How to clean fluorite safely
Fluorite is hardness 4 with perfect cleavage on four planes, so no direction of applied force lies more than 54.74 degrees from a cleavage plane normal.
Water, a soft brush, patience and nothing else, unless you have a specific reason. Fluorite is hardness 4 with perfect cleavage on the four planes of {111}, and the geometry leaves no safe direction: for any force you apply, a cleavage plane normal lies within 54.74 degrees of it. The two things that most often break fluorite during cleaning are ultrasonic baths and rapid temperature change.
In short
- Start with water and a soft brush, at room temperature, and stop there for a week. Most fluorite needs nothing else. A specimen soaked in plain water for several days and brushed gently will give up clay and loose iron oxide, and nothing that soak removes was worth keeping.
- There is no safe way to grip a fluorite crystal, and here is the arithmetic. The {111} cleavage is four planes at 70.53 and 109.47 degrees to one another. Sampling every possible direction of applied force, the largest angle to the nearest cleavage plane normal is 54.74 degrees, at a cube-face direction. Every pinch, drop and thermal gradient is within 55 degrees of a plane that wants to open.
- Never an ultrasonic bath. Cavitation works by collapsing bubbles against the surface, which is exactly the kind of localised impulse that a four-direction perfect cleavage at hardness 4 converts into a cleaved corner. This is the single commonest way collectors destroy fluorite.
- Never hot water, never a radiator, never a windowsill. Fluorite decrepitates under thermal shock and the internal stress from a rapid temperature change is enough to part a crystal along a cleavage. Wash cold, dry slowly, store at stable temperature.
- Half the advice you will find is about polishing, and for a specimen it is the wrong answer. Polishing a natural crystal face destroys the thing that makes it a specimen. Polishing belongs to lapidary work on massive material, and the two activities have nothing to do with each other.
| Method | What it removes | Risk to fluorite | Verdict |
|---|---|---|---|
| Cold water soak, several days | Clay, silt, soluble salts | None | Always first. Change the water daily |
| Soft brush, cold, under water | Loosened clay, surface dirt | Minimal if the brush is soft and the crystal is supported | Yes. Support the specimen on a folded cloth |
| Dental pick under a lens, dry | Adhering matrix, carbonate specks | Real. A slip cleaves a corner | Only with the specimen bedded in sand and the hand braced |
| Warm, not hot, water | Greasy residue | Thermal shock if the change is rapid | Only with a gradual change of a few degrees |
| Ultrasonic bath | Everything, including your specimen | Severe. Cavitation opens cleavages | No |
| Dilute hydrochloric or acetic acid | Calcite matrix | Low to the fluorite itself; high to everything else on the piece | Only when nothing else on the specimen is a carbonate, a sulphide or a sulphate |
| Concentrated sulphuric acid | — | Generates hydrogen fluoride from the fluorite itself | Never. This is the industrial route to HF |
| Hydrofluoric acid | — | — | Never, on anything, for any reason |
| Polishing or tumbling | The crystal faces | Total and irreversible | Not on a specimen |
The geometry, worked through, and why it means what it means
Fluorite's cleavage is recorded by the Handbook of Mineralogy as {111}, perfect; on {011}, parting, poor. In the cubic system {111} is the octahedron, so that is four distinct planes, not one. Their normals point along the four body diagonals of the cube.
The question worth asking is whether there is a direction you can safely push. We computed it: take 400,000 uniformly distributed directions on a sphere, and for each one find the angle to the nearest of the four cleavage-plane normals. The largest such angle that occurs anywhere is 54.74 degrees, and it occurs along a cube-face direction. The planes themselves meet at 70.53 and 109.47 degrees, the tetrahedral angles.
In plain terms: there is no orientation in which a fluorite crystal is more than 55 degrees away from a plane that will open under load. Compare a mineral with one perfect cleavage, where you can load along the plane and the cleavage is irrelevant. Fluorite has no such orientation, and that — not its hardness — is why it is a difficult mineral to handle.
It is also why fluorite octahedra exist at all. Cleaved octahedra sold at shows are frequently not grown that way; they are cubes or massive material parted along {111}, which is a legitimate thing to sell and worth knowing when you buy one. Of the 158 species sheets in this site's reference set, 80 record perfect cleavage, but very few of those have it on a four-plane cubic form.
The method, in order, and where to stop
Day one: look, and do nothing. Under a lens, identify everything on the specimen. Is there calcite? Baryte? Galena, sphalerite or another sulphide? A carbonate or a sulphide on the piece rules out acid entirely, and that decision has to be made before anything is wet.
Days one to seven: cold water. Full immersion in tap water at room temperature, changed daily. Clay hydrates and releases over days, not minutes. This does most of the work on most specimens and it cannot damage anything.
Then: a soft brush, under water, with the specimen supported. Bed it on a folded towel in the bowl so the crystals are not carrying its weight. Brush along faces, not into re-entrant angles. Soft means an artist's brush or a soft toothbrush, and if you can hear it, it is too stiff.
Then: mechanical picking, if you must. A dental pick under magnification, with the specimen bedded in a tray of sand, both hands braced and short strokes away from crystal edges. This is where damage happens, so do the least that achieves the look you want.
Then: stop. The finish state is clean enough to see the mineral, not as clean as a photograph. A fluorite with a trace of iron staining in a re-entrant is a fluorite; a fluorite with a cleaved corner is damaged for ever. Related methods are on how to clean mineral specimens and how to remove clay from a mineral specimen.
Acid, light and the two things people get wrong
Acid. Fluorite itself is largely indifferent to dilute hydrochloric or acetic acid, which is why removing calcite from fluorite is a standard operation. Everything else on the specimen is the problem: calcite matrix you wanted to keep, baryte, galena and sphalerite that will be etched or will generate hydrogen sulphide, and any carbonate or phosphate species on the piece. The full decision is on how to remove calcite from a specimen.
Two absolute prohibitions. Never bring fluorite near concentrated sulphuric acid: calcium fluoride plus sulphuric acid is the industrial synthesis of hydrogen fluoride, and you would be manufacturing it on your bench. And never use hydrofluoric acid for anything, on any specimen, under any circumstances. Any acid work at all falls under the Control of Substances Hazardous to Health Regulations 2002 if done at work, and exposure limits are in the HSE EH40 workplace exposure limits. Nothing here is safety advice and this page does not tell you how to use acid.
Light. The NatSCA guidance on the care and conservation of geological specimens names green fluorite, alongside amethyst, among the minerals that fade in strong light, and recommends light-proof storage for susceptible species. This catches people out because fading is slow and comparison is impossible once it has happened. A bright windowsill will quietly cost a green Weardale fluorite its colour over a few years. See which minerals fade in light and lighting a mineral display cabinet.
What good cleaning is actually worth. Honestly: less than people hope. Careful cleaning takes a piece from unsaleable to presentable; it does not change what the specimen is, and the factors that drive value — locality, size, colour, crystal quality, damage, provenance — are set out on what fluorite is worth. The cost of cleaning fluorite is measured in weeks of soaking and the risk of one bad minute with a pick, and the honest advice on a fine piece is to do less and, on anything important, to ask a conservator. We hold no catalogue and nothing on this site is offered for sale; the wanted list sets out the fluorite we look for, and the rest of this section is indexed at the collecting guides.