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
Vitreous to greasy\|re\|er}}	Vitreous to 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

Short description: Tectosilicate mineral

Danburite is a calcium boron silicate mineral with a chemical formula of CaB₂(SiO₄)₂.^[5]

It has a Mohs hardness of 7 to 7.5 and a specific gravity of 3.0.^[5] The mineral has an orthorhombic crystal form.^[5] It is usually colourless, like quartz, but can also be either pale yellow^[5] 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.^[5]