Why did my fluorite break
Fluorite is hardness 4 with perfect cleavage on four planes. A flat, bright break on a specimen is cleavage; a rough one is fracture. One corner test decides.
Why did my fluorite break? Almost certainly along its cleavage. Fluorite is hardness 4 and brittle, with perfect cleavage on {111}: four planes at 70.53 and 109.47 degrees. It is the only one of the 176 Handbook sheets in this site's reference set with a perfect {111} cleavage. A flat, bright break on a crystal is cleavage; a rough one is fracture.
In short
- A flat, bright break on a fluorite crystal is cleavage, and the geometry is fixed. The Handbook sheet records {111}, perfect: four planes, parallel to the faces of an octahedron, meeting at 70.53 and 109.47 degrees. Any knock, squeeze or drop that splits the crystal finds one of those four planes.
- The corner test, stated once: on a cube, a cleavage break takes the corner off as an equilateral triangle. The new face sits at 125.26 degrees to each of the three cube faces it cuts, because the octahedral plane is tilted 54.74 degrees from each cube face. A rough, curved or stepped surface is fracture instead, which the sheet records as subconchoidal to uneven.
- The broken surface tells you how the crystal parted, not why. A knock, a temperature change and an ultrasonic bath all end on the same {111} plane. The history decides the cause, so the order of causes on this page is our judgement of what is common, not a count.
- Fluorite is unusual in this site's reference set, but not the most cleavable species in it. Six of the 22 cubic sheets record a perfect cleavage. Fluorite's is the only octahedral one; galena, halite, sylvite and gersdorffite cleave on the cube in three directions, and sphalerite on the dodecahedron in six.
- Keep every piece. A clean cleavage break is a repair question, and a repaired specimen is a disclosure question. The rest of this page sets out which break you have and where each route leads.
| What you see | What happened | The number that identifies it | Test that separates it | What to do next |
|---|---|---|---|---|
| A flat, mirror-bright face where a cube corner was | Cleavage on {111} from a knock, a drop or a squeeze | Equilateral triangle; 125.26 degrees to each of the three cube faces | Tilt it under one lamp: the whole face flashes at the same moment | Keep the corner; see the repair section below |
| Loose flat fragments with sharp and blunt edges | Several cleavage planes opened together | Faces meet at 70.53 or 109.47 degrees | Every face of a fragment flashes as a single plane | Bag the fragments with the specimen, labelled |
| A rough, curved or stepped surface | Fracture, where the break did not follow one plane | Sheet: subconchoidal to uneven; hardness 4 | Light scatters from it; there is no single flash | Nothing to rejoin cleanly; record the damage |
| A new flat crack inside the crystal after washing or a sunny windowsill | A cleavage plane opened by a temperature change, as this site's cleaning guide treats it | Optical CaF2 expands 18.85 x 10-6 per K; no breaking point is published here | The surface cannot separate this from a knock; only the history can | Room-temperature water from now on |
| A crack or lost corner after an ultrasonic bath | Cavitation, treated as the commonest cleaning cause on this site's cleaning guide | Hardness 4; four cleavage directions | History again: when did you first see it | Do not repeat; the method is on the cleaning page |
| A break along an old line with a film on both faces | A previous repair has let go | Adhesive is not a crystal plane: no single flash, and often a different response under longwave UV | Raking light, then longwave UV, as the repair guide sets out | Disclose it if the specimen is ever passed on |
Why did my fluorite break along a flat face? The cleavage, counted and measured
Fluorite's cleavage is recorded by the Handbook of Mineralogy as {111}, perfect; on {011}, parting, poor, with tenacity brittle and hardness 4. In the cubic system {111} is the octahedron, so the one cleavage is four planes. Worked from the plane normals, the angles between them are 70.53 and 109.47 degrees, and each is tilted 54.74 degrees from each face of a cube. The indices here were read from the rendered sheet, not from extracted text, which drops overbars.
Method. The 176 sheets in this site's reference set were converted with pdftotext -layout and each Cleavage field was parsed by script. 149 sheets have one. Nine name {111} in it, and fluorite is the only one of the nine where {111} is perfect: galena records parting or cleavage on {111}, cuprite interrupted, skutterudite distinct, chalcopyrite poor, bornite in traces, pyrite and cassiterite only partings, and arsenolite gives no quality. Of the 22 cubic sheets, 6 record a perfect cleavage: fluorite on the octahedron, four directions; galena, halite, sylvite and gersdorffite on the cube, three; and sphalerite on the dodecahedron, six.
So by number of cleavage directions, fluorite is not the most cleavable species in the set; sphalerite has more. What fluorite has is an octahedral cleavage on a species whose sheet lists cubes first among its forms, so on a cube the planes run straight across the corners. A knocked cube can lose a corner that way, which is why the fluorite species page says never to wedge or lever one.
Is it cleavage or fracture? The 125-degree corner test
The corner test: if a cube has lost a corner and the new surface is a flat equilateral triangle meeting each of the three cube faces at 125.26 degrees, it cleaved. The triangle is equilateral because the octahedral plane is tilted equally, 54.74 degrees, from all three cube faces; cut 2 mm along each edge and each side of the triangle is 2.83 mm. You do not need a goniometer. An equilateral outline and a face that flashes all at once under a single lamp are enough.
If the surface is rough, curved or stepped, it fractured, which the sheet records as subconchoidal to uneven. Fracture and cleavage can appear together on one break: a flat run of {111} interrupted by a stepped section where the break jumped between parallel planes. A stepped break made of small flat treads at the same angle is still cleavage. The general distinction, with parting as a third case, is on cleavage, parting or fracture: how to tell them, and is not repeated here.
Why fluorite so often breaks into octahedra or triangles: any fragment bounded only by cleavage planes has faces from the four {111} directions, so its edges meet at 70.53 or 109.47 degrees and its outline tends toward an octahedron or a piece of one. The sheet also records octahedra as a grown form, so an octahedron alone does not prove cleavage; this page does not offer a field test that separates the two.
Did I break my fluorite, or did the temperature? What the surface cannot tell you
A crack opened by a temperature change and a crack opened by a knock end on the same {111} plane, so the broken surface cannot tell them apart. The history does. Cracks noticed after hot water, a radiator, direct sun or an ultrasonic bath belong to the cases set out on how to clean fluorite safely, which covers those causes and the method that avoids them; this page does not repeat that method.
The one thermal number verified for this page is an expansion coefficient, not a breaking point. Crystran's data sheet for the optical calcium fluoride it grows gives thermal expansion of 18.85 x 10-6 per K. Worked through, a 50 mm crystal warmed or cooled by 40 K changes length by 37.7 micrometres. That is the whole of what the number says: it sets no threshold, and the page does not claim one. The Handbook sheet records nothing about thermal shock; it notes that fluorite may be thermoluminescent, and that the name comes from the Latin to flow, for its low melting point.
In our judgement, knocks remain the ordinary case. At hardness 4 and brittle, fluorite in a drawer beside harder specimens, or wrapped too loosely for a journey, meets impacts it cannot absorb. Packing is covered on how to pack and ship mineral specimens.
My fluorite broke: what to do with the pieces
Keep every fragment, bag it with the specimen and write the date on the label. A clean {111} break leaves two flat faces that may fit back together, which makes it a repair question, and a repaired specimen is a disclosure question when it is sold, swapped or inherited. How repairs are found, with raking light and longwave UV, is on how to tell if a specimen is repaired; this page gives no adhesive method.
What a break does to the specimen's standing is a separate matter, set out on what fluorite is worth, where damage sits beside locality and colour. British fluorite localities named on the sheet include Weardale, Castleton in Derbyshire and many in Cornwall; see Weardale and Derbyshire.
The damage and conservation pages are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.