Grandidierite

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Grandidierite
Grandidierite-169886.jpg
Tabular crystals of grandidierite from the type locality in Madagascar
General
CategoryNesosilicate
Formula
(repeating unit)
(Mg,Fe2+)Al3(BO3)(SiO4)O2
Strunz classification9.AJ.05
Dana classification54.01.01.01
Crystal systemOrthorhombic
Crystal classDipyramidal
H-M symbol: (2/m 2/m 2/m)
Space groupPbnm
Identification
ColorCyan to turquoise; bluish green
Crystal habitTabular
Twinningnone
CleavageGood
FractureBrittle
Mohs scale hardness7.5
LusterVitreous, glassy
StreakWhite
DiaphaneityTransparent to translucent
Density2.976
Optical propertiesBiaxial (-)
Refractive index1.583 - 1.639
PleochroismVisible
2V angleMeasured: 24° to 32°, calculated: 32°
Dispersionstrong r < v
References[1][2][3]

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]

A large, faceted 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).[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, with a 7.5 on the Mohs scale.

Large transparant faceted grandidierite specimens are extremely rare. The largest cut specimen currently known to the GIA weighs in at 763.5 carats.[5]

See also[edit]

References[edit]

  1. ^ a b Grandidierite: Grandidierite mineral information on Mindat
  2. ^ Grandidierite mineral data on Webmineral
  3. ^ Grandidierite on GemSociety
  4. ^ Ominelite on Mindat.org
  5. ^ "Unusually Large Grandidierite | Gems & Gemology". www.gia.edu. Retrieved 2020-09-30.