Cummingtonite: The fibrous amphibole hidden within metamorphosed iron-rich rocks across Wisconsin
In short: Named for its unique crystalline structure, cummingtonite is a fascinating member of the amphibole family. Often appearing as elongated or fibrous green to brown grains, this mineral serves as a geological marker in metamorphosed iron formations, frequently coexisting with a complex array of other minerals in diverse rock assemblages.
Cummingtonite is defined by its monoclinic structure and the chemical formula (Mg,Fe)7Si8O22(OH)2. As a member of the amphibole group, it is primarily identified in metamorphosed iron-rich environments. In Wisconsin, its presence is well-documented across several counties, where it appears in various geological contexts. For instance, in Ashland County, it is found within the Ironwood Iron Formation, having undergone contact metamorphism due to gabbroic intrusions. In this specific setting, it is associated with minerals such as minnesotaite, stilpnomelane, grunerite, and various iron-rich pyroxenes.
The mineral's occurrence is not limited to iron formations. In Eau Claire County, researchers have identified cummingtonite as tiny grains and exsolution lamellae within hornblende in amphibolite at Big Falls County Park. Similarly, in Oconto County, it appears alongside hornblende and plagioclase in metamorphosed mafic lava flows of the Waupee Formation. These varied associations highlight the mineral's versatility in different metamorphic environments, ranging from massive sulfide deposits in Marinette and Oneida counties to reaction rims replacing augite in the Tigerton Anorthosite of Shawano County.
The complexity of these assemblages is further illustrated by the Jackson County Iron Mine, where pale green cummingtonite exists in a sophisticated mixture of amphiboles, including grunerite, riebeckite, hastingsite, ferroactinolite, quartz, and biotite. Whether appearing as radial sprays of crystals in talc schist or as a component of reaction rims, cummingtonite remains a critical subject for understanding the metamorphic history of the regions where it is found. Its presence provides geologists with essential clues about the temperature, pressure, and chemical conditions that shaped these ancient rock formations over geological time.
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