Lecture Notes: Alex Strekeisen on Perthite

Introduction to Perthite

• Location: Named after Perth, Ontario.
• Type Locality: 9.5 km southwest of Perth.
• Definition: Perthite is an intergrowth of sodic and potassic feldspar resulting from subsolidus exsolution.
  • Perthite: Blebs or irregular lamellae of sodic feldspar within potassic alkali feldspar.
  • Antiperthite: Potassic feldspar within a sodic feldspar.
  • Mesoperthite: Equal abundance of sodic and potassic feldspars.
  • Microperthite: Only observable with a microscope.

Exsolution Phenomenon

• Occurrence: Happens in minerals showing complete solid solution at higher temperatures.
• Process: Slow cooling of plutonic rocks allows alkali feldspar crystals to form stable structures at low temperatures.
• Contrast with Volcanic Rocks: Intra-crystalline textures in alkali feldspar are not seen in volcanic rocks.

Orthoclase - Albite System

• Components: Orthoclase (KAlSi₃O₈) and Albite (NaAlSi₃O₈).
• Conditions:
  • Dry/Low Water: Stability field of leucite (KAlSi₂O₆).
  • Water-Saturated: Stability field of leucite disappears, becomes binary between NaAlSi₃O₈ and KAlSi₃O₈.
• Solid Solution: Complete between NaAlSi₃O₈ and KAlSi₃O₈.

Temperature and Pressure Impact

• PH₂O = 200 MPa: Homogeneous alkali feldspar forms, exsolves into perthite or antiperthite upon cooling.
• PH₂O > 500 MPa: No homogeneous feldspars; separates into K-rich and Na-rich feldspar.
• Granitic Compositions: Characterized by hypersolvus or subsolvus granites based on feldspar populations.

Role of Solvus

• KAlSi₃O₈ - NaAlSi₃O₈ Solvus: Highly temperature-sensitive mutual solubility.
• High Temperatures (>700°C): Complete solid solution in sanidine-anorthoclase range.
• Slow Cooling: Leads to unmixing in perthite form.

Summary

• Subsolidus Exsolution: Central to perthite formation.
• Pressure and Water Presence: Significant in determining feldspar textures in granitic rocks.
• Granites: Differentiated as hypersolvus or subsolvus based on cooling and crystallization conditions.

Additional Notes

• Images: Various images of perthite unmixing in microcline and orthoclase.

References

• Cox et al. (1979), Howie, R. A., Zussman, J., & Deer, W. (1992), Le Maitre, R. W., et al. (2002), Middlemost, E. A. (1986), Shelley, D. (1993), Vernon, R. H. & Clarke, G. L. (2008).