Fundamental Minerals

Sialic Minerals

Feldspars

Feldspathoids

Quartz

Femic Minerals

Spodumene

Accessory Minerals

Rare Accessory

Alteration Products

Microstructures

Rocks

Jacupiranga complex

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.

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.

Orthose
Orthose altered by sericite. PPL image, 2x (Field of view = 7mm)	Orthose with carlsbad twinning. XPL image, 2x (Field of view = 7mm)	Perthite unmixing in orthose. XPL image, 10x (Field of view = 2mm)
Perthite unmixing in orthose. XPL image, 10x (Field of view = 2mm)	Perthite unmixing in orthose. PPL image, 10x (Field of view = 2mm)	Perthite unmixing in orthose. XPL image, 10x (Field of view = 2mm)
Perthite unmixing in orthose. XPL image, 10x (Field of view = 2mm)	Perthite unmixing in orthose. PPL image, 10x (Field of view = 2mm)	Perthite unmixing in orthose. XPL image, 10x (Field of view = 2mm)

Microcline
perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)
perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)
Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)	Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)	Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)
Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)	Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)
perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)
Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)	Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)	Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)
Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)	perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)
perthite unmixing in microcline. XPL image, 10x (Field of view = 2mm)	Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)	Cross-hatched twinning in microcline. XPL image, 10x (Field of view = 2mm)

Adularia
Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)	Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)	Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)	Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)	Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)