Duftite-β

Duftite-beta was found to be a compositional intermediate in the duftite–conichalcite series, which has a modulated structure based on the intergrowth of the two structures in domains of approximately 50 Å: Kharisun, 1998.

The duftite “story” - and the "β-duftite" saga in particular - from Malcolm Southwood. See also https://www.mindat.org/mesg-551160.html.

Guillemin (1956) proposed that compositions close to end member duftite – PbCu(AsO4)(OH) – are isostructural with descloizite. He called duftites of this composition α-duftite and found that it could readily be synthesisd in the laboratory. However, he also found that certain duftites with more than about 20% substitution of calcium for lead were isomorphous with conichalcite and could not be synthesised. He proposed the name β-duftite for these compositions and suggested that β-duftite forms a series with conichalcite.

The distinction between these two “polymorphs” was quite important at Tsumeb in the 1970s and 1980s, because they accounted for a significant proportion of the lead (and to a smaller extent copper) in certain ore types but responded quite differently to the flotation process in the concentrator. Because of this, former Tsumeb Corporation mineralogist John Innes encouraged various technical and academic studies of these minerals. The “duftite problem” has been discussed by several authors, but the most relevant to a simple explanation are as follows:

Jambor et al. (1980) examined a series of calcian duftite and plumbian conichalcite specimens from Tsumeb and concluded that complete solid solution probably exists between the duftite and conichalcite and they proposed that β-duftite is merely an intermediate in the duftite-conichalcite series (as opposed to a distinct polymorph).

Kharisun et al. (1998) studied the structure of duftite, conichalcite and of intermediate compositions. They agreed with Jambor et al. (1980) that “β-duftite” is not a distinct mineral but rather a composition, or series of compositions within the duftite-conichalcite series. However, the relationship is not one of simple solid solution; instead, modulated structures comprising domains of different composition occur. This explains why Guillemin (1956) had been unable to synthesise his beta polymorph, but it also indicates that “β-duftite” is not a valid mineral!

Burke (2006) included “duftite-beta” among minerals officially discredited by the IMA.

So much for “β-duftite”, but its interesting (to us Tsumeb geeks anyway) that Jambor et al. (1980) also demonstrated extensive solid solution between conichalcite and austinite. They noted, though, that no compositions with both Pb>Ca and Zn>Cu had been found at that time, but that any such compositions found in the future would be an “as-yet-undescribed new member of the adelite group”. Such compositions were later found, leading to the description of a new mineral – arsenodescloizite (Keller and Dunn, 1980) for which Tsumeb is the type locality.

My good friend Paul Carr and I’ve recently run about 250 quantitative analyses of Tsumeb duftites, conichalcites and austinites. The compositions are shown in the accompanying chart. Duftite plots in the bottom left quadrant; conichalcite in the bottom right; austinite in the top right, and (much rarer) arsenodescloizite in the top left quadrant. As you can see, a very wide range of compositions are present.




While it's usually possible to identify austinite by visual means, the distinction between conichalcite and duftite is much more difficult. I’ve learned that in the absence of an analysis it’s much safer to label these things as “duftite / conichalcite series”.