Danburite

Danburite
Cluster of creamy crystals of danburite
General
Category Tectosilicates
Formula
(repeating unit)		 CaB2(SiO4)2
Strunz classification 9.FA.65
Dana classification 56.3.1.1
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Pnam
Unit cell a = 8.038(3), b = 8.752(5)
c = 7.73 [Å]; Z = 4
Identification
Colour Colourless, white, gray, brownish white, straw yellow
Crystal habit Euhedral prismatic crystals; disseminated masses
Cleavage {001} Poor
Fracture Subconchoidal to uneven
Tenacity Brittle
Mohs scale hardness 7 – 7.5
Lustre Vitreous – greasy
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.93 - 3.02
Optical properties Biaxial (+/-)
Refractive index nα = 1.627 – 1.633 nβ = 1.630 – 1.636 nγ = 1.633 – 1.639
Birefringence δ = 0.006
2V angle 88 to 90° measured
Dispersion r < v strong
Ultraviolet fluorescence Fluorescent and thermoluminescent (red); Short UV=violet blue; Long UV=blue to blue-green
References [1][2][3]

Danburite is a calcium boron silicate mineral with a chemical formula of CaB2(SiO4)2.[4]

It has a Mohs hardness of 7 to 7.5 and a specific gravity of 3.0.[4] The mineral has an orthorhombic crystal form.[4] It is usually colourless, like quartz, but can also be either pale yellow[4] or yellowish-brown. It typically occurs in contact metamorphic rocks.

The Dana classification of minerals categorizes danburite as a sorosilicate, while the Strunz classification scheme lists it as a tectosilicate;[3] its structure can be interpreted as either.

Its crystal symmetry and form are similar to topaz; however, topaz is a calcium fluorine bearing nesosilicate. The clarity, resilience, and strong dispersion of danburite make it valuable as cut stones for jewelry.

It is named for Danbury, Connecticut, United States, where it was first discovered in 1839 by Charles Upham Shephard.[4]

Danburite from Mexico, ≈ 4 cm in height

References

  1. ↑ Danburite data on Webmineral
  2. ↑ Danburite in The Handbook of Mineralogy
  3. 1 2 Danburite on Mindat.org
  4. 1 2 3 4 5 Wikisource Chisholm, Hugh, ed. (1911). "Danburite". Encyclopædia Britannica. 7 (11th ed.). Cambridge University Press. p. 793.
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