High-pressure minerals and new lunar mineral changesite-(Y)in Chang'e-5 regolithOA北大核心

Forty-five years after the Apollo and Luna missions,China's Chang'e-5(CE-5)mission collected～1.73 kg of new lunar materials from one of the youngest basalt units on the Moon.The CE-5 lunar samples provide opportunities to address some key scientific questions related to the Moon,including the discovery of high-pressure silica polymorphs(seifertite and stishovite)and a new lunar mineral,changesite-(Y).Seifertite was found to be coexist with stishovite in a silica fragment from CE-5 lunar regolith.This is the first confirmed seifertite in returned lunar samples.Seifertite has two space group symmetries(Pnc2 and Pbcn)and formed from an α-cristobalite-like phase during"cold"compression during a shock event.The aftershock heating process changes some seifertite to stishovite.Thus,this silica fragment records different stages of an impact process,and the peak shock pressure is estimated to be～11 to 40 GPa,which is much lower than the pressure condition for coexistence of seifertite and stishovite on the phase diagram.Changesite-(Y),with ideal formula(Ca8Y)□Fe2+(PO4)7(where □ denotes a vacancy)is the first new lunar mineral to be discovered in CE-5 regolith samples.This newly identified phosphate mineral is in the form of columnar crystals and was found in CE-5 basalt fragments.It contains high concentrations of Y and rare earth elements(REE),reaching up to～14 wt.％(Y,REE)2O3.The occurrence of changesite-(Y)marks the late-stage fractional crystallization processes of CE-5 basalts combined with silicate liquid immiscibility.These new findings demonstrate the significance of studies on high-pressure minerals in lunar materials and the special nature of lunar magmatic evolution.