Alkali Fesldspar - KAlSi₃O₈

Unlike plagioclase feldspars, alkali feldspars are divided into separate minerals not only based on their chemistry, but more importantly, based on optical properties determined by a petrographic microscope. Most alkali feldspars are compositionally closer to Or end-member which explains why we often refer to them as K-feldspars (potassium feldspars).

Microcline is a triclinic K-feldspar and has a characteristic cross-hatched tartan pattern of twinning seen with a petrographic microscope.

Orthoclase and sanidine are both optically monoclinic K-feldspars. Sanidine occurs in volcanic and subvolcanic rocks. Orthoclase occurs in plutonic rocks.

Adularia is a K-feldspar produced by low-temperature hydrothermal (veins) or authigenic processes. It is structurally similar to microcline but without the cross-hatched twinning.

Anorthoclase is the only alkali feldspar that is not K-feldspar. It is sodic in composition, optically triclinic and characterized by a twinning similar to microcline but on a smaller scale.

The structural state which is the basis of the alkali feldspar classification, is dependent on the temperature of crystallization and subsequent cooling history. Volcanic feldspars tend to retain their high temperature ordering because the cooling is very rapid (sanidine). Low-temperature feldspars either crystallized at lower temperatures (adularia) or had ample amount of time to cool down (microcline).

Fig.1: Feldspar classification. This diagram shows how feldspar minerals are classified on the basis of their chemical composition. The sequence of minerals along the base of the triangle represents the solid solution series of plagioclase between albite and anorthite.

Optical Properties:
• Relief: low
• Color: colorless
• Interference colors: 1° gray to 1° white interference colors
• Form: Sanidine crystals are commonly euhedral in phenocrysts, either essentially square in section or elongated parallel to a axis.
• twinning: Only Carlsbad twinning is common in Sanidine, bisecting phenocrysts or microlites parallel to either length. Anorthoclase is distinguished by combined albite and pericline twinning, which form a diffuse grid, or quadrille pattern, like in Microcline.

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Sanidine
Sanidine crystals in a Trachyte. PPL image , 2x (Field of view = 7mm)	Sanidine crystal in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystal in a Trachyte. XPL image , 2x (Field of view = 7mm)
Sanidine crystal in a Trachyte. PPL image , 2x (Field of view = 7mm)	Sanidine and Aegirine crystals in a Trachyte. PPL image , 2x (Field of view = 7mm)	Sanidine crystal in a Trachyte. XPL image , 2x (Field of view = 7mm)
Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystal in a Trachyte. PPL image , 2x (Field of view = 7mm)
Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystals in a Trachyte. PPL image , 2x (Field of view = 7mm)
Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
Sanidine crystals in a Trachyte. PPL image , 2x (Field of view = 7mm)	Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
Sanidine crystal in a Trachyte. XPL image , 2x (Field of view = 7mm)	Sanidine crystal in a Trachyte. XPL image , 10x (Field of view = 2mm)

Anorthoclase
Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 10x (Field of view = 2mm)	Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 10x (Field of view = 2mm)	Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 10x (Field of view = 2mm)
Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)	Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)	Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)
Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)	Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)