Tourmaline chemistry, crystal system, physical & optical properties and pictures

Tourmaline

Chemistry:

Complex boro-silicate of aluminum, magnesium and iron.

(Na,Ca) (Mg,Li,Al,Fe²⁺,Fe³⁺)₃ (Al,Mg,Cr)₆ B₃ Si₆ (OH,O,F)₄

  Crystal system and habit:

Trigonal: Habit usually 3 or 6-sided elongated prisms with a slightly rounded triangular cross-section. Tourmaline is a hemimorphic mineral: the two ends of the crystal have different terminations.

 

Varieties: Transparent to opaque.

Tourmaline: Almost all shades of colour are represented by the species. Red (pink), orange, yellow, green, blue, violet, purple, colorless and black.

 

·      Achroite: Colorless variety.

 

·       Indigolite or Indicolite: Blue variety.

 

·      Rubellite: Pink and red variety.

 

·      Schorl: Opaque black variety.

 

·      Watermelon tourmaline: Pink in the centre and green around the outside

 

·      Bi, tricolour tourmaline or Parti colour tourmaline: Tourmaline with more than one colour

 

·      Paraiba Tourmaline - neon-blue variety (found in Paraiba, Brazil)

liddicoatite tourmaline

 

 

Phenomena:

 

Cat's-eye and Color-Changing (rare) Tourmaline

Pleochroism: Strong dichroism and moderate doubling of the back facets in stones cut with the table facet parallel to the length of the crystal is of aid in identification. The dichroic colours seen usually are different hues of the body colour.

Inclusion:

Trichites (hair-like irregular liquid-filled canals); tube-like channels (parallel to the 'C' axis); moderate doubling of back facets (D.R. 0.018).

Fluorescence:

 

Majority inert. Pink - nil to weak red (LW/SW).

Synthesis

Tourmaline has not been grown in ornamental-quality sizes.

Enhancement:

·       Heat treatment causes dark blue and green to lighten to blue/green or yellow/green. Brownish red stones may turn pink. (All colours are stable.) Detection: None as it duplicates the natural process. (Reduction of strong dichroism noted).

·         Irradiation (gamma-ray) deepens reds and darkens pinks (Results are stable). Irradiation is also said to alter pale tourmalines (both solid and parti-coloured material) to deeper colours and change some greens and yellows to browns. 

 Detection: Not possible to establish by routine gemological tests.

Fashioning/ cut:

Faceted, usually step or mixed, cabochon, beads, carvings. Chatoyant or heavily included stones cut en cabochon. Due to strong pleochroism dark stones maybe cut with the table parallel to the 'C' axis (i.e., in the lighter direction), while paler stones are oriented so that their table facet is at right angles to the 'C' axis in order to give as much depth of color as possible.