"Early partial melting" in annealed natural cordierites

Natural cordierite crystals with a range of Fe/Mg ratios have been annealed over a range of T between 1200 and 1430 degrees C (under controlled fO (sub 2) ), after previously being annealed at 900 degrees C to drive off any volatiles contained within their channels. TEM showed no evidence of an orthorhombic to hexagonal transformation, but all the heat-treated samples contained pervasive glassy needles [001], each including a small bubble. These became larger and less regular in shape with increase in annealing T, and are interpreted in terms of early partial melting of cordierite at T well below the macroscopic melting point. Larger glassy veins contain crystals of mullite, and in two samples annealed at approximately 1400 degrees C or below, pockets of glass with prismatic spinel crystals were seen. Chemical analysis showed that the glass phase was generally enriched in SiO (sub 2) and depleted in Al (sub 2) O (sub 3) with respect to the cordierite matrix. It is thought that these areas of more extensive melting probably occurred in regions of altered cordierite or around inclusions.