The origin of chloritoid – 3‐mica pseudomorph growth in staurolite–muscovite schist,Bangriposi (Eastern India)

At Bangriposi, variable stages in replacement of staurolite by chloritoid – Na–K–Ca mica shimmer aggregates in muscovite schists provides insight into the complex interplay between fluid flow, mass transfer, and dissolution–precipitation during pseudomorph growth. Idioblastic chloritoid growing into mica caps without causing visible deformation, and monomineralic chloritoid veins (up to 300 μm wide) within shimmer aggregates replacing staurolite attest to chloritoid nucleation in fluid‐filled conduits along staurolite grain boundaries and crystallographic planes. The growth of shimmer aggregates initiated along staurolite margins, and advanced inwards into decomposing staurolite along networks of crystallographically controlled fluid‐filled conduits. Coalescence among alteration zones adjacent to channel fills led to dismemberment and the eventual demise of staurolite. Mass balance calculation within a volume‐fixed, silica‐conserved reference frame indicate the shimmer aggregates grew via precipitation from fluids in response to mass transport that led to the addition of H₂O, K₂O, Na₂O and CaO in the reaction zone, and Al₂O₃ was transported outward from the inward‐retreating margin of decomposing staurolite. This aided precipitation of chloritoid in veins and in the outer collars, and as disseminated grains in the shimmer aggregates at mid‐crustal condition (~520 ± 20 °C, 5.5 ± 2.0 kbar). Computation using one‐dimensional transport equation suggests that staurolite decomposition involved advection dominating over diffusive transport; the permeation of externally derived H₂O caused flattening of chemical potential gradients in H₂O and aqueous species, for example, and , computed using the Gibbs method. This suggests that staurolite decomposition was promoted by the infiltration of a large volume of H₂O that flattened existing chemical potential gradients. In the initial stages of replacement, chloritoid super‐saturation in fluid caused preferential nucleation and growth of chloritoid at staurolite grain boundaries and in crystallographic planes. As reaction progressed, further chloritoid nucleation was halted, but chloritoid continued to grow as the 3‐mica aggregates continued to replace the remaining staurolite in situ, while the chloritoid‐compatible elements were transported in the water‐rich phase facilitating continued growth of the existing chloritoid grains.