Descloizite
Descloizite is PbZn(VO4)(OH), hardness 3 to 3.5, density near 6.2. Its calculated density sits within 0.24% of mottramite, so weight cannot separate the two.
Descloizite is a lead zinc vanadate, PbZn(VO4)(OH), orthorhombic, hardness 3 to 3.5, with a measured density of about 6.2 and an orange to brownish red streak. It forms a series with mottramite, PbCu(VO4)(OH), whose calculated density of 6.187 differs from descloizite's 6.202 by 0.24% — so heft cannot separate the two.
In short
- The two end-members differ by 0.46% in formula mass and 0.24% in calculated density. Swapping zinc (65.38) for copper (63.55) in a formula unit weighing about 404 changes almost nothing. Any heft test on this series is measuring the specimen's porosity, not its chemistry.
- The measured densities differ by 5.08% and that difference is misleading. The Handbook gives descloizite about 6.2 and mottramite about 5.9, both with a tilde. Both species are commonly botryoidal and drusy, and botryoidal material contains voids — so the measured gap reflects habit, not composition.
- The streak is the field test that does work. Descloizite streaks orange to brownish red; mottramite streaks yellowish green. On a porcelain plate that is an unambiguous difference, and it costs nothing.
- Descloizite is among the densest species this site covers, at about 6.2 — behind gold at 19.3, uraninite 10.63, galena 7.58, mendipite and mimetite 7.24, safflorite 7.2, pyromorphite 7.04, vanadinite 6.88, chloroxiphite 6.76 and cerussite 6.55.
- Both end-members carry a type locality of note. Descloizite was described from Argentina in 1854; mottramite from Mottram St Andrew in Cheshire in 1876, one of eleven United Kingdom type localities among the species on this site.
| Species | Formula | Hardness | Density measured | Density calculated | Streak |
|---|---|---|---|---|---|
| Descloizite | PbZn(VO4)(OH) | 3 to 3.5 | About 6.2 | 6.202 | Orange to brownish red |
| Mottramite | PbCu(VO4)(OH) | 3 to 3.5 | About 5.9 | 6.187 | Yellowish green |
| Vanadinite | Pb5(VO4)3Cl | 2.5 to 3 | 6.88 | 6.95 | White to yellowish |
| Pyromorphite | Pb5(PO4)3Cl | 3.5 to 4 | 7.04 | 7.10 | White |
| Mimetite | Pb5(AsO4)3Cl | 3.5 to 4 | 7.24 | 7.28 | White |
| Wulfenite | Pb(MoO4) | 3 | 6.5 | 6.82 | White |
The arithmetic that kills the heft test on this series
Descloizite and mottramite are the zinc and copper end-members of a continuous series. The substitution is zinc for copper in one site, and the Handbook of Mineralogy reports both.
Formula masses. Descloizite: Pb 207.2 + Zn 65.38 + V 50.94 + 4O 64.00 + OH 17.01 = 404.52. Mottramite: Pb 207.2 + Cu 63.55 + V 50.94 + 4O 64.00 + OH 17.01 = 402.69. The difference is 0.46%.
Calculated densities. Descloizite 6.202, mottramite 6.187 — a difference of 0.24%. That is the real, structural answer, because a calculated density is derived from the unit cell and the composition and contains no voids.
Measured densities. Descloizite about 6.2, mottramite about 5.9 — a difference of 5.08%, which is twenty times the structural difference. Both figures carry a tilde in the Handbook, and both species are commonly drusy, botryoidal, stalactitic or coarsely fibrous. Porous material weighs less than the crystal it is made of.
So the conclusion is firm and slightly counter-intuitive: the measured gap is real, it is reproducible, and it is not telling you about the chemistry. A hydrostatic measurement on a botryoidal crust from this series returns the density of the crust. The method and its assumptions are on how to measure specific gravity at home, which states the same limitation in general terms: the constraint is not the scale, it is getting a clean single-species fragment.
The streak plate settles what the balance cannot
Descloizite streaks orange to brownish red. Mottramite streaks yellowish green. Both are well below the hardness at which a streak plate stops working, at 3 to 3.5 against the plate's practical limit near 6.5.
This is one of the cases where the streak test earns its keep. A survey of this site's reference set found that of 108 species sheets, 78 record a streak and exactly 39 of those are white — so the likeliest outcome of a streak test is a white mark that eliminates nothing. Descloizite and mottramite are in the useful minority, and the two colours are not close.
A caution on intermediate material. The series is continuous, so a specimen part-way along it will streak part-way between: brownish, olive, or an indefinite dark yellow. A clear orange-red streak is good evidence for descloizite and a clear yellow-green streak is good evidence for mottramite, but an ambiguous streak is evidence of an intermediate composition rather than of a failed test. The streak method in full is on how to use a streak plate.
Where descloizite comes from, and why Tsumeb and Berg Aukas dominate
Descloizite is a secondary mineral of the oxidised zone of vanadium-bearing base metal deposits, and rarely an ore of vanadium. Its listed associates are mottramite, vanadinite, pyromorphite, mimetite, wulfenite and cerussite — the full oxidised-lead suite.
The Handbook singles out large crystals from an economic deposit at Berg Aukas near Grootfontein, at Abenab, at Tsumeb and elsewhere in the Otavi district of Namibia. Also the Sierra de Córdoba in Córdoba Province, Argentina, which is the type area; Los Lamentos and Santa Eulalia in Chihuahua; the Mammoth-St Anthony mine at Tiger and Bisbee in Arizona; the Caballo Mountains and mines around Georgetown in New Mexico; the Chalk Mountain mine in Nevada; Kabwe, formerly Broken Hill, in Zambia; M'fouti and Ruwe in the Congo; the Gowde mine in Iran; Obir in Carinthia; Mežica in Slovenia; and the Prequica mine in Portugal.
Note that the Zambian Kabwe is not the Australian Broken Hill. Both were called Broken Hill, both are famous lead-zinc orebodies, and old labels do not always say which. It is one of the commoner locality ambiguities a collector meets, and it is worth resolving before it is inherited — see is a specimen without a locality worth buying.
A note on the density ladder, and a coincidence worth keeping
Descloizite sits high on this site's density ladder without being at the top of it. Measured densities across the reference set run from chrysocolla at 1.93 to gold at 19.3.
Those two happen to be in a ratio of exactly ten: 19.3 / 1.93 = 10.000. It is a coincidence of rounding rather than anything structural, but it is a useful anchor — the whole density range a collector encounters spans one order of magnitude, and nothing more. Everything else sits between those two, which is why a density measured to two decimal places is usually enough to identify a species and a density measured to one is usually not.
Descloizite at about 6.2 is in the crowded upper band, with vanadinite at 6.88, chloroxiphite 6.76, cerussite 6.55, wulfenite 6.5 and anglesite 6.38 all within a few per cent of each other. In that band density stops discriminating and colour, streak and habit take over — which is the whole argument of this page, stated the other way round.
Nothing here is for sale and there is no catalogue on this site. A Berg Aukas or Tsumeb descloizite is a wanted list matter, and the rest of the section is indexed under mineral species.