Danburite

Danburite
Danburite 35×28×21 cm
General
Category	Tectosilicates
Formula (repeating unit)	CaB2(SiO4)2
Strunz classification	9.FA.65
Dana classification	56.3.1.1
Crystal system	Orthorhombic
Unit cell	a = 8.038(3) Å, b = 8.752(5) Å, c = 7.73Å; Z=4
Identification
Colour	Colourless, white, gray, brownish white, straw yellow
Crystal habit	Euhedral prismatic crystals; disseminated masses
Crystal symmetry	Orthorhombic dipyramidal H-M symbol: (2/m 2/m 2/m) Space group: Pnam
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.

It has a Mohs hardness of 7 to 7.5 and a specific gravity of 3.0. The mineral has an orthorhombic crystal form. It is usually colourless, like quartz, but can also be either pale yellow 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. Danubrite data on Webmineral
2. Danubrite in The Handbook of Mineralogy
3. Danubrite on Mindat.org
4.  Chisholm, Hugh, ed. (1911). "Danburite". Encyclopædia Britannica 7 (11th ed.). Cambridge University Press. p. 793.