Hemimorphite
Hemimorphite is a zinc silicate, hardness 4.5 to 5, whose crystals are terminated differently at each end. Habit, the calamine confusion, and identification.
Hemimorphite is a hydrated zinc silicate, hardness 4.5 to 5, density 3.475, and it is named for the property that identifies it: the two ends of a crystal are terminated differently. Thin tabular crystals reach 10 cm. It is strongly pyroelectric and may fluoresce bluish under shortwave ultraviolet.
In short
- The name is the test. Hemimorphic crystals have no centre of symmetry, so a doubly terminated crystal is different at each end. Nothing else in the oxidised zinc suite does this.
- It was calamine, and so was smithsonite. Both were traded under that one name for zinc ore until they were distinguished in the nineteenth century, which is why old labels on this material are so often ambiguous.
- Density separates it from smithsonite decisively: 3.475 against 4.43. Hardness does not, at 4.5 to 5 against 4 to 4.5.
- Habit is typically sheaf-like or fan-shaped aggregates of thin tabular striated crystals, but it is equally common as stalactitic, mammillary and botryoidal crusts, and as chalky coatings.
- It is strongly pyroelectric — it develops an electric charge when heated — which is a direct consequence of the same asymmetry that gives it its name.
| Species | Formula | Hardness | Density | The tell |
|---|---|---|---|---|
| Hemimorphite | Zn4Si2O7(OH)2 H2O | 4.5 to 5 | 3.475 | Doubly terminated crystals differ end to end; no acid reaction |
| Smithsonite | ZnCO3 | 4 to 4.5 | 4.43 | Much heavier; effervesces in cold dilute acid |
| Hydrozincite | Zn5(CO3)2(OH)6 | 2 to 2.5 | 3.5 to 4 | Soft, chalky, white; bright blue-white fluorescence |
| Aurichalcite | (Zn,Cu)5(CO3)2(OH)6 | 1 to 2 | 3.9 | Pale blue-green acicular tufts; very soft |
| Rosasite | (Cu,Zn)2(CO3)(OH)2 | 4.5 | 4.0 to 4.2 | Blue-green spherules; effervesces |
| Willemite | Zn2SiO4 | 5.5 | 4.0 | Harder; the classic green fluorescence at Franklin |
Hemimorphism, and what it looks like on a specimen
Most crystals have a centre of symmetry, so whatever appears at one end of an axis appears at the other. Hemimorphite does not. Its point group is mm2, which permits a polar axis, and the result is that a crystal free to grow at both ends terminates differently at each: typically a flat or pyramidal face at one end and a domed or differently faceted arrangement at the other.
On most specimens you will not see this, because the crystals grew attached to matrix at one end and only the free termination is visible. Where a doubly terminated crystal exists — loose in a cavity, or on a specimen that has been freed — it is unambiguous and it is the fastest identification available for the species.
The same asymmetry produces the pyroelectricity. Heating the crystal changes the polarisation along that axis and develops a charge at the ends. This is a physical curiosity rather than a practical field test, but it is a genuine one, and it is the mechanism, not a coincidence.
Habit otherwise is thin tabular crystals flattened on {010} and striated parallel to [001], reaching 10 cm, gathered into the sheaf-like and fan-shaped aggregates that are the species' most recognisable form. Cleavage is perfect on {110} in one direction, the fracture uneven to subconchoidal, and the tenacity brittle.
Calamine, and the label problem
Before the two were distinguished, hemimorphite and smithsonite were both calamine — a smelter's word for oxidised zinc ore that carried no mineralogical meaning at all. The name survives in British place names and in old collection labels, and it is the reason so much nineteenth-century zinc secondary material is labelled in a way that cannot be relied on.
This is not a small effect. The two species occur together, in the same oxidised zinc orebodies, in the same botryoidal habit, in the same blue-green colour range, and frequently intergrown on the same piece. A label naming one of them for a specimen containing both is neither dishonest nor useful. We set out the tests that resolve it — density first, acid last — on smithsonite vs hemimorphite.
The practical rule when buying old zinc material is to treat the species name on the label as a hypothesis and the locality as the valuable part. A traceable mine name from a district that is finished is worth having whichever of the two the specimen turns out to be.
Colour, and where the blue comes from
Pure hemimorphite is colourless or white. The blue and blue-green material that collectors want — the Chinese and Mexican botryoidal pieces especially — owes its colour to copper substituting into the structure, exactly as the blue and green in smithsonite does. Pale green, grey and brown come from other impurities.
Two consequences. First, colour tells you nothing about species within this suite, because the same trace element colours both minerals the same way. Second, colour intensity is a value driver rather than an identification aid, and a strongly coloured botryoidal hemimorphite is a specimen bought for its appearance.
Lustre is more informative: vitreous, subpearly, sometimes adamantine on crystals and occasionally silky on fibrous material. That is a different impression from smithsonite's characteristic silky-to-pearly botryoidal surface, and once you have handled both, lustre is a reasonable first guess — to be confirmed by weighing it.
Fluorescence is weak evidence. Hemimorphite may fluoresce bluish under shortwave ultraviolet, but the response is inconsistent, and the associated hydrozincite that is often present on the same specimen fluoresces a bright blue-white far more reliably — which is a common source of confusion, and one we cover in our notes on shortwave and longwave fluorescence.
Localities, and buying
The Handbook of Mineralogy lists relatively few localities for fine material: Băiţa in Romania; Banská Štiavnica in Slovakia; the Caldbeck Fells in Cumbria; Moresnet in Belgium; Freiberg and Altenberg in Saxony; the Sa Duchessa mine in Sardinia; large crystals from Nerchinsk in Siberia; Tchah Kuh in Iran; Franklin and Sterling Hill in New Jersey; Bisbee and the 79 mine in Arizona; the Emma mine in Utah; the Ibex and Wolftone mines at Leadville; the Elkhorn mine in Montana; large crystals from Santa Eulalia in Chihuahua; and the Ojuela mine at Mapimí in Durango.
The British occurrence to know is the Caldbeck Fells, which is also the reference for so much else in British lead and zinc secondaries. The Mendip calamine workings are the other British zinc story, and their name records precisely the confusion described above.
What to look for when buying: crystals over crust, because crystallised hemimorphite is much less common than botryoidal material and the fan-shaped aggregates are the species at its best; a clean unbroken fan, since the thin tabular crystals chip easily and a sheared aggregate is obvious once you look for it; and a locality that means something, because good botryoidal blue material is abundant and its price rests on appearance while a Caldbeck or Sterling Hill piece rests on provenance.
Storage is undemanding: it is stable, moderately hard and needs only an individual box. If you want a crystallised specimen or a documented British one rather than the plentiful modern botryoidal material, the wanted list is how to say so. We hold no stock; what we can offer is having weighed it and read the label first.