Skip to content

Geology is the Way

Alkali feldspar granite

Alkali feldspar granites are the alkali feldspar-rich end.member of granites. They are felsic plutonic igneous rocks consisting essentially of quartz (20-60% of the felsic minerals) and alkali feldspar (> 90% of  feldspars). Due to the high K-feldspar content, this rock tends to display intense red to pink hues. Plagioclase, when present, occurs in small amounts and with Na-rich composition. The typical color index (M) of an alkali feldspar granite ranges between 0 and 20 %. Biotite and hornblende representing the commonest mafic minerals. If M < 10% the rock can be named alaskite (i.e. a leucocratic variety of alkali feldspar granite). Some alkali feldspar granites may be so rich in alkaline elements (Na and K) to contain Na-amphibole (richterite) and/or Na-pyroxene (aegirine): in this case, they should be classified as peralkaline granites.

alkali feldspar granite

Alkali feldspar granite with large amounts of K-feldspar (pink-reddish), quartz (vitreous, transparent), biotite, and hornblende (black). Åland archipelago, Finland. Photo by Hildegard Wilske.

Dakota Mahogany Granite

Dakota Mahogany Granite – a porphyric alkali feldspar granite with K-feldspar (pink), quartz (grey, transparent) and mafic minerals (black). Milbank Granite (late Neoarchean). Milbank, Grant County, South Dakota, USA. Photo by James St. John.

Alkali feldspar granite
Plutonic igneous rock
Felsic minerals:
quartz
alkali feldspar
• sodic plagioclase
Mafic minerals:
biotite
hornblende

QAPF classification:
Q = 20 – 60%
Plagioclase/feldspars = 0 – 10%
Colored varieties:
• alaskite (M < 10%)
• alkali feldspar melagranite (M > 20%)
Other varieties:
peralkaline granite
Extrusive equivalent: alkali feldspar rhyolite

alkali feldspar granite

Alkali feldspar granite with red K-feldspar and transparent quartz. Very few mafic minerals are visible. Åland archipelago, Finland. Photo by Hildegard Wilske.

alkali feldspar granite

Alkali feldspar granite with dark grey, transparent quartz and reddish K-feldspar. Åland archipelago, Finland. Photo by Hildegard Wilske.

alkali feldspar granite

Alkali feldspar granite. Åland archipelago, Finland. Photo by Hildegard Wilske.

alaskite

Pegmatitic alaskite. Note the lack of mafic minerals. 7.8 cm across at its widest. Mount Evans Batholith, Mesoproterozoic, central Colorado, USA. Photo by James St. John.

References
Barbey, P., Faure, F., Paquette, J. L., Pistre, K., Delangle, C., & Gremilliet, J. P. (2019). Skeletal quartz and dendritic biotite: Witnesses of primary disequilibrium growth textures in an alkali-feldspar granite. Lithos348, 105202.
Kwon, S. T., Shin, K. B., Park, H. K., & Mertzman, S. A. (1995). Geochemistry of the Kwanaksan alkali feldspar granite: A-type granite. Jour. Petrol. Soc. Korea4(1), 31-48.
Lan, T. G., Fan, H. R., Yang, K. F., Cai, Y. C., Wen, B. J., & Zhang, W. (2015). Geochronology, mineralogy and geochemistry of alkali-feldspar granite and albite granite association from the Changyi area of Jiao-Liao-Ji Belt: Implications for Paleoproterozoic rifting of eastern North China Craton. Precambrian Research266, 86-107.
Lee, M. R., Waldron, K. A., & Parsons, I. (1995). Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite. Mineralogical Magazine59(394), 63-78.
Mackenzie, D. E., Black, L. P., & Sun, S. S. (1988). Origin of alkali-feldspar granites: An example from the Poimena Granite, northeastern Tasmania, Australia. Geochimica et Cosmochimica Acta52(10), 2507-2524.

        

See also
Alkali feldspar granite – Alexstrekeisen.it
Peralkaline granite – Alexstreisen.it.

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

Do you like this page?
en_USEnglish