What is a pseudomorph
A pseudomorph is one mineral in the crystal shape of another. Of 109 reference sheets we read, just one mentions the word — it is not a species property.
A pseudomorph is one mineral species occupying the crystal shape of a different one — the form of the original survives while the substance is replaced. We read the Handbook of Mineralogy sheet for 109 species and exactly one uses the word. Malachite after azurite and quartz after barite are the familiar cases. A pseudomorph is a property of a specimen, not of a species, so no reference can tell you whether yours is one.
In short
- The word means false form, and that is literally what it is. The outward crystal shape belongs to one species and the material inside belongs to another. A cube of goethite is a cube because it used to be pyrite.
- No reference book can confirm your specimen is a pseudomorph. Of 109 Handbook of Mineralogy sheets we read, exactly one mentions pseudomorphism — meta-autunite, formed as dehydration pseudomorphs after autunite. It is a specimen-level phenomenon, not a species-level one.
- There are four mechanisms and they are not interchangeable. Substitution replaces the material atom by atom; alteration changes it chemically; incrustation coats a crystal that later dissolves; a paramorph keeps the same chemistry and changes only the structure.
- “After” is the convention and the order matters. Goethite after pyrite means goethite now, pyrite before. Reading it the wrong way round is the commonest mistake on old labels.
- They are genuinely uncommon. In the 285-product archived stock list of the previous business at this domain, 7 items are pseudomorphs — about one in forty.
| Mechanism | What happens | Chemistry | Shape | Example |
|---|---|---|---|---|
| Substitution | Material is carried away and different material deposited in its place, volume for volume | Changes completely | Preserved, often in fine detail | Quartz after barite; quartz after scheelite |
| Alteration | The original reacts in place, gaining or losing components | Partly inherited | Preserved but often roughened | Malachite after azurite |
| Incrustation | A crust forms over a crystal which later dissolves, leaving a hollow mould | Unrelated to the original | Preserved as a shell, hollow inside | Quartz casts after fluorite or calcite |
| Paramorph | The structure inverts to a different polymorph of the same compound | Unchanged | Preserved | Calcite after aragonite |
| Dehydration | Water leaves the structure and a lower hydrate is left in the same form | Loses water only | Preserved, usually cracked | Meta-autunite after autunite |
Why no book can tell me whether my specimen is a pseudomorph
This page began with a search that returned almost nothing, and the nothing turned out to be the answer.
Method. On 18 September 2026 we read the full Handbook of Mineralogy sheet for each of the 109 species this site covers, and searched every one for the string pseudomorph.
One sheet uses the word. Meta-autunite, whose occurrence paragraph reads, in full: A secondary mineral, formed as dehydration pseudomorphs after autunite. That is the entire harvest from 109 sheets of the primary mineralogical reference.
This is not a gap in the Handbook. It is a statement about what a pseudomorph is. A species sheet records formula, hardness, density, habit, cleavage, optics and occurrence — the properties a species has wherever it is found. Pseudomorphism is none of those. It is a history that happened to one lump of rock: this crystal grew, and then something else took its place. Two specimens of the same species from the same mine can differ on it absolutely.
The practical consequence is the useful part. There is no reference you can consult to check a pseudomorph claim on a label. Unlike a hardness or a type locality, it cannot be verified against a book — only against the specimen, by someone who can read what the shape and the substance are each telling them. That makes it one of the claims on an old label most worth examining yourself, and our note on what a label can be trusted for applies here with particular force.
The four mechanisms, and why the distinction is not pedantry
“Pseudomorph” covers several quite different processes, and the difference shows up in what the specimen looks like and how much of the original detail survived.
Substitution is the one people picture: the original dissolves and something else is deposited in the space, volume for volume, often preserving growth striations and face detail. Quartz after barite and quartz after scheelite — both in the archived list discussed below — are of this kind, and they can be startlingly sharp.
Alteration is chemical change in place: the original reacts with circulating solutions and becomes a related species, keeping some of its own atoms. Malachite after azurite is the classic, and it is very common, because azurite is the less stable of the two copper carbonates. Altered pseudomorphs usually look softer and rougher than substituted ones, because the reaction proceeds inward from every surface at once.
Incrustation is not a replacement at all: a crust grows over a crystal, the crystal dissolves, and a hollow shell is left. These are properly casts or moulds rather than pseudomorphs in the strict sense, and they give themselves away by being hollow.
A paramorph is the subtlest — the chemistry does not change at all, only the crystal structure, as one polymorph inverts to another. Calcite after aragonite is the standard case; both are CaCO3, and the Handbook records that aragonite converts to calcite over geologic time. Our aragonite note covers the pair.
Dehydration is the mechanism the Handbook actually documents, and it is the one happening fastest. See the next section.
The one pseudomorph forming in collections right now
Most pseudomorphs took geological time. One does not, and it is in a great many collections.
Autunite converts irreversibly to meta-autunite at room temperature, losing five of its eleven waters. The Handbook's autunite sheet says simply Dehydrates in air; the meta-autunite sheet says Dehydrates irreversibly from autunite under ambient conditions and calls the result dehydration pseudomorphs after autunite. Its distribution note adds that all autunite localities probably contain meta-autunite.
The numbers explain why the crystals crack. Autunite is Ca(UO2)2(PO4)2(H2O)11 and meta-autunite Ca(UO2)2(PO4)2·6H2O. Losing five water molecules costs about 9.3% of the mass, and the measured density rises from 3.05–3.2 to 3.35–3.55 — roughly 10%. Mass down 9% and density up 10% means the crystal occupies about 18% less volume than it did, and a tabular crystal that shrinks by that much while sitting on its matrix has nowhere to go but apart.
So a yellow autunite that has gone dull, pale and flaky has not been damaged by anything you did. It has become a pseudomorph of itself. Our note on whether autunite is dangerous works through what that means for handling it, because the flaking is also the mechanism by which a uranium mineral becomes a dust question.
How common are they, really
Pseudomorphs have a reputation as a curiosity rather than a staple, and one measurement on this domain supports it.
Method, and the disclosure that goes with it. This domain was previously used by a different mineral business, and the archived list of its 285 products survives. We are not that business, we hold none of that stock, and none of its descriptions or photographs are reproduced here. What the list is good for is one thing: a snapshot of what a real dealer's inventory contained.
Seven of the 285 are pseudomorphs — about one in forty. They are: malachite after azurite; barite after alstonite; dolomite pseudomorphing calcite; quartz after barite; pyrite after pyrrhotite; bornite after chalcocite; and quartz after scheelite.
Two things are worth drawing out of that short list. Every one names both species, in the right order — which is the convention, and the thing to check on any label you are offered. And the mechanisms are spread across the categories above: substitution (quartz after barite, quartz after scheelite), alteration (malachite after azurite, bornite after chalcocite), and replacement in carbonates (dolomite after calcite).
The rate matters for buying. At roughly one specimen in forty, a pseudomorph is uncommon enough to be interesting and common enough that you will meet one. Our notes on buying online and on old collection material cover the general checks; the pseudomorph-specific one is to ask what evidence supports the “after” part, because that is the half of the label nobody can verify from a book.
We keep no catalogue and nothing here is offered for sale. Our wanted list sets out what we look for, and well-documented pseudomorphs with old labels are on it. The rest of our collecting notes cover reading an old label and what it can be trusted for.