Danburite

Tectosilicate mineral
(repeating unit)CaB2(SiO4)2IMA symbolDbu[1]Strunz classification9.FA.65Dana classification56.3.1.1Crystal systemOrthorhombicCrystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)Space groupPnamUnit cella = 8.038(3), b = 8.752(5)
c = 7.73 [Å]; Z = 4IdentificationColourColourless, white, gray, brownish white, straw yellowCrystal habitEuhedral prismatic crystals; disseminated massesCleavage{001} PoorFractureSubconchoidal to unevenTenacityBrittleMohs scale hardness7 – 7.5LustreVitreous to greasyStreakWhiteDiaphaneityTransparent to translucentSpecific gravity2.93 – 3.02Optical propertiesBiaxial (+/-)Refractive indexnα = 1.627 – 1.633 nβ = 1.630 – 1.636 nγ = 1.633 – 1.639Birefringenceδ = 0.0062V angle88 to 90° measuredDispersionr < v strongUltraviolet fluorescenceFluorescent and thermoluminescent (red); Short UV=violet blue; Long UV=blue to blue-greenReferences[2][3][4]

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;[4] 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]

Danburite from Mexico, ≈ 4 cm in height

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Danburite data on Webmineral
  3. ^ "Danburite in The Handbook of Mineralogy" (PDF). Archived from the original (PDF) on 2022-05-11. Retrieved 2014-03-24.
  4. ^ a b Danburite on Mindat.org
  5. ^ a b c d e Chisholm, Hugh, ed. (1911). "Danburite" . Encyclopædia Britannica. Vol. 7 (11th ed.). Cambridge University Press. p. 793.
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