It is a pentagonal prism
Removing all the face-centered atoms along one of the axes in a NaCl crystal removes 1/4 of the original atoms, leaving behind only the atoms at the corners. Since each corner atom is shared between 8 unit cells, the resultant stoichiometry is A:8B.
That's the principle behind RADAR (Radio Detection And Ranging). A brief, high-powerpulse of radio is transmitted, then the transmitter goes silent, and the receiver listens forthe small bit of signal reflected back from any solid object. The size and distance of thesolid object are calculated from the strength and time-lag of the reflected signal.