  Numerous contemporary investigations in condensed matter physics are devoted
to high temperature (high-$T_c$ ) cuprate superconductors. Despite its unique
effulgence among research subjects, the enigma of the high-$T_c$ mechanism
still persists. One way to advance its understanding is to discover and study
new analogous systems. Here we begin a novel exploration of the natural mineral
murunskite, K$_2$FeCu$_3$S$_4$, as an interpolation compound between cuprates
and ferropnictides, the only known high-$T_c$ superconductors at ambient
pressure. Because in-depth studies can be carried out only on single crystals,
we have mastered the synthesis and growth of high quality specimens. Similar to
the cuprate parent compounds, these show semiconducting behavior in resistivity
and optical transmittance, and an antiferromagnetic ordering at 100 K.
Spectroscopy (XPS) and calculations (DFT) concur that the sulfur 3$p$ orbitals
are partially open, making them accessible for charge manipulation, which is a
prerequisite for superconductivity in analogous layered structures. DFT
indicates that the valence band is more cuprate-like, while the conduction band
is more pnictide-like. With appropriate doping strategies, this parent compound
promises exciting future developments.
