Danburite
| 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) |
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.
Contents
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
