Where is Altered Feldspar found?
Feldspar (likely K-feldspar) with secondary alteration/mineralization (e.g., chlorite or epidote)
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Open the appAltered 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 Altered Feldspar
Porphyry Copper Deposits
Worldwide, e.g., Chuquicamata (Chile), Bingham Canyon (USA), Grasberg (Indonesia). Sericitic alteration (feldspar to sericite) is a key indicator.
Geothermal Fields
Iceland, New Zealand, USA (e.g., Salton Sea). Alteration of feldspars to clay minerals, epidote, and chlorite is common due to hot water circulation.
Greenschist Facies Metamorphic Terrains
Many ancient mountain belts, e.g., Appalachian Mountains (USA), Scottish Highlands (UK). Plagioclase often alters to epidote and chlorite (saussuritization).
Weathered Granitic and Volcanic Rocks
Ubiquitous globally. Feldspars alter to clay minerals (kaolinite, illite) due to meteoric water interaction.
Collecting tips
- Look for Color Changes. Unusual green, yellowish-green, dull white, or silvery patches within otherwise typical feldspar-bearing rocks often indicate alteration.
- Check for Reduced Hardness. Altered feldspar, especially if rich in clay minerals or sericite, will be softer than fresh feldspar (Mohs 6-6.5). Test with a steel knife (hardness 5.5).
- Observe Luster Changes. A dull, earthy, silky, or waxy luster replacing the typical vitreous luster of feldspar is a strong indicator of alteration.
- Examine Textural Features. Cloudiness, opacity, or fine-grained aggregates replacing clear feldspar crystals are characteristic. Pseudomorphic textures can be diagnostic.
- Consider the Geological Context. If you are in an area known for hydrothermal activity, metamorphism, or significant weathering, the likelihood of finding altered feldspar increases.
Quick Check
- Color:
- Variable (green, yellowish-green, white, gray, silvery) often mottled or cloudy.
- Luster:
- Dull, earthy, silky, waxy, or greasy, often lacking the vitreous luster of fresh feldspar.
- Streak:
- White to pale green/yellow (depending on secondary mineral), often lighter than the apparent body color.
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