This mineral is often called potassium feldspar or K-feldspar, because by definition potassium always exceeds sodium in its formula. It comes in three different crystal structures that depend on the temperature it formed at. Microcline is the stable form below about 400° C. Orthoclase and sanidine are stable above 500° C and 900° C, respectively. Being in a plutonic rock that cooled very slowly to yield these large mineral grains, it's safe to assume that this is microcline.
In the field, workers generally just write down "K-spar" and leave it at that until they can get to the laboratory. Alkali feldspar is generally white, buff or reddish and is not transparent, nor does it show the striations of plagioclase. A green feldspar is always microcline, the variety called amazonite.
Learn more about the geology of the feldspars
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