Apatite

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Apatite
Chemical composition Ca5(PO4)3(F,Cl,OH)
Crystal system Hexagonal
Habit Prismatic
Cleavage Imperfect, basal
Fracture Conchoidal
Hardness 5
Optic nature Uniaxial -
Refractive index 1.63-1.64
Birefringence 0.003
Dispersion Low, 0.013
Specific gravity 3.2
Lustre Vitreous
Pleochroism Weak
Faceted Apatite
Photo courtesty of The Gem Trader


Apatite is named "the deceiver" as it's color, refractometer and specific gravity readings are similar to gemstones as tourmaline, topaz, andalusite and danburite.


Diagnostics

Refractometer

Refractometer readings are in the range of tourmaline, topaz, andalusite and danburite but usually with a very low birefringence of 0.003.
Full range: nω = 1.629 - 1.667 and nε = 1.624 - 1.666 with a full birefringence of 0.001-0.007.

Spectroscope

The spectroscope is a good diagnostic tool in seperating apatite from gemstones as tourmaline and topaz.

Blue apatite will, usually, show a rare earth spectrum line in the blue (praseodymium).
Blue-green stones may have added spectrum lines in the yellow (didymium) and line(s) in the green (praseodymium).
Yellow gems show the rare earth spectrum of greenish-blue without the line in the blue.
Neon blue stones may show rare earth spectra in the yellow.

Chelsea Colour Filter

Green-blue stones: green

Dichroscope

Apatite has weak pleochroism, so this tool may not be very useful except for blue stones where the dichroism is more profound (blue and pale yellow).

Polariscope

A nice uniaxial negative interference figure should be found.

Magnification

Clouds of tiny white crystal inclusions may be easily observed.

Phenomena

  • Cat's-eyes

Sources

  • A Students' Guide to Spectroscopy (2003) - Colin Winter FGA, DGA ISBN 0954485300
  • Introduction to Optical Mineralogy (2004) - William D. Nesse ISBN 0195149106