metamorphic
Garnet in Schist (Mica Schist with Garnet Inclusion)
Garnetiferous Mica Schist (X3Al2(SiO4)3)
Hardness 6.5-7.5 for the garnet crystal, 2.5-3.5 for the surrounding mica matrix. Color ranges from silvery-grey matrix with deep reddish-brown dodecahedral garnet crystals embedded within. Luster is vitreous to resinous on the garnet and pearly to metallic on the mica matrix.
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Physical properties
Hardness 6.5-7.5 for the garnet crystal, 2.5-3.5 for the surrounding mica matrix. Color ranges from silvery-grey matrix with deep reddish-brown dodecahedral garnet crystals embedded within. Luster is vitreous to resinous on the garnet and pearly to metallic on the mica matrix.
Formation & geological history
Formed during regional metamorphism of clay-rich sedimentary rocks (shales or mudstones) under medium to high temperatures and pressures, typical of collisional mountain-building events.
Uses & applications
Used primarily as mineral specimens for education and collecting. Garnet-bearing rocks are also crushed for use as industrial abrasives (sandblasting, waterjet cutting). Fine garnets are faceted into gemstones.
Geological facts
Garnets grow as porphyroblasts within metamorphic rocks, pushing aside or surrounding existing mineral grains as they form. They are often used by geologists as index minerals to determine the metamorphic grade of a rock formation.
Field identification & locations
Look for a glittering, micaceous grey metamorphic matrix containing small, hard, rounded to 12-sided reddish-brown crystals. Commonly found in metamorphic terranes worldwide, such as the Appalachian Mountains, Alps, and Himalayas.
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