Danburite

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Danburite
Danburite-162634.jpg
Cluster of creamy crystals of danburite
General
CategoryTectosilicates
Formula
(repeating unit)
CaB2(SiO4)2
Strunz classification9.FA.65
Dana classification56.3.1.1
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPnam
Unit cella = 8.038(3), b = 8.752(5)
c = 7.73 [Å]; Z = 4
Identification
ColourColourless, white, gray, brownish white, straw yellow
Crystal habitEuhedral prismatic crystals; disseminated masses
Cleavage{001} Poor
FractureSubconchoidal to uneven
TenacityBrittle
Mohs scale hardness7 – 7.5
LustreVitreous – greasy
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.93 - 3.02
Optical propertiesBiaxial (+/-)
Refractive indexnα = 1.627 – 1.633 nβ = 1.630 – 1.636 nγ = 1.633 – 1.639
Birefringenceδ = 0.006
2V angle88 to 90° measured
Dispersionr < v strong
Ultraviolet fluorescenceFluorescent 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[edit]

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