  This chapter begins with some basic concepts regarding the structure and
mineralogy of rocky planets, how to read and construct ternary diagrams, and
why partial melting occurs when plate tectonics is operative. Partial melting
is a key concept in that it governs crust and core formation, which in turn
control mineralogy. These sections are for astronomers, or geologists new to
the study of igneous petrology. From there, computational approaches for
estimating planetary mineral assemblages will be introduced. These quantitative
methods are simple, consonant with the level of information currently available
on exoplanet compositions, and while largely intended for mineralogists, should
be accessible to non-specialists as well. Such methods are followed by a study
of error when plotting mineral abundances in ternary diagrams, for
mineralogists and petrologists who construct such diagrams. The chapter
concludes with caveats, and the ways in which exoplanets might surprise us.
