Duftite

duftite

segnitite

adelite

descloizite

Images

Formula: PbCu(AsO4)(OH)
Anhydrous arsenate containing hydroxyl, adelite - descloizite group, forms series with conichalcite and with mottramite.
Specific gravity: 6.12
Hardness: 4½
Streak: Pale green, white
Colour: Olive-green, grey-green; light apple-green in transmitted light.
Environments:

Hydrothermal 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 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).
stability Cu Pb.jpg

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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