  Propagation-based phase-contrast X-ray imaging (PB-PCXI) generates image
contrast by utilizing sample-imposed phase-shifts. This has proven useful when
imaging weakly-attenuating samples, as conventional attenuation-based imaging
does not always provide adequate contrast. We present a PB-PCXI algorithm
capable of extracting the X-ray attenuation, $\beta$, and refraction, $\delta$,
components of the complex refractive index of distinct materials within an
unknown sample. The method involves curve-fitting an error-function-based model
to a phase-retrieved interface in a PB-PCXI tomographic reconstruction, which
is obtained when Paganin-type phase-retrieval is applied with incorrect values
of $\delta$ and $\beta$. The fit parameters can then be used to calculate true
$\delta$ and $\beta$ values for composite materials. This approach requires no
a priori sample information, making it broadly applicable. Our PB-PCXI
reconstruction is single distance, requiring only one exposure per tomographic
angle, which is important for radiosensitive samples. We apply this approach to
a breast-tissue sample, recovering the refraction component, $\delta$, with 0.6
- 2.4\% accuracy compared to theoretical values.
