Chiastolite

The chiastolite variety of andalusite contains a cross of inclusions of quartz and graphite in sections through prismatic porphyroblasts parallel to (001). The cross is always accompanied by roughly 5 μm diameter rod-shaped quartz inclusions that formed by co-precipitation of andalusite and quartz during growth, a process similar to one described by Burton (1986) to explain garnet-quartz intergrowths in regional metamorphic rocks. The metamorphic fluid consisted of a mixture of H2O, CO2 and minor amounts of other gases produced by internal buffering by graphite, which was always present during crystal growth. Chiastolite porphyroblasts grew alongside sector zoned garnets like Burtonʹs during contact metamorphism of the Zhoukoudian aureole, Beijing, China. Crystal growth accompanied by deformation gave rise to curved quartz rods, sometimes alongside curved cracks that are not exactly parallel to cleavage. Individual porphyroblasts had multi-stage growth histories during deformation accompanied by removal of large volumes of Si in solution from the matrix. The resulting loss of volume and mechanical weakening of country rocks facilitated emplacement of the Zhoukoudian intrusive complex by ballooning.