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 magnesiowüstite-or quartz-saturated, and we present a new classification scheme based on the weight % ratio (FeO+MgO)/SiO2, 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 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, Fe varies from 0 (Mercury) to about 0.54 (Mars)]. Remaining variations in exoplanet mantle mineralogy result from non-trivial variations in star compositions. But we also find that Earth is decidedly non-solar (non-chondritic); this is not a new result, but appears worth re-emphasizing, given that current discussions often still use carbonaceous or enstatite chondrites as models of bulk Earth. We conclude that such models are untenable, regardless of the close overlap of some isotope ratios between certain meteoritic and terrestrial (Earth-derived) samples. There is also the possibility that Earth contains a hidden component, that if added to known reservoirs would yield a solar/chondritic Earth. We test that idea using a mass balance of major oxides using known reservoirs, so that the sum of upper mantle, metallic core and crust, plus a hidden component, yield a solar bulk composition. Here, the fractions of crust and core are fixed and the hidden mantle component is some unknown fraction of the entire mantle, Fh (so if FDM is the fraction of depleted mantle, then Fh + FDM = 1). Such mass balance shows that if a hidden mantle component were to exist, it must comprise >28% of Earth's mantle, otherwise it would have negative major oxide abundances. There is no clear upper limit for such a component, so it could comprise the entire mantle, but all estimates from Fh = 0.28 to Fh = 1.0 yield a hidden fraction that does not match the sources of mantle plumes or midocean ridge basalt (MORB). The putative component is also geologically unusual, being enriched in Na and Fe, and depleted in Ca and Mg, compared to familiar mantle components. We conclude that such a hidden component does not exist.
