Grandidierite

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.

Grandidierite are blue in color, the intensity of which increases with their iron (Fe) content. They exhibit strong trichroic pleochroism (dark blue-green, colorless (sometimes very light yellow) and dark green), which can generally help distinguish them from other gemstones such as lazulites.

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

Grandidierite

General

Category Nesosilicate

Formula (Mg,Fe2+)Al3(BO3)(SiO4)O2

IMA symbol Gdd

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

Crystal habit Tabular

Twinning none

Cleavage Good

Fracture Brittle

Mohs scale hardness 7.5

Luster Vitreous, glassy

Streak White

Diaphaneity Transparent to translucent

Specific gravity 2.85 to 3.00

Density 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

Etymology

Discovered in 1902 at Cape Andrahomana in southern Madagascar by mineralogist Alfred Lacroix, it is named in honor of explorer and naturalist Alfred Grandidier (1836–1912), the first authority on the natural history of Madagascar. Cape Andrahomana was the only known source until the 2000s when transparent grandidierite was discovered in 2000 in Sri Lanka, and another gem-quality deposit also in Madagascar near Tranomaro in 2014, and then another at Tulear in 2015.

In 2002, geologists discovered an iron-rich bluestone called ominelite that forms a series with grandidierite as its iron-dominant counterpart (with grandidierite as its magnesium-dominant analogue). The formula of ominelite is (Fe 2+, Mg)(Al, Fe 3+) 3 (SiO 4)(BO 3)O 2 and its density is slightly higher than that of grandidierite.

Classification

According to the Nickel-Strunz classification, grandidierite belongs to "09.AJ: Nesosilicate structures (isolated tetrahedra), with triangles of BO₃ and/or B, tetrahedra of Be, sharing a vertex with SiO₄" together with the following minerals: ominelite, dumortierite, holtite, magnesiodumortierite, garrelsite, bakerite, datolite, gadolinite-(Ce), gadolinite-(Y), hingganite-(Ce), hingganite-(Y), hingganite-(Yb), homilite, melanocerite-(Ce), minasgeraisite-(Y), calcibeborosilite-(Y), stillwellite-(Ce), cappelenite-(Y), okanoganite-(Y), vicanite-(Ce), hundholmenite-(Y), proschenkoite-(Y) and jadarite.

Features

Grandidierite is a nesosilicate with the chemical formula MgAl₃O₂(BO₃)(SiO₄). It crystallizes in the orthorhombic system. Its hardness on the Mohs scale is 7.5. It is the magnesium analogue of ominelite. Chemically it is a species close to kornerupine. 

Color

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).

Grandidierite 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 grandidierite 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.

Formation and deposits

It was discovered in 1902 at Cape Andrahomana, in the Tôlanaro district, in the Anosy region (Toliara Province, Madagascar).

Sourced from Wikipedia