Where is Magnetite in Garnet found?
Fe3O4 in (Ca,Mg,Fe,Mn)3(Al,Fe)2(SiO4)3
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Open the appMagnetite in Garnet occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Magnetite in Garnet
Worldwide Metamorphic Terrains
Magnetite in garnet can be found in numerous metamorphic belts globally, wherever garnet-bearing metamorphic rocks occur. Notable examples include the Appalachian Mountains (USA), the Alps (Europe), the Himalayas (Asia), and various Precambrian shields.
Skarn Deposits
In contact metamorphic environments, particularly skarns, where iron-rich fluids interact with carbonate rocks, magnetite can be abundant and may be found as inclusions within associated garnets (e.g., andradite-grossular series).
Igneous Rocks
Certain igneous rocks, such as some granites, pegmatites, and kimberlites, can host garnets with magnetite inclusions, reflecting the crystallization conditions of the magma.
Collecting tips
- Examine Metamorphic Rocks. Focus on medium to high-grade metamorphic rocks like schists, gneisses, and amphibolites, which are common hosts for garnet. Look for dark, opaque specks within garnet crystals.
- Use a Hand Lens or Microscope. Many magnetite inclusions are small and require magnification for clear observation. A 10x hand lens is often sufficient, but a petrographic microscope is ideal for detailed study.
- Check for Magnetic Properties. While the host garnet is non-magnetic, magnetite inclusions are strongly magnetic. If a garnet crystal contains abundant magnetite, it might exhibit a weak magnetic attraction to a strong magnet. This is a key differentiating feature from other dark inclusions like ilmenite or graphite.
- Look for Contrasting Luster. The metallic luster of magnetite inclusions will stand out against the vitreous luster of the host garnet.
Quick Check
- Color:
- Black inclusions within a typically red, brown, or green host garnet.
- Luster:
- Metallic to submetallic for inclusions, vitreous for host garnet.
- Streak:
- Black for magnetite inclusions (if tested on a separate piece), colorless for garnet.
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