Garnet

GARNET “Pillar” with ALBITE Gilgit District, Northern Areas, Pakistan, Asia Size: 3.9 x 2.3 x 1.6 cm (Miniature) Owner: Crystal Classics

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different species are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: pyrope-almandine-spessartine and uvarovite-grossular-andradite.

Physical properties

Properties

Garnet species are found in many colors including red, orange, yellow, green, purple, brown, blue, black, pink and colorless. The rarest of these is the blue garnet, discovered in the late 1990s in Bekily, Madagascar. It is also found in parts of the United States, Russia, Kenya, Tanzania, and Turkey. It changes color from blue-green in the daylight to purple in incandescent light, as a result of the relatively high amounts of vanadium (about 1 wt.% V2O3). Other varieties of color-changing garnets exist. In daylight, their color ranges from shades of green, beige, brown, gray, and blue, but in incandescent light, they appear a reddish or purplish/pink color. Because of their color-changing quality, this kind of garnet is often mistaken for Alexandrite.

Garnet species’ light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives. The mineral’s luster is categorized as vitreous (glass-like) or resinous (amber-like).

Crystal structure

Garnets are nesosilicates having the general formula X3Y2(Si O4)3. The X site is usually occupied by divalent cations (Ca, Mg, Fe, Mn)2+ and the Y site by trivalent cations (Al, Fe, Cr)3+ in an octahedral/tetrahedral framework with[SiO4]4− occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit. (Note: the word “trapezohedron” as used here and in most mineral texts refers to the shape called a Deltoidal icositetrahedron in solid geometry.) They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, sharp irregular pieces are formed (conchoidal).

Hardness

Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.5 to 7.5. The harder species like almandine are often used for abrasive purposes.

Photos :

GROSSULAR GARNET var “HESSONITE” Saint-Marcel, Valle d’Aosta, Italy, Europe Size: 3 x 2.8 x 1.7 cm (Miniature) Owner: Crystal Classics
GARNET var ANDRADITE Serifos Island, Aegean Islands, Greece, Europe Size: 4.3 x 3 x 2.6 cm (Small Cabinet) Owner: Kristalle and Crystal Classics
GARNET and CALCITE Valle d’Aosta, Italy, Europe Size: 4.5 x 3 x 2 cm (Small Cabinet) Owner: Kristalle and Crystal Classics
Andradite Marki Khel, Spin Ghar range (Speen Ghar range), Khogyani District, Nangarhar Province (Ningarhar Province), Afghanistan © 2001 John H. Betts
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