We receive a lot of inquiries at our 'Ask a Geologist' service (http://azgs.arizona.edu/ask-a-geologist). Jerry from central Arizona send us several pictures of a rock that contained a square mineral. The mineral is almost certainly pyrite, which is a cubic iron sulfide (FeS2). In this case, the pyrite is oxidized and transformed into the iron oxide mineral limonite (FeO(OH). Identifying the mineral was the easy part. Getting the rock name is more difficult. Because I only have a picture to look at, the best I can come up with is a fine-grain, silica-rick, hypabyssal igneous rock. This specimen was recovered from the Peeples Valley of central Arizona.
Photographer: Anonymous
Photo Date: August 2018
Photo Location Accuracy: Approximately Located
Photo Tags: pyrite, limonite, igneous rock, hypabyssal rock, Peeples Valley, Ask a geologist 