Article: Pyrite and marcasite intergrowths from Northern Illinois.

Excellent microcrystals of pyrite and marcasite are found in vugs in Ordovician limestone at several quarries in northern Illinois. The crystals include pyrite "bars" and marcasite blades which are epitactic on early-formed marcasite whisker crystals.

INTRODUCTION

Three quarries in northern Illinois [ILLUSTRATION FOR FIGURE 1 OMITTED] have produced very similar microcrystals of marcasite and pyrite from rocks of the middle Ordovician Galena Group. The Irene quarry of the Rockford Sand and Gravel Company has provided the majority of the specimens described in this paper; it is located 2 miles north of Irene, Boone County, Illinois in the NW 1/4 of Section 9, ...

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