Yellow Dog peridotite - magnetite grains in euhedral olivine crystal

Click on image to enlarge.          Photo © Daniel R. Snyder

Split image of a partially-serpentinized, euhedral olivine crystal (center) enclosing several small grains of magnetite, with other magnetite grains nearby.  Olivine crystals are on the left (XPL) side of the image, orthopyroxene is on the right (PPL) side. The olivine in the Yellow Dog peridotite is relatively iron-rich, with and average composition of Fo81 (Klasner et al., 1979). When olivine alters to serpentine (filling fractures in olivine), the serpentine can't accommodate the the iron, and it is taken up as newly-formed iron-oxide minerals, mainly hematite and magnetite. Some of the other images show stringers of tiny magnetite grains occupying fractures in olivine. In this example, the iron has formed several larger grains of magnetite, many with the diamond shape characteristic of some euhedral magnetite. Yellow Dog peridotite, Marquette County, northern Michigan. XPL and PPL. Imaged area 1.3 mm x 2 mm.

*John S. Klasner, David. W. Snider, W. F. Cannon, and John F. Slack (1979), The Yellow Dog Peridotite and a Possible Buried Igneous Complex of Lower Keweenawan Age in the Northern Peninsula of Michigan, Geological Survey Division, Michigan Department of Natural Resources, 38 p.