Within the Ca-Al-silicate system, dense, layered hexagonal phases occur at high temperatures and pressures between 20 and 23 GPa. They have been observed both in nature and in experiments. In this study, we describe the endmember with a dominant sixfold coordinated Si as a mineral zagamiite (IMA 2015-022a). This new mineral identified in Martian meteorites has a general formula of (Ca,Na)(Al,Fe,Mg)sub.2(Si,Al,□)sub.4Osub.11, thus defining CaAlsub.2Sisub.3.5Osub.11 as a previously unknown endmember of the hexagonal CAS phases. Zagamiite assumes space group P6sub.3/mmc with a unit cell of a = 5.403(2) Å, c = 12.77(3) Å, V = 322.9(11) Åsup.3, and Z = 2. Zagamiite contains significant Fe and Mg and a substantial deficit of Na relative to plagioclase of an equivalent Al/Si, suggesting that it was formed through crystallization from a melt that was derived from a plagioclase-dominant mixture of plagioclase and clinopyroxene above the solidus beyond 20 GPa.