Garnetite is a crystalline-textured metamorphic rock that is overwhelmingly composed of garnet.  Garnetite is quite hard (H = 7) and easily scratches glass.  It is a high-grade metamorphic rock that can form in deep subduction zone environments or by contact metamorphism.  The sample shown below is a contact metamorphic garnetite.


 Garnetite (reddish-brown) with molybdenite (silver, MoS2) & ferrimolybdite (yellow, Fe2(MoO4)37-8H2O) from near Brownes Lake, northeastern Beaverhead County, southwestern Montana, USA.  This is a contact metamorphic rock (skarn) produced by intrusion of the Pioneer Batholith (Late Cretaceous, 69-77 million years) into Paleozoic limestones.



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