  The two primary observable quantities of an exoplanet--its mass and
radius--alone are not sufficient to probe a rocky exoplanet's interior
composition and mineralogy. To overcome this, host-star abundances of the
primary planet-building elements (Mg, Si, Fe) are typically used as a proxy for
the planet's bulk composition. The majority of small exoplanet hosts, however,
do not have available abundance data. Here we present the open-source ExoPlex
mass-radius-composition solver. Unlike previous open-source mass-radius
solvers, ExoPlex calculates the core chemistry and equilibrium mantle
mineralogy for a bulk composition, including effects of mantle FeO content,
core light elements and surface water/ice. We utilize ExoPlex to calculate the
planetary radii, surface gravities and bulk densities for 10$^6$ model planets
up to 2 R$_\oplus$ across these geochemistries, adopting the distribution of
FGK stellar abundances to estimate of the range of bulk exoplanet compositions.
We outline the $99.7\%$ distribution of radii, surface gravity and bulk
densities that define planets as "nominally rocky." Planets outside this range
require compositions outside those expected from stellar abundance data, likely
making them either Fe-enriched super-Mercuries, or volatile-enriched
mini-Neptunes. We apply our classification scheme to a sample of 85
well-resolved exoplanets without available host-star abundances. We estimate
only 9 planets are within the "nominally rocky planet zone" at $>70\%$
confidence, while $\sim20\%$ and $\sim30\%$ of this sample can be reasonably
classified as super-Mercuries or volatile-rich, respectively. Our results
provide observers with a self-consistent way to broadly classify a planet as
likely rocky, Mercury-like or volatile-enriched, using mass and radius
measurements alone.
