  X-ray spectra in the range $1.5-8.5$~keV have been analyzed for 526 large
flares detected with the Solar Assembly for X-rays (SAX) on the Mercury {\em
MESSENGER} spacecraft between 2007 and 2013. For each flare, the temperature
and emission measure of the emitting plasma were determined from the spectrum
of the continuum. In addition, with the SAX energy resolution of 0.6 keV (FWHM)
at 6~keV, the intensities of the clearly resolved Fe-line complex at 6.7~keV
and the Ca-line complex at 3.9~keV were determined, along with those of
unresolved line complexes from S, Si, and Ar at lower energies. Comparisons of
these line intensities with theoretical spectra allow the abundances of these
elements relative to hydrogen to be derived, with uncertainties due to
instrument calibration and the unknown temperature distribution of the emitting
plasma. While significant deviations are found for the abundances of Fe and Ca
from flare to flare, the abundances averaged over all flares are found to be
enhanced over photospheric values by factors of $1.66 \pm 0.34$ (Fe),
$3.89~\pm~0.76$ (Ca), $1.23~\pm~0.45$ (S), $1.64~\pm~0.66$ (Si), and
$2.48~\pm~0.90$ (Ar). These factors differ from previous reported values for Fe
and Si at least. They suggest a more complex relation of abundance enhancement
with the first ionization potential (FIP) of the element than previously
considered, with the possibility that fractionation occurs in flares for
elements with a FIP of less than $\sim$7~eV rather than $\sim10$~eV.

Solar and Stellar Astrophysics