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Formula: PbCu(AsO4)(OH)
Anhydrous arsenate,
adelite-descloizite group, forms
series with conichalcite
and with mottramite.
Crystal System: Orthorhombic
Specific gravity: 6.12 measured, 6.602 calculated
Hardness: 4½
Streak: Pale green, white
Colour: Olive-green, grey-green; light apple-green in transmitted light.
Environments:
Duftite is a secondary mineral that occurs in the oxidised zone of
hydrothermal metal ore deposits. At the type
locality it is associated with azurite (Mindat).
Localities
At the type locality, the Tsumeb Mine, Tsumeb, Oshikoto Region, Namibia, duftite occurs as crusts, coatings and
olive-green to grey-green and dark green crystals to 3 mm. It forms a series with green
conichalcite. It was common in the top 100 meters of the first oxidation
zone, but more recently was found in the third oxidation zone as deep blackish green crystals to 5 mm, from Level 43 as
olive-green crystals to 5 mm with adamite, and as blocky crystals to 2 mm with
malachite. Associations can also include
wulfenite, dioptase,
mimetite, malachite,
olivenite, minrecordite,
arsentsumebite, tsumcorite,
jamesite, rosasite,
minium, hydrocerussite,
azurite and beudantite
(Minrec 55.6 supplement p105).
Duftite from the Tsumeb Mine - Image
At Arm O'Grain, Cumbria, England, mottramite and duftite are almost
indistinguishable, due to much arsenate substitution for vanadate in
mottramite. It occurs with
mimetite (JRS 9.48).
At Driggith and Sandbed Mines, Cumbria, England, duftite is associated with
mottramite (JRS 9.20).
At Short Grain, Cumbria, England, duftite occurs rarely in vugs in
quartz and baryte, commonly
overgrown by mimetite (JRS 12.53).
At the San Rafael Mine, Nevada, USA, duftite occurs in pods of limonite
associated with adamite, segnitite,
wulfenite and mimetite
(R&M 88.6.517-518).
Alteration
duftite (s) and H2CO3 (aq) to cerussite (s),
malachite (s), H2AsO4- (aq) and H+ (aq)
2PbCuAsO4(OH) + 3H2CO3 ⇌ 2PbCO3 + Cu2CO3(OH)2
+ 2H2AsO4- + 2H+
(MM 52.688)
The Activity-pH diagram below was calculated at 298.2 K for the main
Cu2+ and Pb2+ arsenate minerals.
Boundaries are calculated for constant activities (roughly equivalent to concentrations) of Pb2+ and Cl-
ions in solution, over a range of values of pH and of Cu2+ activity. Higher chloride activites make the stability
fields of bayldonite and duftite vanish,
with respect to the encroachment of that of mimetite
(LMW p269).
The arsenate mineral formulae are:
philipsbornite PbAl3(AsO4)(AsO3OH)(OH)6
mimetite Pb5(AsO4)3Cl
duftite PbCu(AsO4)(OH)
bayldonite Cu3PbO(AsO3OH)2(OH)2
olivenite Cu2(AsO4)(OH)
cornwallite Cu5(AsO4)2(OH)4
clinoclase Cu3(AsO4)(OH)3
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