Where is Potassium Feldspar found?
Potassium Feldspar (K-feldspar)
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Open the appPotassium Feldspar 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 Potassium Feldspar
Pegmatites
Large, well-formed crystals of K-feldspar (often microcline or orthoclase) are frequently found in pegmatite dikes, which are coarse-grained igneous rocks. Notable localities include the Black Hills of South Dakota (USA), Minas Gerais (Brazil), and various regions in Norway and Russia.
Granitic Intrusions
K-feldspar is a primary constituent of granites and granodiorites worldwide. Excellent exposures can be found in batholiths and plutons in regions like the Sierra Nevada (USA), the Scottish Highlands (UK), and the Canadian Shield.
Volcanic Rocks
Sanidine, the high-temperature K-feldspar polymorph, is common in felsic volcanic rocks such as rhyolites and trachytes. Examples include volcanic fields in the Western USA (e.g., Yellowstone), Italy, and Japan.
Metamorphic Terrains
K-feldspar is a common mineral in high-grade metamorphic rocks like gneisses and schists. Regions with extensive metamorphic complexes, such as the Appalachian Mountains (USA), the Alps (Europe), and parts of Scandinavia, are good places to find it.
Collecting tips
- Look for Pink or Orange Grains. In granitic rocks, K-feldspar often appears as distinct pink, salmon-orange, or reddish-brown grains, contrasting with the white or gray plagioclase and clear quartz.
- Check Cleavage Angles. Use a hand lens to observe cleavage surfaces. The near 90-degree intersection of two cleavage planes is a strong indicator of K-feldspar. Plagioclase will have cleavages intersecting at a slightly oblique angle (~94 degrees).
- Absence of Striations. Unlike plagioclase feldspar, K-feldspar typically lacks polysynthetic twinning striations on its cleavage surfaces. This is a key macroscopic differentiator.
- Hardness Test. K-feldspar has a Mohs hardness of 6. It will scratch glass but can be scratched by quartz. This helps distinguish it from softer minerals.
Quick Check
- Color:
- Pink, salmon-orange, white, or reddish-brown
- Luster:
- Vitreous to pearly
- Streak:
- White
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