  We use above-bandgap optical excitation, via a 1047 nm laser, to
hyperpolarize the $^{31}$P spins in low-doped (N$_D =6\times10^{15}$ cm$^{-3}$)
natural abundance silicon at 4.2 K and 6.7 T, and inductively detect the
resulting NMR signal. The $30$ kHz spectral linewidth observed is dramatically
larger than the 600 Hz linewidth observed from a $^{28}$Si-enriched silicon
crystal. We show that the observed broadening is consistent with previous ENDOR
results showing discrete isotope mass effect contributions to the donor
hyperfine coupling. A secondary source of broadening is likely due to
variations in the local strain, induced by the random distribution of different
isotopes in natural silicon. The nuclear spin T$_1$ and the build-up time for
the optically-induced $^{31}$P hyperpolarization in the natural abundance
silicon sample were observed to be $178\pm47$ s and $69\pm6$ s respectively,
significantly shorter than the values previously measured in $^{28}$Si-enriched
samples under the same conditions. We also measured the T$_1$ and
hyperpolarization build-up time for the $^{31}$P signal in natural abundance
silicon at 9.4 T to be $54\pm31$ s and $13\pm2$ s respectively. The shorter
build-up and nuclear spin T$_1$ times at high field are likely due to the
shorter electron-spin T$_1$, which drives nuclear spin relaxation via
non-secular hyperfine interactions. At 6.7 T, the phosphorus nuclear spin
T$_{2}$ was measured to be $16.7\pm1.6$ ms at 4.2 K, a factor of 4 shorter than
in $^{28}$Si-enriched crystals. This was observed to further shorten to
$1.9\pm0.4$ ms in the presence of the infra-red laser.
