Idocrase

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From The Gemology Project
Idocrase
Chemical composition Complex calcium aluminum silicate
Crystal system Tetragonal
Habit Prismatic terminated by truncated pyramids, massive, fibrous
Cleavage Poor
Fracture Sub-conchoidal
Hardness 6.5
Optic nature Uniaxial +/-
Refractive index 1.700 - 1.725
Birefringence 0.005
Dispersion Low, 0.019
Specific gravity 3.25-3.50
Lustre Vitreous
Pleochroism Weak to distinct (shades of body color)
Faceted idocrase

Idocrase image gallery

Synonyms: Vesuvianite, Ambosselite
Idocrase was first discovered on Mount Vesuvius and originally named vesuvianite in 1795 by the famous mineralogist Abraham Gottlob Werner . The French mineralogist Rene Just Hauy suggested it should be named Idocrase four years later. Both names are used interchangeably. The Gemological Institute of America prefers idocrase.

Variental Names: Cyprine, Californite

Cyprine is a blue copper containing variety of idocrase (vesuvianite) that was first described from Norway about 1821 by the famous mineralogist Berzelius; the name is derived from Cyprium, the ancient term for copper.

Californite is a compact massive form of idocrase that has an appearance to poor quality jade. It has historically be used as a jade simulant.

Contents

Crystal System

Tetragonal or monoclinic

Crystal Habit

Prismatic to equant stubby crystals; massive granular to fine grained.

Chemical composition

Ca19(Al,Mg)13B0-5(Si2O7)4(SiO4)10(O,OH,F)10 often containing trace to minor amounts of iron, copper up to 2.1% (as Cu in cyprine), chromium, manganese, titanium, boron or beryllium.

Diagnostics

Recently, idocrase has been misrepresented and sold as transparent hydrogrossular garnet. It's properties can be essentially indistinguishable from hydrogrossular garnet with standard gem testing equipment, with the possible exception of the polariscope.

Peridot has a lower refractive index (1.654 to 1.670).The californite variety of vesuvianite (idocrase) floats or sinks slowly in methylene iodide; jadeite sinks (specific gravity less than 3.3). Nephrite has a lower specific gravity (3.295).

===Color=== yellow, yellowish green, green, emerald green, yellow brown, brown, blue, violet to magneta; sometimes displays distinct color zoning.

Magnification

Idocrase often shows a "heat-wave" effect similar to grossular garnet.
Under magnification, the cyprine variety may exhibit a distinctly fibrous appearance.

===Cleavage=== poor on {110}, {100} and very poor on {001}

===Specific Gravity=== 3.32 - 3.34 (measured); 3.42 (calculated)

Optical Nature

Uniaxial (+) or Uniaxial (-); biaxial idocrase has been observed. Idocrase appearing singly refractive has been observed.

Refractive Index

nω = 1.703 – 1.752 nε = 1.700 – 1.746. The variety cyprine from Franklin New Jersey demonstrates a lower range of refractive indices, with nω = 1.696 – 1.712 nε = 1.710 – 1.719.

Birefringence: approximately 0.005; it can be so extremely low (0.001), that it is often indistinguisable with standard refractometers.

The optical character of idocrase is uniaxial with mostly a negative optic sign. Hydroxyl rich gemstones may be uniaxial +.
Sometimes idocrase can be isotropic or biaxial +, in some zoned crystals the optic nature can be biaxial in some areas and uniaxial in other.

Pleochroism

Weak; nω = colorless to yellowish, nε = yellowish, greenish, brownish

Spectroscope

Spectrum of idocrase: absorption band at 464nm.

Treatments

None known

Phenomenon

None known

Synthetics

None known

Imitations

None known

Occurrence

See Vesuvianite

Sources

  • Deer, Howie & Zussman: An Introduction to Rock Forming Minerals
  • Anthony, John W., Bideaux, Richard A., Bladh, Kenneth W., and Nichols, Monte C. (1990): Handbook of Mineralogy: Mineral Data Publishing, Tucson, Arizona
  • Kunz, George Frederick (1903), American Journal of Science, 4th. Series, Vol. 16 pp397-398 “Californite (vesuvianite); a new ornamental stone”
  • Palache, Charles, Harry Berman & Clifford Frondel (1944): The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837-1892, Seventh edition
  • Shannon, E. V. (1922) Note on the cyprine from Franklin Furnace, New Jersey. American Mineralogist, 7, 140-142.
  • Introduction to Optical Mineralogy 3rd edition (2003), Prof. W.D. Nesse
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