Danburite

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Danburite
Chemical composition CaB2(SiO4)2

Calcium boro-silicate

Crystal system Orthorhombic
Habit Prismatic
Cleavage Indistinct, basal
Fracture Sub-conchoidal
Hardness 7.0-7.5
Optic nature Biaxial +/-
Refractive index 1.630-1.636
Birefringence 0.006
Dispersion low, 0.017
Specific gravity 3.00
Lustre Vitreous
Pleochroism Weak/not determinable
Fluorescence Light blue (LW)
A colorless danburite.
Photo courtesy of Chaman Golecha


Danburite was named for the town of Danbury, Fairfield County, Conneticut, where it was first reported in 1839. This initial discovery was of no gemological interest, but by 1880, an additional occurrence in Russell, St Lawrence County, New York was producing facetable rough. Although danburite is now known from many localities around the world, only a few have produced material of sufficient clarity to be facetted.

Chemical composition

Calcium boron silicate, CaB2(SiO4)2 , sometimes with minor to trace amounts of iron, manganese, sodium, and magnesium.

Diagnostics

Danburite may be confused with topaz, tourmaline, andalusite and apatite.

Color

Colorless to pale pink; golden yellow, yellow-brown.

Refractive index

Danburite is biaxial with either a negative or a positve optical sign, depending on the wavelength of the lightsource. For sodium light (nD) it will be negative. The use of blue light will give a positive optical sign.
Beta is halfway between alpha and gamma.

Refractive index: nα =1.630, nβ =1.633, nγ =1.636.
Birefringence: 0.006

Topaz will have lower readings in RI but a higher birefringence.

Specific gravity

The SG range of danburite goes from 2.93 to 3.02 (mean = 3.0) and it will float in heavy liquids 3.06 and 3.33 while andalusite will sink in 3.06 and topaz will sink in both.

Fluorescence

The majority of danburites fluoresce a blue-white color in longwave ultraviolet radiation. This must not, however, be considered wholly diagnostic.

Spectroscope

Danburite may show a rare earth spectrum due to didymium (lines in the yellow).

Treatments

Danburite may turn pink on irradiation.

Occurrence

Danburite occurs in pegmatites and in carbonate rocks that have been metamorphosed and exposed with hydrothermal activity.

Large quantities of water-clear to pale-pink danburite have been obtained from Charcas, San Luis Potosí, Mexico since the late 1950's; Mogok, Mandalay Division, Myanmar (Burma), has produced rough exhibiting a very pleasing straw-yellow or golden color; and distinctly golden-brownish to straw yellow to amber-yellow rough has been obtained from the Anjanabonoina pegmatites and the Madagascar Sahatany Pegmatite Field and other sites in Antananarivo Province, Madagascar. Sherry to golden colored danburite rough has been produced from Dal'negorsk (Dalnegorsk), Primorskiy Kray, Russia.

Sources

  • Anthony, John W., Bideaux, Richard A., Bladh, Kenneth W., and Nichols, Monte C. (1990): Handbook of Mineralogy: Mineral Data Publishing, Tucson, Arizona
  • 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