monzonite is one of three rocks dominated by feldspar (the others being anorthosite and syenite) with occasional other minerals thrown in in lesser amounts (although the rock can go 100% plagioclase.) The plagioclase is sodium rich, unlike anorthosite which is calcium rich.
     

Because monzonite is light colored it is often confused with granite but granite has quartz >20%. monzonites with quartz 5-20% are "quartz monzonites." Less than 5% quartz and the rock is just monzonite.

 

PLAGIOCLASE - 50-100%
orthoclase - 10-35%
Mafics - < 10%
quartz - 0-5%

 

monzonite is not a common rock, but is typically found around the edges of other felsic plutons, such as plagiogranites or granodiorites. That is, it is typical of continents.

 

 

 

 

  

 

This light colored rock might almost be confused for a granite, but it has less than 20% quartz, removing it from that classification. Even in this view pink orthoclase can be seen, but the most common mineral is plagioclase (which with quartz <20%) makes it a monzonite. Mafics are clearly rare.

 

  

anorthosite is a very unusual rock, not only because it is composed only of calcium plagiclase, but also because it is relatively rare, and forms by mechanisms we do not fully understand. 
     

anorthosite is common on the moon, however, composing much of the rugged upland areas (not the smooth mare.)

It is likely that when the earth formed anorthosite was one of the more common constituents, just because calcium is a common element in interstellar gas clouds, and planetary nebulae. 
     

This specimen was collected from a road cut that was about 50 feet high and a couple of hundred meters long, and it was all anorthosite just like this. In fact, this rock could be seen in the canyon walls for miles around. 
    

Typically when we find anorthosite it is in large masses covering hundreds of square miles at least.