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CRYSTALLOGRAPHICALLY ORIENTED POLYPHASE RODS IN RED WILLEMITE FROM FRANKLIN, NEW JERSEY

Department of Geological Sciences, Brigham Young University, Provo, Utah 84602, U.S.A.

E-mail address: dana_griffen{at}byu.edu

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 5–20 µm in diameter, and found them to contain multiple phases. Polarized-light microscopy shows the rods to be oriented uniformly parallel to the c axis of willemite, and to have hexagonal or pseudohexagonal cross-sections. Phases identified by standardless EDS analysis and electron back-scatter diffraction include franklinite, friedelite, zincian kutnohorite, serpentine and rhodonite, as well as a sparse, unidentified K-bearing phase. Possible explanations for the origin of the rods include exsolution, primary fluid inclusions, and epitactic growth contemporaneous with willemite crystallization. Although rod-like exsolution is known in some geological settings, and an original exsolved phase might have been altered to the present assemblage, some of the elements in that assemblage (e.g., H, C, Cl, K, and Ca) are incompatible with the willemite structure at any temperature. An origin as fluid inclusions is supported by the shapes of the rods (i.e., negative crystals with a hexagonal cross-sections), the presence of phases containing OH, Cl, Ca, K, and CO₃, and void space visible around solid phases. The tapered shapes of the rods seems inconsistent with epitaxy. 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.

Keywords: willemite, inclusions, electron back-scatter diffraction, Franklin, New Jersey.