Hornblende Anorthoclase Syenite
Syenite is a felsic intrusive (coarse grained) rock that has very little to no quartz and in which the majority of the feldspar is alkali (potassium or Na rich or a mixture) as opposed to plagioclase. This sample (fig 1) from Wausau WI is distinguished by having the majority of the minerals being alkali feldspar, likely Anorthoclase, a mineral in between pure potassium feldspar and Na-rich feldspar ((Na,K)AlSi3O8). The feldspar shows classic nearly right angle cleavage and polysynthetic twinning characteristic of alkali feldspars (upper half of the photo) and euhedral hornblende, a Mg/Fe rich mineral, in the lower half on the right (fig 2). Figure 3 shows a closeup of the Hornblende taken with raking (from the left side, shallow angle) light, showing the typical Hornblende dark prismatic crystals and 60/120 degree cleavage. Figure 4 is a reflected light photomicrograph of a polished surface showing a large anorthoclase crystal. It shows a grain boundary zone between anorthoclase (left, blue-gray, polysynthetic twinning, schiller) and probable K-feldspar/perthitic orthoclase (right, warm tan-brown, no schiller, coarser banding).
The key finding is that there is no visible quartz. Only thin section could confirm that quartz is truly zero, but on inspection, macro lens photos, and steromicroscopy, there is none. That and the dominance of alkali feldspar over plagioclase feldspar is what makes this felsic igneous rock a syenite.