Samani - the town that loves peridotites

Samani, a small coastal town on the island of Hokkaido, Japan, has made the most of its proximity to the Horoman peridotite body, which includes Mt. Apoi, site of a natural park devoted to "learning about the earth's transformation from peridotite". The town square in Samani (pictured) has been an open-air petrological museum, displaying a wide variety of locally-occurring peridotites.

Unfortunately, Samani experienced a 3.7-meter wave during the tsunami of March 11, 2011. I don't know more than that about the fate of the town, but since the town square was only few tens of meters from the shoreline, I fear the museum may no longer exist. I've e-mailed the town authorities and am awaiting a reply.

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.