Grandidierite

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

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.

The world's largest known cut grandidierite, a 429.87 carat stone owned by a Filipino collector Benedicto G. Membrere V. GIL Report certificate number STN201811139168

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).

Grandidierites display strong trichroic pleochroism. That means they 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.