Weathered dunite, Newdale, NC

Click on image to enlarge          Photo © Daniel R. Snyder

Above: Weathered dunite, with small grain size and indented grain boundaries resulting from cataclasis and subsequent weathering. All of the blue-green olivine grains clustered at left center go extinct simultaneously, indicatng that they are fragments of a single crystal. Note the dense network of dissolution channels and voids (black). Newdale, Yancey County, North Carolina. XPL digital mosaic. Imaged area 1.3 mm x 3.2 mm.

Below: Optical scan of a freshly-sawn surface of weathered Newdale dunite, same sample as above (NOT same scale). Yellow-orange color is imparted by hematite, disseminated in serpentine layers between and within olivine grains. In the high-magnification view, many of the more robust olivine grains can be seen to retain a light yellowish-green color. Dark grains at lower left are chromite. Dark green grains in cluster at upper left may be remnants of a fragmented hornblende grain. 2400-dpi optical scan. Imaged area 11.4 mm x 17 mm.

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

Talc in weathered Newdale dunite

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

In the above XPL image, a large talc clot (center) is surrounded by weathered olivine grains. The solid black grains at top center, bottom center, and bottom left are chromite. Linear patterns in the lower part of the talc suggest that this part of the talc is partially-altered tremolite/actinolite. Below is a PPL image of the same area at the same scale, showing clearly the chromite and the serpentine meshwork in the dunite. Newdale, Yancey County, western North Carolina. Imaged area 2.7 mm x 4 mm.

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

Talc in pyroxene in Webster dunite

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

A vein of talc (lower left to upper right) bisects two large grains of orthopyroxene, surrounded by olivine, in dunite. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. XPL. Imaged area 2.7 mm x 4 mm.

Deformation bands in olivine

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

In this image of dunite, several large olivine grains (center, top left, top right, bottom right) show deformation banding. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, Western north Carolina. XPL. Imaged area 2.7 mm x 4 mm.

Magnesite in dunite

Click on the image to enlarge.           Photo: Dan Snyder

A cluster of magnesite (MgCO3) grains in Newdale dunite. Magnesite commonly occurs where peridotites are permeated by carbonate-rich fluids.  Light gray, darker gray, and pale yellow grains at center and left are magnesite. Dark gray grains are at partial extinction; pale yellow grains probably contain a small amount of iron carbonate (siderite) in solid solution. The partially extinct (medium gray) grain at the upper edge of the cluster gives an idea of grain size. Colored grains at bottom, upper left, and right are olivine. Newdale, Yancey County, North Carolina. XPL. Imaged area 2.7 mm x 4 mm. Link to Carolina Geological Society 1997 Guidebook (PDF).

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.

Webster-Addie ultramafic body, NC - chromite and chlorite in dunite.

Click on the image to enlarge.           Photo: Dan Snyder

A chromite grain (solid black) in the process of altering to chlorite (gray and gray-brown blades). Surrounding (colored) grains are olivine. Chromite is a ubiquitous accessory in most peridotites of the southern Appalachians. The chlorite mineral is kammererite, a chlorite variety unusually rich in chromium and magnesium. Thus, it grows at the expense of both chromite (FeCr2O4), from which it takes chromium, and olivine, from which it takes magnesium and silica. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. XPL. Imaged area 1.3 mm x 2 mm.

Webster-Addie ultramafic body - sheared dunite, full thin section

Click on the image to enlarge. Click twice to enlarge more- it's a lot more 

interesting close up.      Photo: Dan Snyder.

Tectonized, sheared dunite. Upper 2/3 of image is composed mainly of polygonal olivine grains (more saturated colors) with scattered pyroxene grains (grays and pale yellows), especially at top left; lower 1/3 is composed of sheared talc clots with crushed and elongated grains of olivine and pyroxene. By itself, this thin section probably wouldn't fit the conventional definition of dunite - too much pyroxene - but it was part of a more extensive exposure that is clearly dunite. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. XPL. Imaged area 22 mm x 39 mm.

Pyroxene in dunite

Click on the image to enlarge.           Photo: Dan Snyder

A large orthopyroxene grain in dunite. According to the IUGS classification, a dunite is a rock made up of ferromagnesian minerals, mainly olivine and pyroxene, of which more than 90% is olivine (discounting accessory minerals). In practice, serpentine inferred to be derived from olivine is counted as olivine. Although a pyroxene grain dominates this photograph, there are only a few such grains in the sample.  Webster-Addie ultramafic body, Jackson County, North Carolina. Reflected-light photomicrograph of polished block. Ordinary light. Imaged area 3.3 mm x 4 mm.

Olivine weathering microtexture in dunite

Click on the image to enlarge.      Sample:Michael Velbel; Photo: Dan Snyder

Characteristic diamond-shaped etch pits in olivine. The etch pit field is adjacent to a microfracture in the olivine, through which fluid has permeated, creating the etch pits. Orange areas are iron-oxide mineral stains. Webster-Addie ultramafic body, Jackson County, western North Carolina. Plane-polarized light (PPL). Magnification 400X.

Thanks to Dr. Michael Velbel, Michigan State U., for the loan of this and many other thin sections.

Etch pits in olivine - Webster-Addie ultramafic body

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

The dark specks that appear to be mineral grains or polishing grit are actually microscopic etch pits  along fractures in and around this olivine grain. These fractures provided access to fluids that developed weathering textures in the olivine. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. XPL. Imaged area 0.5 mm x 0.8 mm.

The etch pits stand out more clearly in this PPL image of the same feature (same scale as above):

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

Weathering microtexture in olivine.

Click on the image to enlarge. Sample: Michael Velbel; photo: Dan Snyder

Coalescing funnel-shaped etch pits forming serrated edges in olivine (light gray). Darker gray at top and right is serpentine. Webster-Addie ultramafic body, Jackson County, western North Carolina. Scanning electron microscope (SEM) image of polished thin section (backscattered electron image), magnification 650x.

Thanks to Dr. Michael Velbel, Michigan State U., for the sample, and thanks to Dr. Velbel and NASA for the SEM time. Thanks also to E. Danielewicz for assistance with electron microscopy.

Pyroxene lamellae, amphibole in dunite

Click on image to enlarge.           Photo: Dan Snyder

Exsolution lamellae in pyroxene (pale yellow grains at left and bottom, gray grain at right); ampohibole mineral (grains with diagonal cleavage patterns: center, top center); olivine (brightly colored grains at  center and top center). Webster-Addie utlramafic body, western North Carolina. XPL. Imaged area 1.3 mm x 2 mm.

Thanks to Dr. Michael Velbel, Michigan State U., for the loan of this and many other thin sections.

Polyhedral texture in dunite - Scanning Electron Microscope Image

Click on the image to enlarge.  Sample: Michael Velbel; photo: Dan Snyder

Granular (recrystallized) olivine, fractured dunite fragment, Webster-Addie ultramafic body, Jackson County, western North Carolina. Scanning electron microscope (SEM) image (secondary electron image). Magnification 40x.

Thanks to Dr. Michael Velbel, Michigan State U., for the sample, and thanks to Dr. Velbel and NASA for the SEM time. Thanks also to E. Danielewicz for assistance with the SEM.

Polyhedral texture (mosaic texture) in dunite

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

Classic polyhedral mosaic in dunite. This texture results from recrystallization in the solid state, and is common in metamorphic rocks composed mainly of one mineral, such as dunite. Grains tend to have fairly straight sides, and many grain boundaries intersect in triple junctions at approximately 120˚ angles. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. Macrophotograph, XPL. Imaged area 8 mm x 14 mm.

Buck Creek ultramafic body, NC - chloritization in dunite

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

Advanced stage of chloritization in dunite, Buck Creek ultramafic body, Blue Ridge Mountains, Clay County, western North Carolina. Pratt and Lewis (1905)* described the chloritization of the Buck Creek dunite thus: "In some cases the olivine alters extensively to chlorite, which penetrates the grains generally along cleavage cracks first, though frequently in diagonal directions, in long slender laths, sometimes passing uninterruptedly through two or three grains." XPL. Imaged area 1.3 mm x 2 mm.

Thanks to Dr. Brannon Andersen of Furman University for giving me detailed instructions on reaching this exposure.

*Joseph Hyde Pratt and Joseph Volney Lewis, Corundum and the Peridotites of North Carolina.

Buck Creek ultramafic body - unaltered dunite grading into chloritized dunite

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

Portion of a thin section showing unaltered dunite (left) grading to heavily chloritized dunite (right). Buck Creek ultramafic body, Blue Ridge Mountains, Clay County, western North Carolina. Macrophoto, XPL. Imaged area 10 mm x 20 mm.

Buck Creek dunite - serpentinized and chloritized

Click on image to enlarge. See image below for enlargement of outlined area.

Polished slab of partially serpentinized and chloritized dunite. Buck Creek ultramafic body, Blue Ridge Mountains, Clay County, western North Carolina. Optical scan at 1524 pixels/cm (600 dpi). Original width 3400 pixels, reduced by Blogger to 1600 pixels at maximum magnification. Slab dimensions 8 cm x 14.5 cm. Below is a higher-resolution image (6096 pixels/cm.) of the 3.5-cm-by-3.5-cm area outlined in the larger image.

Click on image enlarge. Click again to get really close up.