In a nutshell the difference in density between Continental Crust and Oceanic Crust is derived from their different mineralogical compositions. Continental Crust is made up of alkali rich feldspar (K-spar) and quartz, whereas oceanic crust is made up of Calcium-rich feldspar (plagioclase) and pyroxene.

In terms of compositions Oceanic Crust is much richer in dense elements like iron, calcium , titanium, and magnesium, whereas the Continental Crust is made up of light elements like potassium (K) sodium (Na) and silicon (SI) plus Alumina A.

So, in a nutshell the Continental Crust has a typical density of 2600 kg per cubic meter (or 2.6 g/cc) whereas the rock making up Oceanic Crust is called gabbro, and gabbro has a density around 3000 kg/m3 (or 3 g/cc).