Apatite

From The Gemology Project

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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

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

Contents 1 Diagnostics 1.1 Appearance 1.2 Optic Character 1.3 Refractometer 1.4 Spectroscope 1.5 UV Reactions 1.6 Chelsea Colour Filter 1.7 Dichroscope 1.8 Polariscope 1.9 Primary Sources 1.10 Magnification 2 Phenomena 3 Sources

Diagnostics

Appearance

Apatite occurs in yellow, green, violet, purple, blue, brown, colorless and pink and can be transparent to translucent.

Optic Character

DR, Uniaxial negative

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: n_ω = 1.629 - 1.667 and n_ε = 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.

UV Reactions

• Blue: Blue to light blue (LW and SW)
  
• Yellow: Purplish Pink (stronger in LW)
  
• Green: Greenish-yellow (stronger in LW)
  
• Violet: Greenish-yellow (LW);Light purple (SW)

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.

Primary Sources

Brazil, Burma, Mexico
Violet and Purple are found in Maine and Afghanistan.

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 3rd edition (2003), Prof. W.D. Nesse

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