  We present a survey of >4,000 star compositions from the Hypatia Catalog to
examine whether rocky exoplanets (i.e., those with rocky surfaces, dominated by
silicates) might be geologically similar to Earth, at least with respect to
composition and mineralogy. To do so, we explore the variety of reported
stellar compositions to then determine a possible range of exoplanetary mantle
mineralogies. We find that exoplanetary mantles will likely be dominated by
olivine and/or orthopyroxene, depending upon Fe partitioning during core
formation. Some exoplanets may be magnesiowustite- or quartz-saturated, and we
present a new classification scheme based on the weight $\%$ ratio
(FeO+MgO)/SiO$_{2}$, to differentiate rock types. But wholly exotic
mineralogies should be rare to absent. We find that half or more of the range
of exoplanet mantle mineralogy is controlled by core formation, which we model
using $\alpha_{Fe}$ = $Fe^{BSP}$/$Fe^{BP}$, where $Fe^{BSP}$ is Fe in a Bulk
Silicate Planet (bulk planet, minus core), on a cation weight $\%$ basis
(elemental weight proportions, absent anions) and $Fe^{BP}$ is the cation
weight $\%$ of Fe for a Bulk Planet. In our solar system, $\alpha_{Fe}$ varies
from 0 (Mercury) to about 0.54 (Mars). Remaining variations in exoplanet mantle
mineralogy result from non-trivial variations in star compositions. Our major
oxide analysis also verifies earlier, isotopic studies indicating that Earth is
non-solar (non-chondritic). We also find that major oxide estimates for Earth's
mantle appear to preclude a hidden component in the deep mantle that would
allow for a bulk solar/chondritic Earth. If it did exist, such a hidden
component must comprise at least 28% of the mass of the total mantle (to avoid
negative concentrations of some oxides) and would not look anything like the
sources of ocean island or mid-ocean ridge basalts.
