Mottramite

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Environment

Secondary (weathered) zone of ore deposits.

Usually in small to very small, transparent, yellow-brown, short spearlike blades, or in velvety black druses of microscopic crystals. Large solid crystals at one locality. Also stalactitic as mammillary crusts.

Cherry red to yellow-brown, chestnut brown, green, or black. Luster greasy; hardness 3Ɖ; specific gravity 5.9 (mottramite) to 6.2 (descloizite); streak yellowish orange to brownish red; fracture small conchoidal areas; cleavage none. Brittle; transparent to translucent.

Alkaline lead, zinc-copper, vanadates of variable composition making a series (approximately 55.4% PbO, 22.7% V ₂ O ₅ , 2.2% H ₂ O, and about 20% divided between Cu and Zn); the Zn member is descloizite, but when copper is more abundant it is mottramite.

Fuse readily on charcoal, boiling at first, even after flame is removed, eventually making a ball of lead surrounded by a black slag. The powder is dissolved by hydrochloric acid and makes a yellow-green solution. A piece of pure zinc added to this solution turns it blue, then violet (vanadium test).

They are usually recognized by their color, crystal form, and associations, and do not greatly resemble any other mineral in their kind of associations.

The descloizite-mottramite series is commonly associated with wulfenite and vanadinite, in the usual secondary mineral suites from oxidized areas of ore deposits. It is especially widespread in both Arizona and New Mexico, and is commonly found crusting the rock matrix of specimens of wulfenite and vanadinite. Black velvety crusts of descloizite came from Sierra Co., New Mexico, and also from Bisbee and Tombstone, Arizona. The most remarkable occurrence known was in Otavi, Namibia, where some mammoth crystals of mottramite--over an inch (3 cm) in size, dark brown in color, and resembling sphalerite--were found, forming great crystal clusters. Unfortunately, they are not as aesthetically appealing as the small brown or green "trees" from Berg Aukas, Grootfontein, Namibia.

Mottramite
Mottramite (yellow) with fluorite and quartz. From Lower Silesia, Poland.
General
Category	Vanadate minerals
Chemical formula	PbCu(VO4)(OH)
Strunz classification	08.BH.40
Crystal symmetry	mmm (2/m 2/m 2/m) - Dipyramidal
Unit cell	a = 7.68Å, b = 9.27Å, c = 6.03Å
Identification
Color	Grass-green, olive-green, yellow-green, siskin-green, blackish brown, nearly black
Crystal system	Orthorhombic
Cleavage	None observed
Fracture	Irregular/uneven, sub-conchoidal
Tenacity	Brittle
Mohs scale hardness	3 - 3½
Luster	Greasy
Streak	Yellowish green
Diaphaneity	Transparent, opaque
Specific gravity	5.9
Optical properties	Biaxial (-)
Refractive index	nα = 2.170(2) nβ = 2.260(2) nγ = 2.320(2)
Birefringence	δ = 0.150
Pleochroism	Visible
2V angle	Measured: 73° , Calculated: 46°
Dispersion	strong r > v rarely r < v

Mottramite is an orthorhombic mineral, PbCu(VO₄)(OH), at the copper end of the descloizite group. It was formerly called cuprodescloizite or psittacinite (this mineral characterized in 1868 by Frederick Augustus Genth).

Gallery

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  Pseudomorph of Mottramite after dendritic Copper, Tsumeb mine, Namibia. Size 3.1 x 1.2 x .2 cm.

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  Mottramite, Olifantsfontain, Namibia

References

• Mindat.org
• Webmineral.com
• Mineral galleries

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