How to tell pyromorphite from mimetite
Both are hardness 3.5 to 4 and their densities differ by only 0.20. No hand test separates them, because they form a continuous series. What the tests can do.
You usually cannot, and the honest answer is worth more than a false rule. Both are apatite-group lead halides at hardness 3.5 to 4. Pyromorphite's density is 7.04 and mimetite's 7.24 — a difference of 2.8% — and they form a continuous phosphate-arsenate series, so intermediates exist.
In short
- Hardness is identical. The Handbook of Mineralogy gives both species 3.5 to 4. There is no hardness test, and any guide that offers one is wrong.
- Density differs by 0.20, which sounds usable and is not. Pyromorphite 7.04, mimetite 7.24. On a 20 ml specimen that is 140.8 g against 144.8 g — a 4 g gap that a measuring-cylinder displacement reading cannot resolve.
- They form a series, arsenate substituting for phosphate continuously. So even a perfect density measurement returns a composition, not a species name, unless the value sits near an end member.
- Colour and habit give a probability, not an identification. Dark grass-green with hoppered terminations leans pyromorphite; pale to bright yellow, barrel-shaped, leans mimetite. Both statements have well-known exceptions, and campylite is the famous one.
- Locality is the strongest single clue available to a collector, because the phosphate-to-arsenate balance is a property of the deposit. What a locality cannot do is settle an individual specimen.
| Test | Pyromorphite | Mimetite | Does it separate them? |
|---|---|---|---|
| Hardness | 3.5–4 | 3.5–4 | No. Identical |
| Density | 7.04 | 7.24 | Only with a hydrostatic balance reading to 0.01 g. Displacement in a cylinder cannot |
| Streak | White | White | No |
| Lustre | Resinous to subadamantine | Resinous to subadamantine | No |
| Crystal system | Hexagonal, 6/m, apatite group | Hexagonal, 6/m, apatite group | No |
| Typical colour | Dark grass-green, green, yellow, orange, brown | Pale to bright yellow, yellowish brown, yellow-orange | Suggestive only. Both range widely and overlap |
| Typical habit | Prismatic with hoppered terminations; radiating, branching, tapering; globular, reniform | Prismatic to acicular; rounded, barrel-shaped, mammillary, stalactitic | Suggestive only. Barrel-shaped leans mimetite |
| Maximum crystal size | 8 cm | 12 cm | No, not on any individual specimen |
| Fluorescence | May fluoresce yellow to orange, LW and SW | May fluoresce reddish yellow, LW or SW | Weakly suggestive. Response is locality-dependent and often absent |
| Chemical analysis | Phosphate dominant | Arsenate dominant | Yes. This is the only test that determines the species |
The worked calculation: why density seems to work and does not
Density is the test everyone reaches for, because 7.04 against 7.24 looks like a comfortable margin. Work it through with real numbers and it stops looking comfortable.
Take a specimen displacing 20 ml. If it is pure pyromorphite it weighs 20 × 7.04 = 140.8 g. If it is pure mimetite, 20 × 7.24 = 144.8 g. The whole difference is 4.0 g, or 2.8% of the mass.
Now consider how that measurement is usually made. A kitchen scale reads to 1 g, which is 0.7% — acceptable. The problem is the volume. Reading a water level in a 100 ml measuring cylinder to better than 1 ml is optimistic, and 1 ml on 20 ml is 5%. The error in the volume is nearly twice the entire difference between the two species. A displacement measurement cannot answer this question, no matter how carefully it is done.
What does work is hydrostatic weighing: weigh the specimen in air, then suspended in water, and divide the dry mass by the difference. For our pyromorphite, 140.8 ÷ (140.8 − 120.8) = 7.04. For the mimetite, 144.8 ÷ (144.8 − 124.8) = 7.24. With a balance reading to 0.01 g the measurement is comfortably good enough to separate 7.04 from 7.24.
And here is why even that does not finish the job. Because the two form a continuous series, a specimen measuring 7.14 is a real and common result, and it is not half of each species — it is a single mineral of intermediate composition whose name depends on which of phosphate and arsenate dominates. Density tells you where on the series you are. It names the species only near the ends. This is the same structural problem as the smithsonite-and-hemimorphite case, where a density measurement does settle it because those two are separate species rather than a series — the contrast is worth understanding, because it explains why one of these questions has a clean answer and the other does not.
What colour and habit are actually telling you
The field guides are not wrong that pyromorphite is usually green and mimetite usually yellow. They are wrong to present it as an identification.
The Handbook of Mineralogy lists pyromorphite as dark grass-green, green, yellow, yellow-orange, reddish orange, yellow-brown, brown, tan, greyish and sometimes colourless. Mimetite: pale to bright yellow, yellowish brown, yellow-orange, white and sometimes colourless. The two colour ranges overlap across most of their width. A yellow-orange crystal could be either. Colour comes from trace chromophores, chiefly chromium and vanadium, not from the phosphate-arsenate ratio, which is the reason it cannot track the species.
Habit is a little better. Pyromorphite characteristically forms prismatic crystals with hoppered terminations — stepped, hollow-looking crystal ends — and radiating or branching groups that taper to a point. Mimetite characteristically forms rounded, barrel-shaped, mammillary and stalactitic aggregates. A cluster of hoppered green prisms is very probably pyromorphite; a group of rounded yellow barrels is very probably mimetite.
The exception every British collector meets is campylite, the barrel-shaped variety from Dry Gill in the Caldbeck Fells, which is mimetite in a habit and often a warm orange-brown colour that neither rule predicts cleanly. It is a variety name, not a species, and it exists precisely because the material did not fit.
Both species also have the same interrupted {1011} cleavage, the same uneven to subconchoidal fracture, the same brittle tenacity and the same white streak. Our page on identifying a mineral specimen covers the order to run tests in; on this pair, the sequence runs out early.
Locality: the strongest clue a collector actually has
Since the difference between the two species is whether phosphorus or arsenic dominates, and since that depends on the chemistry of the deposit, locality carries more information than any physical test available without a laboratory.
Pyromorphite dominates in the classic British lead orefields where the ore fluid was phosphate-rich. Mimetite dominates in oxidised zones of arsenic-bearing lead deposits — its associates, per the Handbook, are cerussite, anglesite, smithsonite, willemite, pyromorphite and wulfenite. Note that pyromorphite appears in mimetite's own association list. They occur together, on the same specimens, and that is the honest end of the locality argument.
So locality tells you the prior probability and no more. A specimen labelled from a well-known pyromorphite locality is probably pyromorphite. It may be a mimetite that grew alongside, or an intermediate. Both species are apatite-group members with essentially identical structures, so they substitute for each other freely and can zone within a single crystal — the core one composition, the rim another.
Two practical implications. First, a locality-based identification should be recorded as such: “pyromorphite (by locality, unanalysed)” preserves the reasoning where a bare species name conceals it. Second, if a specimen came from a locality known for producing both, or has an unusual colour or habit for its label, treat the name as provisional. Reading listings with that in mind is part of buying specimens online well.
If a definitively analysed example of either species is what you need — for a systematic collection, or for teaching — that is a specific and findable thing, and it is worth putting on a wanted list rather than hoping to spot one, because analysed material surfaces rarely and is almost never advertised as such.
When it matters, and when to let it go
It is worth being clear that for a great many collectors this distinction does not need resolving.
It does not matter if you are buying a specimen because it is a fine crystal group from a named mine. The object is the same object whichever name is correct, and the label reflects the trade consensus for that locality. Nobody is being deceived and nothing is lost.
It matters in three cases. If you are building a systematic species collection, where the point is one verified example of each species, an unanalysed lead apatite is not a determination. If you are cataloguing for an institution or for eventual donation, where the label becomes a record that outlives you. And if the specimen is unusual for its locality — a yellow crystal from a green-pyromorphite mine — because that is exactly the case where the trade assumption is most likely to be wrong and where an analysis is most likely to be interesting.
The rule to take away: on the lead apatites, name what you can defend and record how you decided. “Pyromorphite” on its own asserts more than a collector can usually support. “Pyromorphite, by locality and habit, unanalysed” asserts exactly what is true, costs nothing, and is far more useful to whoever holds the specimen next. That is the same discipline our page on reading and writing mineral labels argues for generally, and this pair is the clearest case for it.