danburite

{{Infobox mineral | name = Danburite | boxwidth = | boxbgcolor = | image = Danburite-162634.jpg | imagesize = 260px | alt = | caption = Cluster of creamy crystals of danburite | category = Tectosilicate minerals | formula = CaB2(SiO4)2 | IMAsymbol = Dbu | IMAstatus = Grandfathered (1839) | strunz = 9.FA.65 | dana = 56.3.1.1 | system = Orthorhombic | class = Rhombic dipyramidal (mmm) | symmetry = Pnam (no. 62) | unit cell = a = 8.038(3), b = 8.752(5) c = 7.73 [Å]; Z = 4 | colour = Colourless, white, gray, brownish white, straw yellow | habit = Euhedral prismatic crystals; disseminated masses | twinning = | cleavage = {001} Poor | fracture = Subconchoidal to uneven | tenacity = Brittle | mohs = 7 – 7.5 | lustre = Vitreous to greasy | streak = White | diaphaneity = Transparent to translucent | gravity = 2.93 – 3.02 | density = | polish = | opticalprop = Biaxial (+/-) | refractive = nα = 1.627 – 1.633 nβ = 1.630 – 1.636 nγ = 1.633 – 1.639 | birefringence = δ = 0.006 | pleochroism = | 2V = 88 to 90° measured | dispersion = r 2(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.