16.8.1: Almandine

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gemology
Gemology Project

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Almandine Garnet
Chemical composition	Fe₃Al₂(SiO₄)₃ron aluminum silicate
Crystal system	Cubic
Habit	Dodecahedra
Cleavage	None
Hardness	7.5
Optic nature	Isotropic
Refractive index	1.78-1.81
Birefringence	None
Specific gravity	3.95 - 4.20
Lustre	Vitreous

Diagnostics

File:Almandine.jpg

Figure \(\PageIndex{1}\): Oval, faceted Almandine garnet

Color

Brownish red, red to violet/purple

Spectroscope

Almandine has a typical iron spectrum.

File:Almandine spectrum.jpg

Figure \(\PageIndex{2}\): usually at 504, 520, and 573nm, may also have faint lines at 423, 460, 610 and 680-690nm

Inclusions

Short rutile needles in 3 directions
Crystals surrounded by stress cracks
Irregular and rounded crystals

Polariscope

Almandine is isotropic but may have a small DR.

Specific gravity

Almandine sinks in all common heavy liquids.

Optical effects

Almandine may show asterism and chatoyancy.

Varieties

Rhodolite

Rhodolite is the purple (pinkish/violet) variety of pyrope-almandine.

Localities

India, USA, Brazil

Occurrence

Metamorphic rocks

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