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
Twinningnone
CleavagePerfect
FractureBrittle
Mohs scale hardness7.5
LusterVitreous, pearly
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.976
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.

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[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.