Skip to content

Geology is the Way

Alkali feldspar syenite

An alkali feldspar syenite represent an extreme composition of syenite, plotting on the A vertex of the QAPF classification. This plutonic rock contains almost exclusively alkali feldspar as the only felsic mineral present. Some plagioclase may be present but typically less than 10% of the total feldspars and with a Na-rich composition. Mafic minerals generally constitute less than 25% of the rock and are represented by hornblende or, less commonly, biotite. Peralkaline varieties of alkali feldspar syenite may contain alkali-bearing amphiboles and pyroxenes, such as aegirine and arfvedsonite. Alkali feldspar syenites may be saturated or undersaturated in silica, hence contain either quartz or feldspathoids. Alkali feldspar syenites containing between 5% and 20% quartz are classified as quartz alkali feldspar syenites. Conversely, if they contain feldspathoids up to 10% they are classified as foid-bearing alkali feldspar syenites, substituting ‘foid’ with the name of the most abundant feldspathoid (e.g. leucite-bearing alkali feldspar syenite). Agpaite is a peralkaline variety of nepheline-bearing alkali feldspar syenite containing complex Zr- and Ti-bearing minerals, such as eudialyte, rather than common minerals like zircon and ilmenite.

alkali feldspar syenite

Alkali feldspar syenite (also known as umptekite) from Sweden. The pale red grains are alkali feldspar crystals. Width of sample 12 cm. © Siim Sepp.

alkali feldspar syenite

The red of alkali feldspar dominates the texture of this quartz alkali feldspar syenite from Estonia. Width of sample 8 cm. © Siim Sepp.

Alkali feldspar syenite
Plutonic igneous rock
Felsic minerals:
alkali feldspar
quartz
feldspathoids
Mafic minerals:
hornblende
pyroxene

QAPF classification:
Q = 0 – 20% or F < 10%
Plagioclase/feldspars < 10%
Colored varieties:
• alkali feldspar melasyenite (M > 25%)
Other varieties: agpaite
Extrusive equivalent: alkali feldspar trachyte

agpaite

Agpaite with pink-red eudialyte, greenish-black aegirine and grey-whitish albite and nepheline. Khibina Massif, Kola Peninsula, Russia. Width of sample 8 cm. © James St. John.

References

        

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

Do you like this page?
en_USEnglish