The crystals found in deformed Sevier Shale at Walland are an exception to the more general presence of bipyramidal crystals in that formation. They are found in a road exposure just below the Great Smoky Fault.  These crystals are small, but some are very unusual.

Folds in bedded shale and siltstone. This locality is often visited by geology students for its good examples of folding.


Deformed rock contains vugs with crystals in extension cracks.


Extension cracks result from a ductility contrast between the more silty layers and the clay-rich layers.  One might use the analogy of layers of peanut butter cookie separated by layers of peanut butter.   If you sit on it, the peanut butter may be forced into the cracks formed as the more brittle cookie cracks and the entire mass spreads out.


Siltstone and deformed beds of shale.



Another view.


Closer view of the sheared shale.  The elongated gray surface to left of center is a calcite-lined extension crack surface with several small quartz crystals.


The crack surfaces in this photograph are somewhat dirty, but calcite-lined and crystal bearing.


Close-up showing calcite-lined extension cracks with quartz crystals.  The objects in the very center are dew drops trapped in a spider web however.  The next photograph looks down into the small crack to the upper right.


Deformed layers with crystals growing in extension cracks.


Each of the shiny objects in this extension crack is a quartz crystal.


Vug in extension crack.









Quartz crystals in vug, small specimen.

Microphotographs

Close examination reveals some interesting features

“Skyscraper” tapered form.



Yellow material in inclusion is probably a petroleum or derivative.


Phantoms defined by the dark inclusions indicate they were separate crystals, not in contact at that time.



Inclusion bubbles. See below.


Another view with ultraviolet illumination.  Blue fluorescence indicates petroleum.


Unique terminations occur on some crystals from this locality.  This crystal shows that they developed as late stage overgrowths.



Detail looking down ath the termination.  These are the ‘pi and ‘omega faces, obtuse rhombohedral terminations that are generally rare in quartz.



Specimen with unusual faces growing from extension crack surface.  This specimen has a well-developed obtuse rhombohedral termination.


Details of above crystal with ‘ pi and ‘ omega termination faces.

What if anything can we say about these crystals?
First, it seems that the ones in the Knox Group formations show greater variety.  There’s always both elongate prismatic crystals and bipyramidal crystals, and everything in between.  Transitional forms or “scepters”, with clear bipyramidal quartz growing over milky fractured prismatic quartz are common.  With the one exception, the occurrence near the Great Smoky Fault at Walland, no Sevier Shale crystals are strongly prismatic, and none show scepter overgrowth formation.  In the next section we will see if any conclusions can be drawn.

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Index