PSEUDORUTILE IN THE BARABOO RANGE, WISCONSIN: FIRST RECOGNITION AS A METAMORPHIC MINERAL

摘要

The iron titanate, pseudorutile, is typically an intermediate product in the alteration of ilmenite to rutile and hematite during weathering or diagenesis. In contrast to this common mode of occurrence, pseudorutile in the chlorite-grade Seeley slate (Baraboo Range, Wisconsin) appears to be metamorphic in origin, comprising tabular, poikiloblastic crystals and coexisting with quartz, muscovite, chlorite, tourmaline, apatite, pyrite, and rutile. Individual grains of pseudorutile are compositionally heterogeneous, with Fe_2O_3 ranging from 29.4 to 35.5 wt.% and TiO_2 ranging from 58.3 to 61.9 wt.%. Complete electron microprobe (EMP) analysis for all elements, except H, yields a mean atomic formula for Seeley pseudorutile, based on 3 atoms of Ti + Si per formula unit, of: (Fe_(1.61)Mn_(0.06)Zn_(0.02)Al_(0.01))_(1.70)(Ti_(2.97)Si_(0.03))_(3.00)O_(8.10)(OH)_(0.90)-0.69H_2O in which oxygen in excess of that required for the anhydrous cations is assigned to H, which is apportioned between OH and H_2O based on charge balance considerations. It is anticipated that detailed investigation of tabular, opaque grains in other chlorite-grade slates and metapelites may reveal additional occurrences of metamorphic pseudorutile.