Apatite
Ca₅(PO₄)₃(F,Cl,OH) · Hexagonal
Apatite is the hardness-5 reference mineral and the mineral your bones and teeth are built from. Its usual mistake is being taken for beryl or tourmaline — the knife test settles it, because apatite is the softest of the three by a wide margin.
Properties
- Mohs hardness
- 5
- Streak
- white
- Lustre
- vitreous, greasy
- Colour
- Often a distinctive sea-green or blue-green; also yellow and violet.
- Cleavage
- Poor basal cleavage.
- Fracture
- Conchoidal to uneven
- Specific gravity
- 3.1–3.2 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Some specimens fluoresce
Tests that confirm apatite
- Hardness exactly 5 — a steel knife scratches it, and it will not scratch glass properly. This is the whole point of apatite on the Mohs scale.
- Hexagonal prisms, often stubby.
- Many specimens fluoresce yellow or orange under ultraviolet light.
What apatite gets confused with
Each of these is settled by one test. Run that test first.
Beryl resists a knife completely; apatite is cut by one.
Tourmaline is harder and its prisms have a rounded-triangular cross-section.
Where it is found
Igneous rocks as an accessory mineral, pegmatites, and phosphate sedimentary beds.
Is it worth anything?
Gem-quality blue-green apatite has modest value but is too soft for daily-wear jewellery.