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 or “scepters”, with 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 none show scepter overgrowth formation.  In the next slides we will see if any conclusions can be drawn.

This is the Nakaya Diagram, presenting experimental results from the first experiments to grow snow crystals.  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 are found in concretions in northern Ohio, far from any orogenic disturbance.   Also, conodont alteration indices for these rocks do not suggest anything nearly this hot.



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:  Douglas Dam in particular shows evidence of regrowth after cracking.  This suggests that the crystals were present before the end of the orogenic event.  I have not found Sevier Shale crystals showing evidence of cracking and regrowth.



Transport:  In most places, most crystals are completely unworn.  hat suggests that they come from the nearby bedrock, and have not been moved about by water.  But in many places 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, where all the crystals show some degree or erosion and tumbling.  Here they 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 shown above is from Diamond Creek.



Abrasion: Crystals found at Muddy Creek show eroded faces, but it doesn’t have the same look on the edges and points as Diamond Creek specimens.  They may have been in the dirt and soil for several millennia, where animals stepped on them from time to time.



Milling: At Douglas Dam, small concentrations of water-worn crystals are found in the residual clay, that are very similar 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 cause by groundwater flow.  In the 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 macrocrystaline, and the vertical axis is whether the quartz tends to cover all surfaces or just grow on existing quartz.

“Drusy quartz” refers to when quartz crystals cover all surfaces, even where the crystals have grown large through competition, as in a Brazilian amethyst geode.  In drusy quartz, crystals start out small and numerous, and grow competitively so that there are fewer larger crystals.  Doubly terminated crystals are rare.




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



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




Brazilian agate nodule showing deposition going back and forth from chalcedonic agate to drusy quartz.  Similar features are observed elsewhere, including in Tennessee agates.



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



Not found in the Eastern United States, chalcedony roses occur in silicic volcanic rocks of the West.  Quartz flowers come from locations in Brazil and elsewhere.   For examples search for “quartz flower” or “amethyst flower” For one of the finest – from the Carnegie Museum of Natural History website.  See http://www.carnegiemnh.org/graphics/programs/online/hillman/quartz/27324%20quartz%20amethyst.jpg, online at   http://www.carnegiemnh.org/online/hillman/quartz.html.   One specimen also had anhydrite, suggesting that the sulfate ion may have a role controlling the morphology of quartz.  For photo see http://www.Johnbetts-fineminerals.com.  One internet photo, showed a thunderegg that had white chalcedony, overlying a few quartz crystals on the wall, followed by a white quartz flower in the center, again suggesting alternation from chalcedonic to quartz-flower favoring conditions.




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


Chalcedony roses can be simple,…


Or complex forms.


Chalcedony rose transitioning to crystalline growth.


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




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 other direction yield some of the finest specimens. Some gorgeous examples, with large late-stage amethyst growing uponwhite drusy crystals can be found at http://www.wilenskyminerals.com/about/criteria/ .  Nevertheless, the specific conditions responsible for these changes remain elusive.

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