MULTI-PHASE, CRYSTALLOGRAPHICALLY ORIENTED INCLUSIONS IN RED WILLEMITE FROM FRANKLIN, NEW JERSEY

The color of red willemite (Zn₂SiO₄) from Franklin, New Jersey, has been attributed in the past to inclusions of franklinite. We have examined these inclusions, which are rod-shaped and 5–20μm in diameter, and found them to contain multiple phases. Polarized light microscopy shows the rods to be oriented uniformly parallel to the willemite c axis, and to have hexagonal or pseudohexagonal cross sections. Phases identified by standardless EDS analysis and electron back-scatter diffraction include (with approximate compositions) franklinite, (Fe_1.86Zn_0.75Mn_0.35Mg_0.02)O₄; friedelite, (Mn_6.48Zn_0.90Mg_0.21Fe_0.15)(Si_5.94Al_0.06)O₁₅(OH_9.53Cl_0.48); zincian kutnahorite, (Ca_0.46Mn_1.42Zn_0.11)(CO₃)₂; chlorite, (Mn_3.41Mg_1.31Zn_0.72Fe_0.66)Si_3.95O₁₀(OH)₈; and rhodonite, (Mn_0.87Zn_0.12Fe_0.01)SiO₃. Additionally, a K-bearing phase is as yet unidentified. Possible explanations for the origin of the rods include exsolution and primary fluid inclusions. Although rod-like exsolution is not unknown, and it is possible that an original exsolved phase might have been altered to the present assemblage, some of the elements in that assemblage (e.g., Ca, Cl, C) are incompatible with the willemite structure at any temperature. Moreover, unlike the inclusions, the host willemite is Fe-free. Origin as fluid inclusions is supported by the shapes of the rods (i.e., negative crystals with hexagonal cross sections), the presence of phases containing OH, Cl, and CO₃, and void space visible around solid phases. We suggest that willemite-generating metasomatic fluids were preserved as inclusions rich in elements incompatible with willemite, and that subsequent conditions led to their crystallization, possibly by reaction with the host. Reaction ceased with complete consumption of the fluid, resulting in relict fluid inclusions that impart color on the macro scale.