Day #84 Deskcrop: Sillimanite (Fibrolite) Gneiss

Following Horace Greeley’s advice we continue our journey westward. Today’s deskcrop is a sillimanite (var. fibrolite) gneiss collected from a roadcut along the Beartooth Highway, south of Red Lodge, Montana. I collected this sample during the NAGT Teaching Petrology Workshop field trip in July 2003. I take special pride in this sample because it marks the only sillimanite bearing sample I’ve collected to date that I felt confident of my field identification of that mineral (in this case, the variety fibrolite – best seen in the upper left quadrant of this image). What’s more, I believe it was the only macroscopically visible sillimanite-bearing sample collected by the entire group of petrologists that day. I would not pretend my field skills exceeded that of my many eminent colleagues that day – I’m quite content to chalk it up to good luck.

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Sillimanite (Fibrolite) Gneiss

Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!

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