Canichalcite (#83)

Limonite matrix with a vug of lustrous micro crystalline botryoidal green conichalcite.
North America Mammoth Mine, Tintic District, Juab County, Utah 2" x 2" x 1 ½"

 

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Specimen 57 features the mineral dultite, PbCu(AsO4)(OH). The mineral in this specimen, conichalcite, CaCu(AsO4)(OH), is similar to dultite, but with Ca (calcium) replacing Pb (lead). In fact, there is a solid solution between duftite and conichalicte. This means that compositions between the two are possible. The extreme compositions of a solid solution series are called end-members. Some solid solution series are not complete - that is, compositional intermediates are found only part way from one end-member to the other. The duftite-conichalcite series is complete: any compositional intermediate between PbCu(AsO4)(OH) and CaCu(AsO4)(OH) can occur. This is because the sizes of the lead and copper atoms are not too dissimilar, so substituting one for the other does not disrupt the crystal structure (atomic arrangement).

So how does one tell whether a particular specimen is dultite, conichalcite, or something in between? The answer is, not easily. The colors of the end-members are not exactly the same shade of green, but this is an unreliable way to estimate the composition of an intermediate member of the series. Laboratory analysis to determine the composition is really the only way to tell. For mineral collectors, there is usually no motivation to know the exact composition of a specimen. For scientific work, the compositional analysis might be too important to guess at (depending on the question to be answered), and the laboratory work would proceed.

Incidentally, "botryoidal" in the label describes a mineral that occurs in subspherical masses resembling (more or less) a cluster of grapes.