Where is Corundum in Feldspathic Matrix found?
Corundum (Al2O3) in a feldspathic matrix
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Open the appCorundum in Feldspathic Matrix 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 Corundum in Feldspathic Matrix
North Carolina, USA
Historically significant deposits of corundum in feldspathic matrices (often plagioclase-rich) are found in the Appalachian Mountains, particularly in areas like Cowee Valley. These are typically associated with ultramafic rocks and desilicated pegmatites.
Montana, USA
Some sapphire-bearing rocks, including those with a feldspathic component, are found in Montana, often associated with igneous or metamorphic settings.
Madagascar
Various corundum deposits, including those in metamorphic rocks with feldspar, are known from Madagascar, producing both gem and industrial corundum.
Sri Lanka
While famous for alluvial gem corundum, primary deposits in metamorphic rocks, including those with feldspathic components, are also present.
India
Corundum occurrences in metamorphic rocks, sometimes with feldspar, are found in various regions.
Canada (Ontario, Quebec)
Corundum-bearing rocks, including nepheline syenites and associated desilicated rocks with feldspar, are found in the Bancroft area of Ontario and other parts of the Grenville Province.
Collecting tips
- Look for Metamorphic Terrains. Focus on regions known for high-grade metamorphic rocks, especially those derived from aluminum-rich protoliths or areas with ultramafic intrusions.
- Identify Associated Minerals. Corundum in feldspathic matrix is often found with other minerals like mica (biotite, muscovite), garnet, spinel, magnetite, and sometimes sillimanite or kyanite, depending on the specific formation conditions.
- Check for Hardness. Corundum is extremely hard (Mohs 9). Use a known mineral of lesser hardness (e.g., quartz, Mohs 7) to test the hardness of suspected corundum crystals within the matrix. Corundum will scratch quartz.
- Observe Crystal Habit. Look for characteristic hexagonal or barrel-shaped crystals of corundum embedded in the lighter-colored feldspathic matrix.
- Examine for Parting. While not true cleavage, corundum often exhibits parting planes, which can be visible as parallel lines or steps on crystal faces.
Quick Check
- Color:
- Variable (corundum: red, blue, white, gray; matrix: white, gray, pink)
- Luster:
- Vitreous to adamantine (corundum); Vitreous to pearly (feldspar)
- Streak:
- White (for both corundum and feldspar)
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