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rock classification

From the chart, I would say that plagioclase feldspar is more abundant in coarse-grained igneous rocks than alkali feldspar, as it is found in all three listed, whereas alkali feldspar is only present in diorite and granite.

Apparently I am wrong, can anyone please explain to me why?

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You can't really generalize like that for all igneous rock ranges. The chart actually tells you what the type and (approximate) mineral composition of a rock are as functions of the chemical composition, specifically silica content. You should be looking at how the minerals are distributed as a function of silica content.

To understand this diagram fully, note the following:

  1. The horizontal axis is actually a continuous variable, typically the silica content you would obtain if you were to separate all the individual oxide components. This diagram seems to go roughly from 50% to 70% from left to right; the one quoted here which includes rocks found predominantly in the mantle goes down to 40%.

  2. The rocks are classified according to silica content and crystal or grain size; thus granite and rhyolite are similar in silica content and are regarded in the diagram as having similar mineral compositions; only the grain size is really different.

Thus the curved lines define how much of each mineral is typically in a rock having some level of silica content, which varies across the chart as indicated in point (1). When we have a high silica content more than about 65% (felsic rocks) in a granite or rhyolite, there can be more alkali feldspar than plagioclase feldspar (the alkali feldspar composition range is higher in silica). In contrast, a low-silica (mafic) rock won't have any alkali feldspar and even the more flexible plagioclase feldspar is limited; mafic rocks are higher in other minerals such as pyroxenes.

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Let me just add to the other answer:

Plagioclase feldspar is the most common mineral in the Earth's crust. You will find it in most rocks plutonic rocks, and it is also a very common mineral in volcanic rocks. You will find it in gabbros, diorites, andesites, most basalts, most granites, some rhyolites, all monzonites, some peridotites, and list goes on and on.

Alkali-feldspar isn't as common. The only common rocks which contain alkali-feldspar are most granites and their volcanic chemical equivalent, rhyolites. Other than that, you will find alkali-feldspar only is more rare rocks such as syenites and other weird rocks that you will probably never see or hear about.

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