Final answer:
The rock described with a glassy, vesicular texture and composed of potassium feldspar and quartz does not have a clear match in common igneous rock classifications. A typical igneous rock with such mineral content would have a phaneritic texture, such as granite, or an aphanitic texture, like rhyolite, not a glassy, vesicular one. More information is needed for accurate identification.
Step-by-step explanation:
A rock with a glassy, vesicular texture composed mostly of potassium feldspar and quartz might be initially suggestive of an igneous rock due to the presence of these minerals. However, the texture given is not typical for a rock composed principally of quartz and potassium feldspar, which are typically found in plutonic rocks like granite with a phaneritic texture (coarse-grained). The description might be pointing towards a specific variety of igneous rock but does not align well with the common textures associated with granite or rhyolite, the latter of which tends to have a finer, aphanitic texture. Additionally, the glassy texture description is more akin to volcanic glass, such as obsidian.
Given the information provided, there is no igneous rock that exactly fits the description of being glassy and vesicular with a composition primarily of potassium feldspar and quartz. Pumice is an example of a vesicular and often glassy igneous rock, however, it is usually composed of volcanic glass and is less dense than the rock described. In light of this, the student should re-evaluate the rock's texture and composition for accurate identification or provide additional information for a clearer diagnosis.