Occurrence of “monalbite” in nature: a TEM study

Natural “monalbite” (MA) has been observed on a submicron-scale in jadeite along with high albite (HA) and low albite (LA) in jadeite quartzite in the Dabie ultrahigh-pressure (UHP) metamorphic terrane, China. Transmission electron microscope (TEM) observation of inclusions in jadeite crystals has revealed mineral phases with C2/m and C1̄ structures. The cell parameters of the C2/m crystal structure correspond to those of MA, whereas the cell parameters of C1̄ structure are identical to those of HA and LA. The existence of the C2/m structure shows that the jadeite underwent partial change to MA on a submicron-scale during retrograde metamorphism. Albite transformation twinning of HA suggests the transformation of MA into HA. MA occurrence provides powerful evidence of high-temperature metamorphism during exhumation, probably above 930 °C. The preservation of MA may be due to the armouring by the jadeite and implies a very rapid cooling (quenching) during retrogression.