, typically with acicular habit.[2] Mottramite is a member of the Adelite-Descloizite group.[2] Group members:

Mottramite, which is a copper rich member, forms a series with descloizite, which is a zinc rich member.[2] [4] These two minerals usually contain significant percentages of both copper and zinc and are seldom pure. Mottramite is rarer than descloizite.[5] Mottramite also forms a series with duftite PbCu(AsO4)(OH).[2] It was discovered in 1876[2] and named for the locality, Mottram St Andrew, Cheshire, England, where ore was stockpiled, although it was probably mined from Pim Hill Mine, Shrewsbury, Shropshire, England.[6] [2] [4]

Crystallography

Mottramite is an orthorhombic mineral belonging to the crystal class 2/m 2/m 2/m, with space group Pnma.[3] [2] [4] The unit cell has sides of lengths a = 7.6 to 7.7 Å, b = 9.2 to 9.5 Å and c = 6.0 to 6.1 Å.[3] [2] [4] [7] There are 4 formula units per unit cell (Z = 4), the molar mass is 402.69 g[1] and the calculated density is 6.19 g/cc.[3] [2] [4] The structure is composed of chains of edge-sharing CuO6 octahedra and very distorted Pb(O,OH)8 polyhedra linked through VO4 groups into a tight three-dimensional network.[3]

Appearance

Drusy crusts of tiny intergrown crystals are common, also encrustations and mammillary or botryoidal surfaces.[2] [1] [4] The crystals are equant dipyramids or prisms parallel to the c crystal axis, but always microscopic. The colour is various shades of green, yellow-green, blackish brown or nearly black,[3] [2] [4] and crystals often exhibit zonal growth with varying colours.[2] The streak is yellowish green,[2] [4] yellow[6] or green,[5] and the crystals are transparent to opaque,[3] [2] [4] with a greasy lustre.[3] [2] [4]

Mottramite (green) with Chrysocolla (blue) from Mono County, California, USA.
Orange wulfenite on dark grey mottramite from Gila County, Arizona, USA.
Calcite (white) covered by green mottramite from Tsumeb, Namibia.
Quartz matrix with a partial coverage of dark brown mottramite from Arm O'Grain, Caldbeck Fells, UK.
Mottramite, Olifantsfontein, Grootfontein, Namibia.
Pseudomorph of Mottramite after dendritic Copper, Tsumeb mine, Namibia.

Physical properties

No cleavage has been observed.[2] The mineral is brittle[2] [4] and breaks with a subconchoidal to uneven fracture.[2] [4] It is quite soft, with Mohs hardness 3 to 3½,[3] [2] [4] just a little harder than calcite. The hardness is slightly greater on crystal surfaces.[2] It is a heavy mineral, with specific gravity 5.9,[3] [2] [4] because of the lead content. It is readily soluble in acids.[2]

Optical properties

Orthorhombic crystals (and triclinic and monoclinic crystals) have two directions in which light travels with zero birefringence; these directions are called the optic axes, and the crystal is said to be biaxial. The speed of a ray of light travelling through the crystal differs with direction. The direction of the fastest ray is called the X direction and the direction of the slowest ray is called the Z direction. X and Z are perpendicular to each other, and a third direction Y is defined as perpendicular to both X and Z; light travelling along Y has an intermediate speed. Refractive index is inversely proportional to speed, so the refractive indices for the X, Y and Z directions decrease from X to Z.[8]
For mottramite the orientation with respect to the crystal axes a, b and c is X = c, Y = b and Z = a.[4] The refractive indices are nα = 2.170(2), nβ = 2.260(2) and nγ = 2.320(2).[3] [2] [4] The maximum birefringence δ is the difference between the highest and lowest refractive index; for mottramite δ = 0.150.[2] The angle between the two optic axes is called the optic angle, 2V, and it is always acute, and bisected either by X or by Z. If Z is the bisector then the crystal is said to be positive, and if X is the bisector it is said to be negative.[8] Mottramite is usually biaxial (–), and rarely biaxial (+).[3] [4] The measured value of 2V is 73°.[3] [2] [4] Also 2V can be calculated from the values of the refractive indices, giving a value of 46°, which differs considerably from the measured value.[2] 2V depends on the refractive indices, but refractive index varies with wavelength, and hence with colour. So 2V also depends on the colour, and is different for red and for violet light. This effect is called dispersion of the optic axes, or just dispersion (not to be confused with chromatic dispersion). If 2V is greater for red light than for violet light the dispersion is designated r > v, and vice versa. For mottramite dispersion is strong, usually with r > v, and rarely with r < v. [2] [4] The mineral is pleochroic; when viewed along the X or Y direction it appears canary yellow to greenish yellow and when viewed along the Z direction it appears brownish yellow.[3] [2] [4]

Occurrence

The type locality is Mottram St Andrew, Cheshire, England, UK[2] and type material is conserved at the Natural History Museum, London 52314-52315.[2] [4] Mottramite is a secondary, supergene mineral found principally in the oxidized zones of vanadium bearing base metal deposits,[3] [2] [4] especially sandstones.[3] Associated minerals are descloizite PbZn(VO4)(OH), duftite PbCu(AsO4)(OH), mimetite Pb5(AsO4)3Cl, wulfenite PbMoO4, cerussite PbCO3, azurite Cu3(CO3)2(OH)2 and dioptase CuSiO3.H2O.[4]

Localities

Australia, Kintore Open Cut

Mottramite is the only secondary mineral with essential vanadium recorded from the Kintore open cut, Broken Hill, Yancowinna County, New South Wales. It has been found scattered on greenish to yellowish drusy plumbogummite PbAl3(PO4)(PO3OH)(OH)6 -hinsdalite PbAl3(PO4)(SO4)(OH)6, as tiny black glossy pyramids or aggregates of dull black, flattened bipyramids up to 0.4 mm across.[9]

Australia, Braeside Station

Braeside station is in the Gregory Ranges District, Shire of East Pilbara, Western Australia. Both mottramite PbCu(VO4)(OH)and descloizite PbZn(VO4)(OH)are common in the central part of the Braeside field. They occur with vanadinite Pb5(VO4)3Cl, pyromorphite Pb5(PO4)3Cl and cerussite PbCO3. Descloizite-mottramite was the last to crystallise showing a variety of colours and habits. These include yellow-orange drusy or botryoidal opaque crusts on chalcedony, brown, olive or yellow flaky wedge-shaped transparent crystals up to 100 microns wide that form rosettes in association with coronadite Pb(Mn4+6Mn3+2)O16 and hemimorphite Zn4(Si2O7)(OH)2.H2O, and short prismatic and bipyramidal green crystals that form a 10 to 15 micron thick crust on quartz SiO2.[10]

Australia, Shangri La

At Shangri La, Kununurra, Wyndham-East Kimberley Shire, Western Australia, green to brown mottramite PbCu(VO4)(OH) forms thin botryoidal crusts of fine-grained, platy crystals on quartz SiO2 and iron oxides, and may be associated with vanadinite Pb5(VO4)3Cl. Some crusts have a radiating internal texture and show a variation in composition from mottramite at their core, to descloizite PbZn(VO4)(OH) at their rim. Mottramite generally grew at the same time as vanadinite.[11]

Morocco, Bou Azer

Mottramite PbCu(VO4)(OH) is the only vanadium mineral known from the Bou Azer district, Tazenakht, Ouarzazate Province, Souss-Massa-Draâ Region, Morocco. It has been found as tiny dark brown resinous crystals on a single piece of heavily altered galena PbS. The specimen shows secondary mineralisation containing mimetite Pb5(AsO4)3Cl and possibly wulfenite PbMoO4.[12]

Russia, Berezovskoe Gold Deposit

Mottramite PbCu(VO4)(OH) has been found at the Berezovskoe Gold Deposit, Berezovskii, Ekaterinburg, Sverdlovskaya Oblast', in the Middle Urals. It was found in a quartz SiO2 vein, in a cavity containing galena PbS, tetrahedrite Cu6[Cu4(Fe,Zn)2]Sb4S13 and tennantite Cu6[Cu4(Fe,Zn)2]As4S13, with associated bushmakinite Pb2Al(PO4)(VO4)(OH), cerussite PbCO3, bindheimite Pb2Sb5+2O7, vauquelinite CuPb2(CrO4)(PO4)(OH) and pyromorphite Pb5(PO4)3Cl.[13]

United Kingdom, Caldbeck Fells

Mottramite occurs at several localities in the Caldbeck Fells, Allerdale, Cumbria, England.

Caldbeck Fells, Arm O’Grain

A variety of supergene minerals occur as microscopic crystals at Arm O'Grain, Caldbeck Fells. These include mimetite Pb5(AsO4)3Cl, pyromorphite Pb5(PO4)3Cl, vanadinite Pb5(VO4)3Cl, duftite PbCu(AsO4)(OH), plumbogummite PbAl3(PO4)(PO3OH)(OH)6 and beudantite PbFe3+3(AsO4)(SO4)(OH)6, as well as mottramite PbCu(VO4)(OH). Mottramite is the commonest of them. It occurs as black elongated boat shaped crystals up to about 1 mm long, scattered over white vein quartz. Occasionally, crusts of mottramite occur in cavities in the quartz that appear to have been formed by the dissolution of baryte BaSO4. Mottramite was almost certainly produced by oxidation of galena PbS.<rec name=JRS9>Journal of the Russell Society (2006) 9:44-53</rec>

Caldbeck Fells, Brandy Gill

Mottramite PbCu(VO4)(OH) has been reported from Brandy Gill, Carrock Fell, Caldbeck Fells as minute yellowish green globular aggregates associated with bayldonite Cu3PbO(AsO3OH)2(OH)2, malachite Cu2(CO3)(OH)2 and mimetite Pb5(AsO4)3Cl, and as olive-green pyramidal crystals associated with bayldonite, beaverite and beudantite PbFe3+3(AsO4)(SO4)(OH)6. The primary sulphide minerals at Brandy Gill are galena PbS, chalcopyrite CuFeS2 and arsenopyrite FeAsS.[6]

Caldbeck Fells, Short Grain

Mottramite PbCu(VO4)(OH) is quite common at Short Grain, Deer Hills, Caldbeck Fells. It usually occurs as thin druses of yellow or dark greenish brown crusts associated with pyromorphite Pb5(PO4)3Cl or baryte BaSO4. Less commonly it occurs as bipyramids on quartz SiO2. The crystals are inconspicuous and rarely exceed 0.1 mm. It is sometimes associated with chrysocolla (Cu2-xAlx)H2-xSi2O5(OH)4.nH2O. Most mottramite contains some arsenic As substituting for vanadium V, so there is a gradation toward vanadium-rich duftite PbCu(AsO4)(OH).[14]

Caldbeck Fells, Ingray Gill

Minute drusy yellow to yellow-brown mottramite PbCu(VO4)(OH) crusts made up of characteristic boat-shaped crystals a few tens of micrometres across cover specimens from the mine dump at Ingray Gill, Caldbeck Fells. Mottramite encrusts mimetite Pb5(AsO4)3Cl and white to pale green pyromorphite Pb5(PO4)3Cl epimorphs after galena PbS. It is one of the most common supergene minerals at Ingray Gill, but because of its colour and habit it is easily mistaken for pyromorphite or mimetite.[15]

Caldbeck Fells, Low Pike

Several supergene minerals including bayldonite Cu3PbO(AsO3OH)2(OH)2, beudantite PbFe3+3(AsO4)(SO4)(OH)6, brochantite Cu4(SO4)(OH)6, cornwallite Cu5(AsO4)2(OH)4, duftite PbCu(AsO4)(OH), malachite Cu2(CO3)(OH)2, mimetite Pb5(AsO4)3Cl, philipsburgite (Cu,Zn)6(AsO4, PO4)2(OH)6.H2O and pseudomalachite Cu5(PO4)2(OH)4 as well as mottramite PbCu(VO4)(OH) occur in thin fractures in quartz at Low Pike, Caldbeck fells.[16]

Caldbeck Fells, Balliway Rigg

Mottramite PbCu(VO4)(OH) is rare at Balliway Rigg. It has been found as minute olive green pyramidal crystals on hemimorphite Zn4(Si2O7)(OH)2.H2O and chrysocolla (Cu2-xAlx)H2-xSi2O5(OH)4.nH2O and as scattered yellow to yellow-brown blocky crystals on lavender-blue plumbogummite PbAl3(PO4)(PO3OH)(OH)6. It also occurs as minute brown pyramidal crystals on green pyromorphite Pb5(PO4)3Cl. The largest crystals are a few tenths of a millimetre across.[17]

Caldbeck Fells, Brae Fell Mine

Mottramite PbCu(VO4)(OH) is quite common on the mine dump at Brae Fell Mine, Roughton Gill, Caldbeck Fells, as coatings of minute rice-grain shaped crystals less than 0.1 mm long on quartz SiO2. The crystals are very dark brown to buff in colour, often associated with pyromorphite Pb5(PO4)3Cl, and occasionally associated with cerussite PbCO3.[18]

Caldbeck Fells, Sandbed Mine

A yellow-brown crust on samples from the dumps of the Sandbed mine has been identified as mottramite PbCu(VO4)(OH). [19]

USA, Brown Monster Mine and Reward Mine

Mottramite PbCu(VO4)(OH) is relatively common at the Brown Monster Mine and Reward Mine, Russ District, Inyo County, California. It forms pale yellow-green to tan, brick-red and chocolate brown botryoidal crusts, commonly associated with mimetite Pb5(AsO4)3Cl and wulfenite PbMoO4, or with dark reddish-brown vanadinite Pb5(VO4)3Cl.[20]

USA, Otto Mountain

At Otto Mountain, Baker, San Bernardino County, California, mottramite PbCu(VO4)(OH) is generally seen as crusts of botryoidal green to olive-green spheres in association with white vanadinite Pb5(VO4)3Cl needles.[21]


References

Wikimedia Commons has media related to Mottramite.
  1. ^ a b c d e http://www.webmineral.com/data/Mottramite.shtml#.VH8NKsngXlc
  2. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag http://www.mindat.org/min-2792.html
  3. ^ a b c d e f g h i j k l m n o p Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley
  4. ^ a b c d e f g h i j k l m n o p q r s t u v w x http://www.handbookofmineralogy.org
  5. ^ a b www.galleries.com/Mottramite
  6. ^ a b c Kingsbury and Hartley (1956). New occurrences of vanadium minerals (mottramite, descloizite, (discredited) and vanadinite) in the Caldbeck area of Cumberland. Mineralogical Magazine 31:289
  7. ^ Van der Westhuizen, de Bruiyn, Tordiffe and Botha (1986). The descloizite-mottramite series of vanadates from the Otavi Mountain Land, South West Africa: an X-ray study. Mineralogical Magazine 50:137
  8. ^ a b Klein and Hurlbut (1993) Manual of Mineralogy 21st Edition. Wiley
  9. ^ Australian Journal of Mineralogy (1997) 3-1:66
  10. ^ Australian Journal of Mineralogy (2007) 13-2:59
  11. ^ Australian Journal of Mineralogy (2011) 16-1:21-22
  12. ^ The Mineralogical Record (2007) 38-5:384
  13. ^ The Mineralogical Record (2004) 35-2:175
  14. ^ Journal of the Russell Society (2009)12:57
  15. ^ Journal of the Russell Society (2009) 12:38
  16. ^ Journal of the Russell Society (2003) 8(1):43-44
  17. ^ Journal of the Russell Society (2008) 11:19
  18. ^ Journal of the Russell Society (2006) 9:39-44
  19. ^ Journal of the Russell Society (2006) 9:2-38
  20. ^ The Mineralogical Record (2010) 41-2:189
  21. ^ Rocks & Minerals (2011) 86-2:132
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