Natural NaAlSi3O8-hollandite in the shocked Sixiangkou meteorite

The hollandite high-pressure polymorph of plagioclase has been identified i n shock-induced melt veins of the Sixiangkou 16 chondrite. It is intimately intergrown with feldspathic glass within grains previously thought to be " maskelynite." The crystallographic nature of the mineral was established by Laser micro-Raman spectroscopy and x-ray diffraction. The mineral is tetra gonal with the unit cell parameters a = 9.263 +/- 0.003 angstroms and c = 2 .706 +/- 0.003 angstroms. its occurrence with the Liquidus pair majorite-py rope solid solution plus magnesio-wustite sets constraints on the peak pres sures that prevailed in the shock-induced melt veins. The absence of a calc ium ferrite-structured phase sets an upper bound for the crystallization of the hollandite polymorph near 23 gigapascals.