Second alteration cycle - serpentine altering to anthophyllite (Presque Isle peridotite)

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

The grain shown in this image, originally olivine, was altered to serpentine, most of which has been altered in turn to anthophyllite. That is, I thought it was anthophyllite, but then I read Michael Lewan's (1972)* M. S. thesis, in which he reported NO anthophyllite in the three samples of Presque Isle peridotite for which he carried out modal analysis. However, all three samples contained small amounts of chlorite (7.3, 7.1, and 5.4 percent volume. I've never seen chlorite that looks like this, but maybe... If you know, please send me a comment.

Tiny red or orange anthophyllite(?) crystals occupy most of the former olivine grain, except for the two remaining areas of serpentine, gray-green in the XPL image above (lower left and upper right). The image below, also XPL, shows most of the anthophyllite(?) crystals (those aligned from upper left to lower right, or vice versa) at extinction - as extinct as they get. The serpentine areas show more clearly in this view, while the anthophyllite(?) crystals that are "at extinction" have dark outlines of varying thickness.

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

In the PPL image below, the serpentine areas are relatively featureless, while elsewhere the elongated crystals of anthophyllite(?) show distinct outlines. Yellow-orange color in PPL image is due to iron-oxide staining from hematite. Black spots are magnetite. Presque Isle peridotite, Marquette County, northern Michigan. Imaged area 0.5 mm x 0.8 mm.

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

*Lewan, Michael D., 1972, Metasomatism and Weathering of the Presque Isle Serpentinized Peridotite, Marquette, Michigan, unpublished M.S. thesis, Michigan Technological University.

Anthophyllite in dunite

Click on image to enlarge.            Photo: Dan Snyder

Anthophyllite in Newdale dunite. Irregularly shaped grains are olivine; long, straight grains are anthophyllite. Newdale, Yancey County, North Carolina. XPL. Imaged area 1.3 x 2 mm. Link to Carolina Geological Society 1997 Guidebook (PDF)

Thanks to Dr. Samuel Swanson of the University of Georgia for directing me to this exposure.

Deer Lake peridotite - magnetite traces of ancient olivine crystals

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

The Deer Lake peridotite, thought to be Proterozoioc in age, is composed entirely of secondary minerals. Here, the outlines of two adjacent olivine crystals - and their internal fractures - have been preserved by stringers of magnetite that separated out when the olivine was serpentinized. The gray masses appear to be carbonate, possibly magnesite, throughout which magnetite grains have been disseminated, giving the gray color. Deer et al.* state that magnesite may be formed if serpentinites undergo low-grade metamorphism where carbon dioxide is available. A few orange and red wisps of anthophyllite or tremolite signal the beginning of another round of alteration. Marquette County, northern Michigan. XPL. Imaged area 1.3 mm x 2 mm.

Deer, W. A., Howie, R. A., and Zussman, J., (1992), An Introduction to the Rock-forming Minerals,  2nd edition, Pearson-Prentis Hall, 685 p.