Grandidierite

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Grandidierite
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General
CategoryNesosilicate
Formula
(repeating unit)
(Mg,Fe2+)Al3(BO3)(SiO4)O2
Strunz classification9.AJ.05
Dana classification54.01.01.01
Crystal systemOrthorhombicB
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPbnm
Identification
ColorBluish green
Twinningnone
CleavagePerfect
FractureBrittle
Mohs scale hardness7.5
LusterVitreous, Pearly
StreakWhite
DiaphaneityTransparent, Translucent
Density2.976
Optical propertiesBiaxial (-)
Refractive index1.583 - 1.639
PleochroismVisible
2V angleMeasured: 24° to 32°, Calculated: 32°
Dispersionstrong r < v
References[1][2]
The world's largest known cut grandidierite, a 429.87 carat stone owned by Gloman Merritt. GIL Report certificate number STN201811139168

Grandidierite, a 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 is often touted by commercial parties (eg gemstone mining companies, gem traders- and cutters) as a very rare and expensive gemstone, but with regard to rarity, a clear distinction must be made between pure gem quality and a much more common low quality grandidierite. To be of pure gem quality a stone like grandidierite has to be of high transparency, even color, and with as few internal errors as possible. A much less rare form of grandidierite is one with a low transparency and many internal defects such as fractures and inclusions. This latter form is widely available.

References`[edit]