Tourmaline | |
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General | |
Category | Cyclosilicate |
Formula (repeating unit) | (Ca,K,Na, ▢)(Al,Fe,Li,Mg,Mn)3(Al,Cr,Fe,V)6 (BO3)3(Si,Al,B)6O18(OH,F)4[1][2] |
IMA symbol | Tur[3] |
Crystal system | Trigonal |
Crystal class | Ditrigonal pyramidal (3m) H-M symbol: (3m) |
Space group | R3m (no. 160) |
Identification | |
Color | Most commonly black, but can range from colorless to brown, red, orange, yellow, green, blue, violet, pink, or hues in-between. It can also be bi-colored, or even tri-colored. Rarely, it can be found as neon green or electric blue. |
Crystal habit | Parallel and elongated; acicular prisms, sometimes radiating; massive; scattered grains (in granite) |
Cleavage | Indistinct |
Fracture | Uneven, small conchoidal |
Tenacity | Brittle |
Mohs scale hardness | 7.0–7.5 |
Luster | Vitreous, sometimes resinous |
Streak | White |
Diaphaneity | Translucent to opaque |
Specific gravity | 3.06+0.20–0.06[1] |
Density | 2.82–3.32 |
Polish luster | Vitreous[1] |
Optical properties | Double-refractive, uniaxial negative[1] |
Refractive index | nω = 1.635–1.675 nε = 1.610–1.650 |
Birefringence | −0.018 to −0.040; typically about −0.020 but in dark stones it may reach −0.040[1] |
Pleochroism |
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Dispersion | 0.017[1] |
Ultraviolet fluorescence | Pink stones; inert to very weak red to violet in long and short wave[1] |
Absorption spectra | Strong narrow band at 498 nm, and almost complete absorption of red down to 640 nm in blue and green stones; red and pink stones show lines at 458 and 451 nm, as well as a broad band in the green spectrum[1] |
Tourmaline (/ˈtʊərməlɪn, -ˌliːn/ TOOR-mə-lin, -leen) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a wide variety of colors.
The name is derived from the Sinhalese tōramalli (ටෝරමල්ලි), which refers to the carnelian gemstones.[4]
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