Geology Reference
In-Depth Information
Plagioclase
10-65
0-10
65-90
90-100
percent total feldspar
0
Anorth-
osite
Alkali
10
Syenite
20
Syenite
30
Diorite
40
50
60
70
80
90
100
Figure 3.13
Plot of increasing content of plagioclase feldspar (left to right), as
a percentage of total feldspar, against mafic mineral content for common rocks
with less than 20% of the salic minerals as quartz. The difference between diorite
and gabbro is discussed in the text and the names used here are defined more
precisely in later chapters.
20 - 60% quartz may, provisionally, be termed 'granitoid'. Figure 3.13 illustrates
the usual range of mafic minerals present in granitoid rocks and the areas in
which the prefixes leuco- and mela-would normally be applied. Similarly, the
feldspars of rocks with lesser amounts of quartz, or no quartz and small amounts
