Mimetite
Mimetite is lead arsenate, hardness 3.5 to 4, named for imitating pyromorphite. Campylite from Dry Gill is the British form. Habit, series, localities and care.
Mimetite is a lead arsenate of the apatite supergroup, hardness 3.5 to 4, with a measured density of 7.24 that makes a small specimen feel absurdly heavy. It is named from the Greek for imitator because it imitates pyromorphite, with which it forms a complete series. Crystals reach 12 cm.
In short
- Mimetite and pyromorphite are a continuous series, arsenate to phosphate, so intermediate compositions are the rule and a great many old labels are approximations rather than determinations.
- Campylite is the British form and the reason to care. The barrel-shaped, curved-face variety from Dry Gill in the Caldbeck Fells is unlike mimetite anywhere else in the world.
- The density is diagnostic in the hand: 7.24, which is heavier than galena, and startling on a specimen the size of a matchbox.
- Colour runs pale yellow to bright yellow, orange-yellow and yellowish brown, and can be white or colourless. It may fluoresce reddish yellow under either longwave or shortwave UV.
- It is a lead and arsenic mineral, so it belongs to both hazard groups — safe as a solid object, never to be sawn, ground or dry-brushed.
| Species | Formula | Hardness | Density | Typical habit |
|---|---|---|---|---|
| Mimetite | Pb5(AsO4)3Cl | 3.5 to 4 | 7.24 | Prismatic to acicular, tabular, barrel-shaped, mammillary |
| Pyromorphite | Pb5(PO4)3Cl | 3.5 to 4 | 7.04 | Hoppered hexagonal prisms, barrel forms, botryoidal |
| Vanadinite | Pb5(VO4)3Cl | 3 to 4 | 6.88 | Sharp hexagonal prisms, hollow prisms, tabular |
| Clinomimetite | Pb5(AsO4)3Cl | 3.5 to 4 | — | The monoclinic dimorph of mimetite; not separable by eye |
Campylite, and why Dry Gill matters
Campylite is a variety name, not a species: it is mimetite in a distinctive barrel-shaped habit, the name coming from the Greek kampylos, bent, for the curvature of the crystal faces. The world's defining occurrence is Dry Gill Mine in the Caldbeck Fells of Cumbria, and the material is instantly recognisable — short, fat, orange to yellow-brown barrels, often on a dark manganese-oxide crust that sets the colour off.
It matters for two reasons. First, it is genuinely distinctive: no other locality produces this habit at this quality, so it is one of the small number of British mineral occurrences that are world-class rather than merely local. Second, it is finite. The Caldbeck workings are old, small, and long finished, and the specimens circulating are the ones already in collections. Everything we say about the district on the Caldbeck Fells page applies here with force: pieces are small, typically under 5 cm, and the labels are often as interesting as the mineral.
A note on chemistry that trips people up. Dry Gill campylite is arsenian, and the district's pyromorphite from Roughton Gill is phosphatic, and material intermediate between them exists. Old labels use both names for the same tray. Where it matters, the determination needs analysis, not an opinion.
The series with pyromorphite, and what labels can honestly claim
Mimetite is the arsenate end-member and pyromorphite the phosphate, and the two substitute freely for one another. That means composition is a continuum, and any specimen from a locality producing both may be anywhere along it. Colour is not a guide: mimetite is usually yellow to orange and pyromorphite usually green, but both species produce both colours.
Density is a partial guide, at 7.24 for mimetite against 7.04 for pyromorphite, but the difference is only about 3% and no home measurement will resolve it reliably. Habit helps more: pyromorphite's characteristic hoppered prisms, hollow down the axis, are a pyromorphite trait, while the rounded barrel is a mimetite trait. Neither is exclusive.
The honest position for a collector is that a label reading mimetite on material from a mixed locality is a statement about which end of the series the writer believed it sat nearer, and that this was frequently decided by colour. That is not a scandal; it is how the material was determined for most of the period during which it was collected. Record what the label says, record what you think, and do not merge the two.
Where the good material comes from
Well-crystallised mimetite is not common, and the list of localities that produce it well is short.
- Johanngeorgenstadt, Saxony is the classic German source and the reference for finely prismatic material.
- Tsumeb, Namibia produces extraordinary crystals, as it does for most lead secondaries — see our Tsumeb notes.
- The Mammoth-St Anthony mine at Tiger, Pinal County, Arizona and the Rowley mine in Maricopa County are the American benchmarks, and Tiger is also one of the great wulfenite localities.
- Santa Eulalia and San Pedro Corralitos, Chihuahua, and the Ojuela mine at Mapimí in Durango.
- Broken Hill, New South Wales, alongside the rest of that oxidised zone.
- In Britain: Dry Gill and other Caldbeck Fells mines, a number of Cornish localities, and Leadhills in Lanarkshire.
Association is a useful cross-check when a locality is in doubt: mimetite keeps company with cerussite, anglesite, smithsonite, willemite, pyromorphite and wulfenite — the standard cast of an oxidised lead-zinc orebody.
Buying, condition and care
Three things determine what a mimetite is worth looking at. Colour saturation, because pale washed-out yellow is common and strong orange is not. Crystal definition, because much mimetite is a botryoidal or mammillary crust with no crystal form to speak of. And completeness of the terminations, since at hardness 3.5 to 4 the crystals damage easily and a specimen that has been handled loose in a drawer for a century usually shows it.
Care is straightforward. Keep it out of strong light as a matter of routine rather than urgency — mimetite is not on the list of genuinely light-sensitive species — and box it individually, because it is soft enough to be marked by anything harder sharing a tray. Do not clean it with acid under any circumstances.
On handling: it is a lead arsenate, which puts it in two of the hazard groups on our toxic minerals page. Solid crystals are safe to hold. Do not saw, grind or dry-brush it, keep it away from food, and wash your hands.
Good British campylite comes onto the market rarely and usually from within collections rather than from dealers' new stock. We hold nothing; if that is what you are after, put it on the wanted list and we will tell you when a piece with a decent label appears.