Grandidierite
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.[1]
| Grandidierite | |
|---|---|
 Tabular crystals of grandidierite from the type locality in Madagascar | |
| General | |
| Category | Nesosilicate |
| Formula (repeating unit) | (Mg,Fe2+)Al3(BO3)(SiO4)O2 |
| 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 |
| Twinning | none |
| Cleavage | Perfect |
| Fracture | Brittle |
| Mohs scale hardness | 7.5 |
| Luster | Vitreous, pearly |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 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 | [1][2][3] |
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).[4]
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.
Large faceted grandidierite specimens are extremely rare. The largest cut specimen currently known to the GIA weighs in at 763.5 carats.[5]
See also
References