Natural NaAlSi3O8-hollandite in the shocked Sixiangkou meteorite
The hollandite high-pressure polymorph of plagioclase has been identified in 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 tetragonal 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-pyrope solid solution plus magnesio-wustite sets constraints on the peak pressures that prevailed in the shock-induced melt veins. The absence of a calcium ferrite-structured phase sets an upper bound for the crystallization of the hollandite polymorph near 23 gigapascals.