Aurichalcite vs. rosasite
Can you tell?
One of the discussion groups at MinDat, the online mineral database, recently addressed the question of distinguishing between these two copper-zinc carbonates. Aurichalcite is (Zn,Cu)5(CO3)2(OH)6 while rosasite is (Cu,Zn)2(CO3)(OH)2. They are both in the same monoclinic crystal class, but that’s largely irrelevant because their typical habits make crystallography hard to discern: both are often silky or velvety, radial balls, or fine acicular (needle-like) crystals, although aurichalcite may be more flattened and feathery versus rosasite’s pointy needles. Rosasite might more often show a botryoidal (“grape-like,” or rounded masses) habit than aurichalcite but I wouldn’t take that to the bank.
Rosasite has a hardness of about 4.5 versus aurichalcite at about 1.5, but again that’s often largely irrelevant because of the fine aggregate nature of the material in both cases. You might be able to rub fragments on a bit of copper (hardness 3) and see what happens, but both minerals are somewhat delicate and might just fall apart. Their specific gravities are both close enough to 4.0 that it would (in my opinion) take a rather exacting measurement to use that technique to differentiate them, and since they often occur together, even intimately intergrown, it might be impossible.
People point out subtle differences in color, but to me, saying “more greenish than blue-green” is, even if valid, only really workable (barely, perhaps) if you have two known examples in front of you, and the natural variation of both seems to span all such shades. I’m hard pressed to differentiate between “pale green, sky blue, or greenish blue” (aurichalcite) and “blue, blue-green to green, sky-blue” (rosasite).
Some people seem to use an unvalidated rule of thumb that says if it is compact and botryoidal, it’s rosasite, and if it forms feathery fans and sprays, it’s aurichalcite. Using that rule, in my photo at the top there would be two balls of rosasite, at near center and inside the edges at left, while the rest would be aurichalcite.
You can certainly do accurate analysis, whether of chemistry (apparently the zinc-copper ratios are significant and consistent) or some other technique like x-ray diffraction, but short of that, can you tell? For me, I cannot. At least I think I cannot, but I probably use that rule of thumb above as much as anyone.
Of the 29 photos of rosasite from the 79 Mine (where mine is from) on MinDat, I’d say most by far are relatively compact balls. And most of the 124 aurichalcite photos are radial, feathery balls and sprays, but there is certainly some overlap in appearance among those photos. And I have no idea how many were confirmed analytically.
Irrespective of all that, they are both pretty minerals!
The 79 Mine was opened in Pinal County, Arizona, in 1879. Mike and Pat O’Brien named it for the year. Three years later Gila County was created out of part of Pinal County, and the mine lay in the new county. It was mostly a lead-zinc mine, with production until the 1950s, but its fame today is for the diverse collectable minerals (85 different species according to MinDat). It is the type locality for liudongshengite, a zinc-chromium carbonate, named for Chinese geologist Liu Dongsheng (1917-2008).
It might seem strange for a blue-green mineral to have “rose” in its name, but it reflects its discovery locality, the Rosas Mine, Sulcis, Sardinia, Italy.
Aurichalcite was named by Th. Böttger (or Boettger) in 1839 (Chemische Untersuchung des Aurichalcits, eines neuen Kupfererzes vom Altai: Annalen der Physik und Chemie 48, 495-500) for its zinc and copper content, the components of brass. Greek όρειχαλκος, orichalcos, "mountain brass" or "mountain copper," was the name of a fabulous metal, mentioned in the legend of the lost continent Atlantis. As yellow copper or yellow brass, it was Latinized by Böttger as aurichalcite, “gold [i.e., yellow] copper.” The Greek prefix, ori, mountain, and Latin auri, aurum, gold, have nothing to do with each other etymologically, although they sound alike. Spanish oro, gold, is from the Latin aurum, not of Greek origin as far as I can tell.
Your comments regarding the difficulty in species identification when relying on color alone were spot on! Thanks for sharing.
If geologists can't tell sometimes without rigorous analysis, than I certainly can't but two questions occur to me. How would crystals of the two minerals be interwoven or entwined? Would it be at the same time or would one be formed before the other? Also, in the photographed specimen, how have such delicate structures survive uncrushed? They look as if they would crumble to powder even if brushed with a finger!