(\mathrm{CaPtSi}), midway between the (\mathrm{Zintl}) phase (\mathrm{CaSi}{2}) and the Laves phase (\mathrm{CaPt}{2}), crystallizes at normal pressure in the LaIrSi-type structure, a derivative of the (\mathrm{SrSi}{2})-type structure. Two quenchable polymorphs have been obtained from this phase in a belt apparatus at high pressures ( (3-4 \mathrm{GPa}) ) and high temperatures (\left(800-1100^{\circ} \mathrm{C}\right)). The first high pressure polymorph (EuNiGe-type structure, (m P 12, P 2{1} /) (\mathrm{n}, a=588.9(1), b=581.0(1), c=724.3(1) \mathrm{pm}, \beta=109.25(1)^{\circ}). Rietveld technique) is (7.3 %), the second (TiNiSi-type structure, (o P 12), Pnma, (a=711.5(1), b=433.6(1), c=723.2(1)), single crystal investigation) (11.6 %) more densely packed than the normal pressure phase. The denser packing is correlated with increases of coordination, bond distances. and metallic properties. The transformation from the LalrSi- into the TiNiSi-type structure results in a change from a three-dimensional threeconnected (Pt-Si: (230 \mathrm{pm}) ) into a three-dimensional four-connected net (averaged Pt-Si: (254 \mathrm{pm}) ). The phases of trimorphic CaPtSi show metallic behavior of the electrical resistivity and their magnetic susceptibility extends from diamagnetic (LaIrSi-type structure) to weakly paramagnetic (TiNiSi-type structure). A possible structural path from Zintl to Laves phases is discussed. (1993 Academic Press, Inc.