Smithsonite vs hemimorphite
Two blue-green botryoidal zinc minerals from the same oxidised zones. Density separates them, acid settles it, hardness does not. The tests in order of use.
Density is the honest answer: smithsonite has a measured density of 4.43 and hemimorphite 3.475, so a piece displacing 20 ml of water weighs about 89 g as smithsonite and about 70 g as hemimorphite. Hardness will not separate them, at 4 to 4.5 against 4.5 to 5. Cold dilute acid is decisive.
In short
- They occur together, in the same oxidised zinc zones, in the same botryoidal blue-green habit, and were both traded as calamine until the distinction was made in the 1800s. Confusing them is the historical default, not a beginner's error.
- Heft is the first test and the best free one. A 27% density difference is something a hand can feel once it has felt both.
- Hardness is useless here. The ranges overlap at 4.5, and a scratch test on a botryoidal crust mostly tells you about the crust.
- Cold dilute hydrochloric acid is decisive — smithsonite is a carbonate and effervesces, hemimorphite is a silicate and does not — but it is destructive, so it is a last resort on a tiny inconspicuous point, never on a crystal face.
- Where crystals are present the argument is over: hemimorphite forms thin tabular crystals whose two ends are differently terminated, which is what its name refers to. Smithsonite is rhombohedral or scalenohedral with curved faces.
| Test | Smithsonite | Hemimorphite | How much it settles |
|---|---|---|---|
| Crystal form, where present | Trigonal; rhombohedral or scalenohedral, faces typically curved | Orthorhombic; thin tabular, striated, doubly terminated crystals hemimorphic | Decisive, and non-destructive |
| Density and heft | D(meas.) 4.43 | D(meas.) 3.475 | Very high — a 27% difference |
| Cold dilute HCl | Effervesces; it is a carbonate | No effervescence; gelatinises in strong acid | Decisive but destructive. Use last |
| Cleavage | Nearly perfect rhombohedral, on {1011} | Perfect on {110}, in one direction | Moderate, if you can see a broken face |
| Hardness | 4 to 4.5 | 4.5 to 5 | Almost none. The ranges meet |
| Lustre | Silky to pearly on botryoidal surfaces, vitreous on crystals | Vitreous, subpearly, sometimes adamantine or silky | Suggestive only |
| UV | May fluoresce pale green or pale blue | May fluoresce bluish under shortwave; strongly pyroelectric | Weak. Both can, neither reliably |
Why these two in particular
Both are secondary minerals of the oxidised zone of zinc deposits, so they form in the same place, in the same fluids, from the same primary sphalerite. Both commonly take a botryoidal or reniform habit. Both come in the blue-green range that collectors want, and both come white and dull far more often. They are frequently intergrown on a single specimen, which is why a label giving one species for a piece that contains both is a very common and largely innocent error.
The historical position is worth knowing because it explains the old labels: until the distinction was properly drawn in the nineteenth century, both were calamine, a smelter's term for zinc ore that made no mineralogical distinction at all. Old British collection material from the Mendip calamine workings, or from Caldbeck Fells, may carry a label that is neither wrong nor useful. If you are buying old material this is one of the routine cases of attribution drift we describe under buying old collection material.
The heft test, worked properly
Density is the property that actually separates these two, and you can get at it with a kitchen scale and a measuring jug.
Take a specimen that is essentially monomineralic — no heavy galena inclusions, no light quartz matrix — and measure its volume by displacement. Suppose it displaces 20 ml. Then:
- Smithsonite at 4.43 g/cm³ → 20 × 4.43 = 88.6 g
- Hemimorphite at 3.475 g/cm³ → 20 × 3.475 = 69.5 g
A difference of about 19 g on something that sits in your palm. That is not a subtle laboratory distinction; it is the difference between a specimen that feels like a lump of metal and one that feels like a rock. Two caveats, both real: matrix wrecks the calculation, so this only works on a piece that is nearly all one mineral; and porous, chalky or cavernous botryoidal material reads low because the voids count as volume. If the number lands between the two figures, suspect either matrix or porosity rather than a new species.
Once you have done this twice with known material, you will not need the jug again. Smithsonite is startlingly heavy for its size and hemimorphite is not, and the hand learns it faster than the eye does.
The acid test, and when it is worth it
Smithsonite is zinc carbonate and will fizz under a drop of cold dilute hydrochloric acid. Hemimorphite is a zinc silicate and will not; in stronger acid it decomposes to a gel rather than effervescing. This is a complete and reliable separation.
It is also destructive, and worth treating with more restraint than most identification guides suggest. Acid etches the very lustre that makes a good botryoidal smithsonite worth having, and a test spot on a display face is permanent damage to the specimen. If you are going to do it: use the smallest possible drop, apply it to the back or an inconspicuous edge, use dilute acid rather than concentrated, and rinse and dry the piece immediately afterwards. On a fine specimen, do not do it at all — weigh it instead.
The same restraint applies to scratch testing. A hardness test on botryoidal material is testing the surface of an aggregate rather than a crystal, which is why the results are unreliable here even before the overlapping ranges are considered.
When crystals are present, it is over in a second
The name hemimorphite is the identification. The crystals are orthorhombic and hemimorphic: the two ends of the same crystal are terminated differently, because the structure has no centre of symmetry. On a doubly terminated crystal this is unmistakable, and it is the reason the species is also strongly pyroelectric. Habit is typically thin tabular, striated, and gathered into sheaf-like or fan-shaped aggregates that fan out from a common base — the classic Mexican and Chinese material shows this beautifully.
Smithsonite crystals are trigonal and comparatively rare: rhombohedral or scalenohedral, usually with distinctly curved faces, and typically small. Most of the smithsonite anyone owns is not crystallised at all but botryoidal, and it is the botryoidal material where the lustre — silky to pearly, quite unlike anything hemimorphite does — becomes the useful clue. We go into that on the smithsonite page; the companion notes are on hemimorphite.
Colour is not evidence. Both run through white, grey, brown, blue and green, and in both species the good colours come from trace elements rather than from the essential chemistry — copper for the greens and blues in each case. A blue-green botryoidal crust proves nothing about which of the two you are holding.
What to do when the label and the specimen disagree
This happens constantly with zinc secondaries, and the useful response is not to correct the old label but to add to it. Keep the original — it is the provenance, and separating a specimen from its history permanently reduces what it is worth. Write your own determination on a new label, with the test you used and the date, and keep the two together.
Where it genuinely matters — a type-locality piece, a species new to your collection, an expensive purchase — the honest limit of what a collector can do at home is worth acknowledging. Displacement and a drop of acid will separate smithsonite from hemimorphite, and they will not separate hemimorphite from hydrozincite, or resolve the aurichalcite and rosasite that often sit alongside both. That takes X-ray diffraction, which university geology departments and some mineralogical societies will do, and which is genuinely cheap by the standards of what a good specimen costs.
If you are trying to fill a specific gap — crystallised smithsonite rather than botryoidal, say, or hemimorphite from a British locality rather than the abundant Chinese material — that is a sourcing problem rather than an identification one. Tell us on the wanted list what you are looking for. We hold no stock; what we can do is know the difference before anything is offered to you.