Alkali Feldspar Granite

Granite is a medium-tocoarse-grained acid igneous rock with essential quartz (>20%) and feldspar, where alkali feldspar constitutes between 100 and 35% of the feldspars, and minor mafic minerals. Hornblende and biotite are the commonest mafic minerals, however, muscovite is also frequently encountered. Pyroxene, andalusite, corundum, tourmaline, garnet and topaz are also relatively common mafic minerals. Rhyolite is the fine-grained equivalent of granite.

Granites are subdivided on the basis of the relative proportion of alkali feldspar to plagioclase feldspar into monzogranites (65-35% alkali feldspar), syenogranites (90-65% alkali feldspar), and alkali granites (>90% alkali feldspar). Around 70% of granites are monzogranites. Aplites are equigranular granitic rocks with little mafic component occurring in veins or dykes. Granophyres are granitic rocks in which quartz and feldspar has a granophyric intergrowth.

Granites can be subdivided on the basis of their chemistry into peralkaline, metaluminous, and peraluminous on the basis of the ratio Al₂O₃/(Na₂O + K₂O + CaO). The related terminology is:

• peraluminous: Al₂O₃ > (Na₂O + K₂O + CaO) or Al₂O₃/(Na₂O + K₂O + CaO) > 1.1; Rock will probably have muscovite and may have garnet; will be corundum-normative.
• metaluminous: Al₂O₃ < (Na₂O + K₂O + CaO) but Al₂O₃ > (Na₂O + K₂O) or Al₂O₃/(Na₂O + K₂O + CaO) > 1.0; rock may have hornblende
• subaluminous: Al₂O₃ < (Na₂O + K₂O + CaO) but Al₂O₃ = (Na₂O + K₂O); Al₂O₃/(Na₂O + K₂O + CaO) < 1.0; rock will probably have hornblende; will have Di in the norm.
• peralkaline: Al₂O₃ < (Na₂O + K₂O) or Al₂O₃/(Na₂O + K₂O + CaO) << 1.0; rock will probably have lot of K-feldspar in the norm, probably a feldspathoid or very little, if any quartz.

But Granites can be subdivided also on the basis of the infered genesis into I-type, S-type, M-type and A-type. Chappell & White classification system was proposed initially to divide granites into I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite. Both of these types of granite are formed by melting of high grade metamorphic rocks, either other granite or intrusive mafic rocks, or buried sediment, respectively.
M-type or mantle derived granite was proposed later, to cover those granites which were clearly sourced from crystallized mafic magmas, generally sourced from the mantle. These are rare, because it is difficult to turn basalt into granite via fractional crystallisation.
A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiar mineralogy and geochemistry. These granites are formed by melting of the lower crust under conditions that are usually extremely dry. The rhyolites of the Yellowstone caldera are examples of volcanic equivalents of A-type granite. H-type or hybrid granites are formed following a mixing of two granitic magmas from different sources, e.g. M-type and S-type.

granite(alkalifeldspar)2019(2).jpg

Alkali Feldspar Granite. Alkali Feldspar (pink) and quartz (glassy-grey). From Wikipedia

Alkali Feldspar pegmatite. Alkali Feldspar (pink) and quartz (glassy-grey). From Sand Atlas

Alkali Feldspar Granite. Åland island, Finland. From Hildegard Wilske

Alkali Feldspar Granite. Åland island, Finland. From Hildegard Wilske

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.

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Quartz and microcline. XPL image, 2x (Field of view = 7mm)	Quartz and microcline. XPL image, 2x (Field of view = 7mm)	QQuartz and microcline. XPL image, 2x (Field of view = 7mm)
quartz and microcline. XPL image, 2x (Field of view = 7mm)	Quartz and microcline. XPL image, 2x (Field of view = 7mm)	Orthose with perthite unmixing. XPL image, 2x (Field of view = 7mm)
Orthose with perthite unmixing. XPL image, 2x (Field of view = 7mm)	Orthose with perthite unmixing and pyroxene inclusions. XPL image, 2x (Field of view = 7mm)	Orthose with perthite unmixing. XPL image, 2x (Field of view = 7mm)
Quartz and microcline. XPL image, 2x (Field of view = 7mm)	Quartz and microcline. XPL image, 2x (Field of view = 7mm)	Biotite, quartz and microcline. XPL image, 2x (Field of view = 7mm)
Biotite, quartz and microcline. XPL image, 2x (Field of view = 7mm)	Biotite, quartz and microcline. XPL image, 2x (Field of view = 7mm)	Biotite, quartz and microcline. XPL image, 2x (Field of view = 7mm)
Quartz and microcline. XPL image, 2x (Field of view = 7mm)	Biotite, quartz and microcline. XPL image, 2x (Field of view = 7mm)	Quartz and microcline. XPL image, 2x (Field of view = 7mm)
Biotite, quartz and microcline. XPL image, 2x (Field of view = 7mm)	Quartz and microcline. XPL image, 2x (Field of view = 7mm)	Quartz and microcline. XPL image, 2x (Field of view = 7mm)
Sericitized orthose and aegirine (Green). PPL image, 2x (Field of view = 7mm)	Sericitized orthose and aegirine. XPL image, 2x (Field of view = 7mm)	Sericitized orthose and aegirine (Green). PPL image, 2x (Field of view = 7mm)
Sericitized orthose and aegirine. XPL image, 2x (Field of view = 7mm)	Sericitized orthose and aegirine. XPL image, 2x (Field of view = 7mm)	Sericitized orthose and aegirine (Green). PPL image, 2x (Field of view = 7mm)
Sericitized orthose and aegirine. XPL image, 2x (Field of view = 7mm)	Sericitized orthose and aegirine (Green). PPL image, 2x (Field of view = 7mm)