Fluorapatite vs Chlorapatite
Fluorapatite vs chlorapatite: one calcium phosphate, fluorine or chlorine at one site. Hardness and density overlap; the ω indices meet at 1.650. Worked.
They are two species of the apatite group: fluorapatite, Ca5(PO4)3F, 3.77% fluorine, and chlorapatite, Ca5(PO4)3Cl, 6.81% chlorine. Hardness (5 and about 5) and density (3.1–3.25 and 3.17–3.18) do not separate them. Their printed ω indices meet at 1.650: 1.631–1.650 against 1.650–1.667, and hydroxylapatite straddles that line. Only an analysis of the halogen site decides.
In short
- Two IMA rows, one site. The September 2026 IMA-CNMNC Master List lists fluorapatite Ca5(PO4)3F and chlorapatite Ca5(PO4)3Cl, both Rn, renamed, 2010 s.p.; there is no row for plain apatite, which is the group name on the apatite page. The sheets file them as Apatite group; F > Cl or OH, and Apatite group; Cl > F or OH.
- The 1.650 line. On the rendered Handbook of Mineralogy sheets fluorapatite reads ω 1.631–1.650 and ε 1.627–1.646; chlorapatite ω 1.650–1.667 and ε 1.647–1.665. The ω ranges touch at 1.650 and the ε ranges are 0.001 apart. Below 1.650 points to fluorapatite, above to chlorapatite. Both are uniaxial (−), and chlorapatite may also be biaxial (−) with 2V about 10°.
- The halogen ledger. Worked on standard atomic weights: fluorine is 3.77% of fluorapatite, chlorine 6.81% of chlorapatite, hydroxyl 3.39% of hydroxylapatite. The sheets’ analyses print 3.89% F at Gloserheia, Norway (1.03 of the ideal), 6.2% Cl at Bob’s Lake, Canada (0.91) and 3.22% Cl with 0.84% F at Fairfax, Virginia. Documented failure: the fluorapatite sheet’s ideal column prints F 3.67, but its own −O = F2 (1.59) and 100.00 total need 3.77.
- Documented failure: density, weight and hydroxylapatite. A 20.00 g piece reads 13.55–13.85 g in water as fluorapatite and 13.69–13.71 g as chlorapatite, entirely inside; why the heavier chlorine barely changes the density is worked on chlorapatite. Hydroxylapatite’s ω 1.642–1.658 straddles the 1.650 line. The chlorapatite sheet adds that it may be in zoned intergrowths with fluorapatite.
- The decision. An unanalysed crystal is an apatite; the sheets make fluorapatite the likelier species. A refractive index well clear of 1.650 is a pointer. Only an analysis of the halogen site names the species. Fluorescence does not help either way: see is apatite fluorescent.
| Property | Fluorapatite | Chlorapatite | Hydroxylapatite (reference) | Separates fluor- from chlor-? |
|---|---|---|---|---|
| Formula (IMA); status | Ca5(PO4)3F; Rn, 2010 s.p. | Ca5(PO4)3Cl; Rn, 2010 s.p. | Ca5(PO4)3(OH); Rn, 2010 s.p. | By analysis only |
| Site anion, ideal by mass | F 3.77% | Cl 6.81% | OH 3.39% | By analysis only |
| Sheet’s ideal column for that anion | F 3.67 (misprint: −O = F2 1.59 and the total need 3.77) | Cl 6.81 | H2O 1.79 | — |
| Printed analysis | Gloserheia, Norway: F 3.89%; F1.03 | Bob’s Lake, Canada: Cl 6.2%, F 0.13%; Cl0.91 | Holly Springs, Georgia: F 0.16%, H2O 1.86%; (OH)1.04 | — |
| System; point group | Hexagonal or monoclinic; 6/m or 2/m | Hexagonal or monoclinic; 6/m or 2/m (end-member monoclinic) | Hexagonal; 6/m | No |
| Largest crystals (sheet) | To 2 m | To 5 cm | To 30 cm | No |
| Hardness | 5 | ~5 | 5 | No |
| Cleavage; tenacity | Poor on {0001} and {10-10}; brittle | Poor on {0001} and {10-10}; brittle | Poor on {0001} and {10-10}; brittle | No |
| Fracture | Conchoidal to uneven | Not printed | Conchoidal | No |
| Density (measured) | 3.1–3.25 | 3.17–3.18 | 3.14–3.21 | No: inside |
| 20.00 g piece in water | 13.55–13.85 g | 13.69–13.71 g | 13.63–13.77 g | No: inside |
| Optical class | Uniaxial (−) | Uniaxial (−), may be biaxial (−); 2V ~10° | Uniaxial (−) | Weakly |
| ω | 1.631–1.650 | 1.650–1.667 | 1.642–1.658 | Yes: they meet at 1.650 |
| ε | 1.627–1.646 | 1.647–1.665 | 1.637–1.654 | Yes: 0.001 apart |
| ω − ε at the range ends | 0.004, 0.004 | 0.003, 0.002 | 0.005, 0.004 | Barely |
| Colour (sheet) | Sea-green, violet, purple, blue, pink, yellow, brown, white, colorless, may be zoned | Greenish yellow, pinkish white, light greenish gray | White, wax-yellow, sea-green, brown, black | No |
| Luminescence (sheet) | May be cathodoluminescent, phosphorescent, or fluorescent in UV | None printed | None printed | No |
| Occurrence (sheet) | The most common rock-forming phosphate mineral | Relatively much rarer than fluorapatite or hydroxylapatite; F-deficient environments | Much less common than fluorapatite | Odds only |
Is fluorapatite the same as chlorapatite? Two species, one site
No: they are two species that share the calcium phosphate framework and differ in which anion is dominant at one site, fluorine or chlorine. The September 2026 IMA-CNMNC Master List lists fluorapatite, Ca5(PO4)3F, and chlorapatite, Ca5(PO4)3Cl, both status Rn (renamed), 2010 s.p., and has no row called apatite. The fluorapatite sheet files it as Apatite group; F > Cl or OH, and the chlorapatite sheet as Apatite group; Cl > F or OH. How a measured analysis is turned into a name, including a chlorapatite with chlorine below half the site, is the X-site rule on the apatite page; this page does not repeat it.
The halogen, worked. On standard atomic weights fluorapatite is 504.30 g/mol and 3.77% fluorine; chlorapatite is 520.75 g/mol and 6.81% chlorine. Both sheets’ P2O5 and CaO ideal figures reproduce: 42.22% and 55.60% for fluorapatite, 40.89% and 53.84% for chlorapatite. The sheets’ analyses sit near those ideals: Gloserheia, Norway, 3.89% F, 1.03 of the ideal; Bob’s Lake, Canada, 6.2% Cl, 0.91 of it.
Documented failure: a misprinted 3.67. The fluorapatite sheet’s ideal column prints F as 3.67. Worked, Ca5(PO4)3F is 3.77% fluorine, and the column’s own correction line agrees: −O = F2 of 1.59 is what 3.77 gives (3.67 would give 1.55), and 42.22 + 55.60 + 3.77 − 1.59 = 100.00, the printed total, where 3.67 gives 99.90. This page uses 3.77, as the apatite page does.
How do you tell fluorapatite from chlorapatite? The 1.650 line
Refractive index is the one printed property where the two ranges do not overlap, and they only just miss: ω 1.631–1.650 for fluorapatite against 1.650–1.667 for chlorapatite. The ε ranges are 1.627–1.646 and 1.647–1.665, 0.001 apart. Both symbols are dropped by text extraction, so they were read from the rendered sheets. A reading clearly below 1.650 points to fluorapatite, clearly above it to chlorapatite; a reading at 1.650 is either.
Documented failure: hydroxylapatite sits on the line. The third calcium apatite reads ω 1.642–1.658 and ε 1.637–1.654 on its sheet, so it overlaps both. A specimen whose index points to “not fluorapatite” may still be hydroxylapatite. The line separates two species, not the group, and it is a pointer, not a name.
The rest of the optics. All three are uniaxial (−). Chlorapatite alone may be biaxial (−), with α 1.665, β 1.667, γ about 1.667 and 2V about 10°, which matches the sheet’s statement that end-member chlorapatite is monoclinic. Birefringence is tiny on all three, 0.002 to 0.005 at the ends of the printed ranges, so it does not separate them.
Why don’t hardness and density work? The overlap, worked
Every hand property printed for chlorapatite falls inside fluorapatite’s. Hardness is 5 for fluorapatite and about 5 for chlorapatite. Both sheets print cleavage poor on {0001} and {10-10} and tenacity brittle; the fluorapatite sheet adds fracture conchoidal to uneven, and the chlorapatite sheet prints no fracture. The scratch method is on how to test mineral hardness properly.
Weight in water. On the measured densities a 20.00 g piece reads 13.55–13.85 g in water as fluorapatite, 13.69–13.71 g as chlorapatite and 13.63–13.77 g as hydroxylapatite. Chlorapatite’s whole range sits inside fluorapatite’s. The heavier chlorine is offset by a larger cell, so the density barely moves; the cell arithmetic is on the chlorapatite page and the weighing method on how to measure specific gravity at home.
Colour and glow. Fluorapatite’s colour line runs from sea-green through violet, blue, pink and yellow to colourless; chlorapatite’s is greenish yellow, pinkish white or light greenish gray, inside the first. Only the fluorapatite sheet mentions luminescence, “may be”, and the others are silent, which is not a test result (is apatite fluorescent). UV lamps are covered on toxic minerals in a collection; this page is not advice.
Which is more likely in a collection? What the sheets say about occurrence
The sheets make fluorapatite far the likelier. The fluorapatite sheet calls it the most common rock-forming phosphate mineral: accessory in most igneous rocks, important in syenites, alkaline rocks, carbonatites and granite pegmatites, common in marbles, skarns, Alpine-type fissures and hydrothermal tin veins, and an essential component of sedimentary phosphorites. Its fine-crystal localities include Panasqueira, Portugal, the Morro Velho gold mine and Tapira in Minas Gerais, and Cerro de Mercado, Durango.
Chlorapatite is the exception. Its sheet calls it relatively much rarer than fluorapatite or hydroxylapatite, formed in F-deficient environments: calcsilicate marble, layered mafic intrusions, veins through diabase, replacing triphylite in granite pegmatite, and meteorites. It gives large crystals from Snarum, Norway, and analysed material from Virginia, the Stillwater and Bushveld complexes, Morocco, India and Japan. It may be in zoned intergrowths with fluorapatite, so one crystal can hold both.
What to write on the label. Without an analysis the honest entry is apatite, with the locality; the form is on how to label and catalogue a mineral collection. Neither formula contains uranium or thorium; trace uranium in real apatite is on is apatite radioactive. More identification pages are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.