Grandidierite

{{Short description|Rare mineral}}

{{Infobox mineral

| name = Grandidierite

| image = Grandidierite-169886.jpg

| alt =

| caption = Tabular crystals of grandidierite from the type locality in Madagascar

| category = Nesosilicate

| formula = (Mg,Fe²⁺)Al₃(BO₃)(SiO₄)O₂

| IMAsymbol = Gdd{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| strunz = 9.AJ.05

| dana = 54.01.01.01

| system = Orthorhombic

| class = Dipyramidal
H-M symbol: (2/m 2/m 2/m)

| symmetry = Pbnm

| unit cell =

| color = Cyan to turquoise; bluish green

| habit = Tabular

| twinning = none

| cleavage = Good

| fracture = Brittle

| tenacity =

| mohs = 7.5

| luster = Vitreous, glassy

| streak = White

| diaphaneity = Transparent to translucent

| gravity = 2.85 to 3.00

| density = 2.976

| polish =

| opticalprop = Biaxial (-)

| refractive = 1.583 – 1.639

| birefringence =

| pleochroism = Visible

| 2V = Measured: 24° to 32°, calculated: 32°

| dispersion = strong r < v

| extinction =

| length fast/slow =

| fluorescence =

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| melt =

| fusibility =

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| prop1 =

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| references = [http://www.mindat.org/min-1737.html Grandidierite: Grandidierite mineral information on Mindat][http://webmineral.com/data/Grandidierite.shtml Grandidierite mineral data on Webmineral][https://www.gemsociety.org/article/grandidierite-jewelry-and-gemstone-information/ Grandidierite on GemSociety]

}}

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.

File:World's Largest Cut Grandidierite 429.60 Carats.jpg

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).[https://www.mindat.org/min-10796.html Ominelite on Mindat.org]

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 transparent faceted grandidierite specimens are extremely rare. The largest cut specimen currently known to the GIA weighs in at 763.5 carats.{{Cite web|title=Unusually Large Grandidierite {{!}} Gems & Gemology|url=https://www.gia.edu/gems-gemology/spring-2020-labnotes-unusually-large-grandidierite|access-date=2020-09-30|website=www.gia.edu|language=en}}