 There are two keys here, both of them in Adobe Acrobat pdf files. When you click on the link, if your computer has an Adobe Acrobat reader on it the reader will open automatically. If you do not have an Acrobat reader a window will open directing you to download the reader.  Composition/Texture Key Color/Texture Key
Igneous Home PageOn Keys and Classifications
It is not at all clear why, but minerals at the top of Bowen's Reaction Series tend to be dark in color (e.g. pyroxene and amphibole), and minerals at the bottom tend to be light in color (e.g. na plagioclase and quartz). After all, what could temperature of formation have to do with mineral color?And it is also not clear why, but magma compositions tend to segregate out, mafic magmas at the top of Bowen's Reaction Series, intermediate magmas in the middle, and felsic at the bottom. Result? Mafic magmas produce dark colored rocks made of dark minerals (such as basalt), intermediate magmas intermediate colored rocks (e.g. diorite) and felsic magmas light colored rocks (e.g. granite). But these fortuitous conditions lead to a first estimate, simple classification of igneous rocks based on texture (grain size) and rock color. Go to Color/Texture KeyAs a first approximation, a classification based on color and texture is ok, but can lead to great mistakes. For example, with phaneritic rocks where crystals can be seen, a felsic igneous rock could be made of orthoclase, or sodium plagioclase, or both, with or without quartz. These are dramatic differences in content, and geologically mean different things. And they are all given different names, but these three rocks are all " light colored" coarse grained rocks and in a color/texture classification are all " granite." So, ultimately a color/texture classification is inadequate. A mineral/texture classification captures the essential differences among igneous rocks much better, and allows us to better appreciate the great diversity that exists. But it does make identifying rocks more difficult. Not only must individual minerals must be identified, but their abundances estimated also, skills that are learned only with practice. As a result, this next key is more difficult to use. However, the classification used at this site is based on mineral content/texture. Go to Composition/Texture Key Go to Igneous Rock Classification ChartBut as you might imagine this is not the end of it. Other igneous rock classifications exist, such as ones based on the chemistry of the rocks - but this is enough for here  .However, for more on igneous rock classification go to the page below.
Strategies for Igneous Rock Classification
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