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