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Classification
Crystal System: Isometric
Class: Silicate
Sub Class: Nesosilicate (single silicate)
Group: Garnet group
Structure and Composition
Composition: (Mg,Ca,Fe,Mn)3(Al,Fe,Cr,V)2(SiO4)3
Structure: Garnet has an isometric crystal structure characterized by a dense, three-dimensional framework of isolated silica tetrahedra surrounded by two different cation sites. Large cations in dodecahedral sites, with smaller cations in octahedral sites.
In the on the right the red represents oxygen, the tan represents silicon, the orange represents the smaller cations in octahedral sites, and the green represent the larger cations in dodecahedral sites.
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: Dark red to reddish brown is the most common, varieties may be red-violet, brown, black emerald green, or white
Luster: Vitreous to resinous
Hardness: 6.5 – 7.5
Streak: White
Specific gravity: 3.6 – 4.3
Common Habit: Dodecahedrons
Cleavage: Indistinct
Identification In Thin Section
PPL Properties
Relief: High positive
Habit/Form: Garnets commonly occur as euhedral to subhedral dodecahedral or trapezohedron crystals, Can also occur as granular or irregular masses
Color: Nearly colorless to a pale-brown/pink/green
Cleavage: Absent, irregular fractures are common
Extra: Inclusions of quartz and other minerals are common
XPL Properties
Isotropy/Anisotropy: Isotropic
Interference color: N/A
Extinction angle: N/A
Twins: Absent
Uniaxial/Biaxial: Neither
Garnet PPL thin section (photo by me)
Garnet XPL thinsection (photo by me)
Geologic Occurrence
Garnet is a common mineral in medium to high grade metamorphosed pelitic and mafic rocks.
Significance
Garnets have been used as gemstones for thousands of years. Garnets’ color, hardness, and transparency make them attractive stones for use in jewelry, but they are generally too common to be prized as high-quality gems. Garnet’s main economic value derives from its hardness and lack of cleavage, which makes it an excellent abrasive. Fine garnet dust is used in the detailed finishing and polishing of optical lens, plate glass, and semiconductor materials. Garnet is also used as a filtering medium for water purification systems or wastewater treatment plants.
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