Quartz crystal morphology


What stories can they tell?

What, if anything, can we say about these crystals?

First, it seems that the ones in the Knox Group formations have greater variety.  There’s always both elongate prismatic crystals and bipyramidal crystals, and everything in between.  Transitional forms, so-called “scepters”, are most commonly 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 few show scepter overgrowth formation.  So can we organize and interpret this to see if any conclusions can be drawn?

We will also go far afield in an attempt to place "Herkimer" type doubly terminated crystals into the enormous variety of quartz morphology seen in nature.

This is the Nakaya Diagram, presenting experimental results from the first experiments to grow snow crystals, and partly the inspiration for this project.  It shows crystal morphology plotted against temperature and degree of supersaturation, and shows a “sweet spot” of conditions where dendritic “classical” type snow crystals form.  I wondered if anyone had done the same thing for quartz.



This diagram from Magono and Lee applied the Nakaya Diagram to natural snow.



The closest thing is a plot of morphology, referred to as a “morphodrom” done by Iwasaki and Iwasaki.  Quartz deposition on the prismatic faces increases to the right, and the relative development of the two sets of pyramidal (rhombohedral) faces up and down.



Fields are defined by the presence of specific faces and their relations.  It is important to consider that a crystal is defined by its slowest-growing faces.  For example the relative growth rate of the prismatic “m”  faces increases going to the right – they grow themselves out of existence.



Here are his isotherm lines.  I’m not clear what he based them on, but if he is correct, then we could say that the earlier stages where we find the longer prismatic crystals are generally cooler, and the later stages of quartz deposition and overgrowths; the scepters and the bipyramidal crystals from the Sevier Shale, took place at higher temperatures.  A “path” showing change in morphology over time can be defined.  The problem is that a fluid inclusion analysis for a quartz crystal at Douglas Dam reported by Edwin Roedder showed much lower temperatures.  Unfortunately, he did not say whether they were looking at an early prismatic crystal or a late bipyramidal crystal. The interpretation is questionable, particularly since bipyramidal crystals occur in concretions in northern Ohio, far from any orogenic disturbance.   Also, conodont alteration indices for these rocks do not suggest anything nearly as hot as suggested by their isotherms.



Time line, with crystal morphologies, showing possible paragenetic timing.  The actual truth is probably more complex, as the crystals at Douglas dam show evidence of a stage of “reverse scepter” or long prismatic over stubby prismatic, growth.  Also, crystal formation is likely concentrated nearer the period just before, during, and after the Alleghenian Orogeny.

Post-crystallization
History


What further stories can they tell?

Regrowth:  Large quartz crystal fragment from Douglas Dam showing regrowth after cracking.  This suggests that the crystals were present before the end of the orogenic event.  I have not encountered Sevier Shale crystals showing evidence of regrowth.



Transport:  In most places, most crystals are completely unworn, which suggests that they come from the nearby bedrock, and have not been moved about by water.  But in many places where you find crystals, some of them ARE water-worn. This is somewhat of a mystery, especially since I have never encountered a water-worn crystal in areas where they are not otherwise found very nearby.  It’s like they show evidence of being transported without being moved long distances.  One exception is Diamond Creek, across the river from White Pine. Here, all the crystals show some degree or erosion and tumbling.  These occur in stream gravels in and adjacent to the creek where its course is exposed in the lake bed.  The creek’s watershed is entirely in the Sevier Shale.  The large crystal fragment above is from Diamond Creek.

Most of these show abrasion from impact under fairly violent conditions, consistent with the larges sandstone cobbles in the creek and adjacent alluvium, including the creek bed well away from the rive and present-day lake bed. The cobbles appear to be sourced from the Chillhowie Group and Thunderhead Mountain exposures from the higher ridges and mountains to the east. I have not explored the immediate area, but similar gravels are often found on the lower ridgetops, deposited when rhe rivers ran at higher levels.

This is about the minimum degree of wear seen on these crystals. Right click and open in new tab for detail.

Impact abrasion, predominantly on edges and corners is typical at Diamond Creek.

Abrasion: On the other hand, crystals from Muddy Creek sometimes show scratched and eroded faces. Wear is less concentrated on the edges and corners as on the Diamond Creek specimens, and its appearance is different.  Perhaps they were lying in the soil for several millennia, where animals stepped on them from time to time.

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Scratches, almost as much on faces as corners and edges. Right click and open in new tab for enlarrged images.

Milling: At Douglas Dam, small concentrations of water-worn crystals are found in the residual clay. They were ssimilar to the local unworn specimens, complete with black inclusions.  How could it be that crystals that appear transported are concentrated so near similar fresh ones?  An explanation might involve some kind of in-place milling action in a restricted cavity caused by groundwater flow.  In the East Tennessee zinc mines they have found fragments of zinc ore in vugs that are rounded and eroded, and that is the explanation given for those occurrences.

“Herkimer” type crystals

and other varieties of quartz growing into open space:

Do “Herkimers” fit into a bigger picture?

Classification diagram, more of a doodle.  The horizontal axis is whether the quartz is cryptocrystalline or macrocrystalline, and the vertical axis is whether the quartz tends to cover all surfaces or just grow on existing quartz.

Here, “drusy quartz” refers to when quartz crystals cover all surfaces, even where the crystals have grown large as in a Brazilian amethyst geode.  In drusy quartz, crystals start out small and numerous, and grow competitively, generally along the C axis, so that with time there are fewer larger crystals with single terminations.  Doubly terminated crystals are rare. In true chalcedonic agate, the crystals are elongated not along the C axis, and grow along one of the perpendicular axes.




Where change from one variety to another is observed, paths can be drawn.



In many agate nodules, deposition goes back and forth between drusy quartz and agate. 


Brazilian agate comes as nodules that formed in large vesicles in basaltic lavas from a number of Brazilian states in southeastern region of the country. Chalcedonic agate in shades of gray and tan are most typical, but many show strong banding, often with alternating chalcedony and druzy quartz.

Brazilian nodule showing deposition alternating between chalcedonic agate anddrusy quartz. These occur in basaltic rock, but similar features are observed elsewhere, including in Tennessee agates.



Detail of above.  There are at least four distinct layers of drusy quartz.

Tennessee agate, from the lower part of the Mississippian age Newman Limestone just above the contact with the Fort Payne Chert near LaFollette, TN. Similar agate has been seen in the quarries near Jellico. This agate resembles the "Paint Rock" agate found near the Alabama border, but tends to have many more cracks,



In thin section,it is seen to be a true (length-fast) chalcedonic agate, (the bluish bands here) as revealed by crossed polars and a quarter wave plate. The chalcedony alternates with length-slow drusy quartz (yellowish here). The chalcedony grew from lower right to upper left in this arrangement, except for the chalcedony on the very left end.

The question of exactly what is changing to cause quartz deposition to go from drusy to chalcedonic has never been addressed. Since the morphology is so different, one would expect it to be fairly extreme, but on the other hand, it takes place deep underground where conditions are pretty constant.

Transformations most commonly occur to minimize the amount of energy in the system. I would look for some factor affecting the relative energy states of the crystals versus the relatively disordered states of the crystal boundaries, but have no idea what that factor might be.

In contrast, the Summerville lace agate is an occurrance of "agate" in a chert/gravel quarry near Summerville, Georgia. It occurs in (or over) the older Knox Group cherty dolomites, but may be much younger than the surrounding country rock. There is no unaltered carbonate bedrock exposed in the excavation. All of it is granular length-slow drusy quartz, a form sometimes referred to as "quartzine" or lace agate. It lacks the flinty nature of true agate.

Summerville, georgia "agate". The sphere is about 3 cm in diameter.

Thin section under low magnification, crossed polars.

detail, crossed polars

The same field as above, under rossed polars and a quarter wave plate.

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Close up under crossed polars. Note undulating extinction (the shades of black, gray and white) on the edges of the larger quartz crystals. And under ordinary light. Interestingly, the banding continues as phantom lines through relatively large quartz crystals. Right click and open under new tab for enlarged views.

Quartz flowers come from locations in Brazil and elsewhere.   For examples search for “quartz flower” or “amethyst flower”   See Mindat, https://www.mindat.org/locentry-19482.html. They are another example where quartz shows a preference for depositing on existing quartz formations    One photo also had anhydrite, suggesting that the sulfate ion may have a role in that.  Another internet photo showed a thunderegg with white chalcedony, overlying a few quartz crystals on the wall, with a white quartz flower in the center, again suggesting change from chalcedonic to quartz-flower favoring conditions.

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Amethystine "quartz flower", and attachment side with some of the celadonate from the country rock wall. It demonstrates how the crystals grew out from there without really making contact with the surface

Not found in the Eastern United States, chalcedony roses occur in silicic volcanic rocks of the West. These share in common with the "Herkimers" and quartz flowers that quartz avoids the country rock, and prefers existing quartz formations.

Examples of chalcedony roses, from Tertiary-age silicic volcanic rocks in Clark County, Nevada.


Chalcedony roses can be simple buttons,…


Or fused into more complex forms.


Chalcedony rose transitioning to crystalline growth.


Another instance of fluctuating habit.  This specimen shows a line of chalcedony-covered quartz crystals (slightly darker gray) growing along the lip.

Chalcedony rose in thin section under crossed polars. The quartz is nearly transparent, and without the polarizing filters you see almost nothing.


The quarter wave plate provides more information about the optical orientation of the quartz.


Detail showing how the optical orientation of the chalcedony can change from length-fast to length-slow even within the same layers.

Further detail.

Internet photo, showing change from chalcedonic to quartz-flower favoring conditions.



The so-called "cactus quartz" is a case where larger quartz crystals become covered with finer drusy crystals



Conditions changing in the opposite direction yield some of the most spectacular specimens. For example, large late-stage amethyst growing upon white drusy crystals as can be found in some amethyst occurrences. Photos were at http://www.wilenskyminerals.com/about/criteria/ .  Nevertheless, the conditions responsible for these changes remain elusive.

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