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Rare mineral From Wikipedia, the free encyclopedia
Grandidierite is a rare mineral that was first discovered in 1902 in southern Madagascar. The mineral was named in honor of French explorer Alfred Grandidier (1836–1912) who studied the natural history of Madagascar.[2]
Grandidierite | |
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![]() Tabular crystals of grandidierite from the type locality in Madagascar | |
General | |
Category | Nesosilicate |
Formula (repeating unit) | (Mg,Fe2+)Al3(BO3)(SiO4)O2 |
IMA symbol | Gdd[1] |
Strunz classification | 9.AJ.05 |
Dana classification | 54.01.01.01 |
Crystal system | Orthorhombic |
Crystal class | Dipyramidal H-M symbol: (2/m 2/m 2/m) |
Space group | Pbnm |
Identification | |
Color | Cyan to turquoise; bluish green |
Crystal habit | Tabular |
Twinning | none |
Cleavage | Good |
Fracture | Brittle |
Mohs scale hardness | 7.5 |
Luster | Vitreous, glassy |
Streak | White |
Diaphaneity | Transparent to translucent |
Specific gravity | 2.85 to 3.00 |
Density | 2.976 |
Optical properties | Biaxial (-) |
Refractive index | 1.583 – 1.639 |
Pleochroism | Visible |
2V angle | Measured: 24° to 32°, calculated: 32° |
Dispersion | strong r < v |
References | [2][3][4] |
Grandidierites appear bluer in color the more iron (Fe) they contain. A recently discovered gemstone, blue ominelite, is the Fe-analogue (Fe, Mg) to grandidierite (Mg, Fe).[5]
Grandidierites display strong trichroic pleochroism. That means that it can show three different colors depending on the viewing angle: dark blue-green, colorless (sometimes a very light yellow), or dark green.
While trichroism can usually help distinguish grandidierites from other gems, lazulites can occur with blue-green colors and show colorless/blue/dark blue pleochroism. Nevertheless, lazulites have somewhat higher refractive indices and specific gravity. Grandidierites also have greater hardness, with a 7.5 on the Mohs scale.
Large transparent faceted grandidierite specimens are extremely rare. The largest cut specimen currently known to the GIA weighs in at 763.5 carats.[6]
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