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Why is feldspar important?
The feldspars are a family of silicate minerals which occur in igneous rocks. There are many different members to the feldspar group. Obviously, silicon and oxygen form the foundation for the group, but calcium, sodium, and potassium are also present. One of these elements is usually dominant, but most of the feldspars contain all 3 in varying amounts. It is the proportions of these 3 elements which help determine which specific feldspar is formed. The feldspars are divided into 2 broad categories: plagioclase, which contains calcium and sodium; and orthoclase, which contains potassium. The plagioclase feldspars represent the "continuous branch" of Bowen's Reaction Series, and form a complete series between anorthite (the pure calcium member), and albite (the sodium-rich variety).
I once saw an estimate that said that 60% of the earth's crust is composed of feldspar. This is quite a number, and since feldspar is nearly always present in igneous rocks, most classification schemes depend on the amount and kind of feldspar. In general terms, mafic and intermediate rocks contain plagioclase, with the more calcium present, the more mafic the resulting rock. Orthoclase occurs only in the felsic igneous rocks.
So if we're faced with the need to name an igneous rock, it is important to be able to distinguish between plagioclase and orthoclase. This is obviously harder than it should be, and a final determination is often impossible without chemical and/or optical studies in a lab. We can, however, make an educated guess in the field based on several assumptions, and a few easily identifiable physical features - color and the presence (or absence) of "striations." Striations are often visible on plagioclase, and resemble very fine (almost microscopic) parallel lines cut into the face of a mineral fragment. All feldspars which have striations are plagioclase, but not all plagioclase has striations. Only orthoclase can be pink or blue, and only plagioclase can be dark gray to black.
Unfortunately, both can be light in color, and since most feldspars are nearly white we're often still faced with a nasty identification problem. If it's white and has striations, I call it plagioclase. If it's white but I can't see any striations, it may be either plagioclase or orthoclase. In this case, I just call it feldspar and look for other clues to come up with a name for the rock. The following graphic summarizes the field identification of feldspar:
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