Seifertite is a dense orthorhombic polymorph of silica with the scrutinyite (α-PbO2) type structure that was found as lamellae occurring together with dense silica glass lamellae in composite silica grains in the heavily shocked Martian meteorite Shergotty. The mineral is also intergrown in some grains with minor stishovite and a new unnamed monoclinic dense silica polymorph with a ZrO2-type structure. Seifertite has also been found in the Martian shergottite Zagami and is a minor constituent in other Martian shergottites. Chemical analyses of seifertite in Shergotty indicate major SiO2 with minor concentrations of Al2O3 and Na2O. Selected-area electron diffraction (SAED) and X-ray diffraction can be interpreted in terms of an orthorhombic pattern from a scrutinyite (α-PbO2) structure. The cell parameters are a = 4.097(1) Å, b = 5.0462(9) Å, c = 4.4946(8) Å, V = 92.92 Å3, Z = 4, and the space group is Pbcn or Pb2n. Density is (calc.) = 4.294 g/cm3 (with pure SiO2), 4.309 g/cm3 (with empirical formula). It is inferred that seifertite was formed by shock-induced solid-state transformation of either tridymite or cristobalite on Mars at an estimated minimum equilibrium shock pressure in excess of 35 GPa. The new mineral is named after Friedrich A. Seifert (b. 1941), founding Director of the Bayerisches Geoinstitut, Universität Bayreuth, Germany, for his seminal contributions to high-pressure geoscience.