The Geology of Arkansas Quartz Crystal
By Robert Beauford
The state of Arkansas offers many geological treasures, and presents rockhounds with several nice opportunities to go out and collect their own specimens. The quartz mines, located in the center of the state, are among the finest such opportunities.
Although uncommon in its crystal form and rarely found as fine, perfectly formed specimens like those in central Arkansas, quartz is the second most common mineral in the earth’s crust, second only to a class of minerals called feldspars. Quartz makes up a significant percentage of the grains in sand, sandstone, and granite, and is found in almost every other rock as well.
The quartz crystals found in and around the Mount Ida area of central Arkansas started as shale and sandstone on the bottom of a seafloor many millions of years ago. Much of the Quartz bearing rock is almost a half-billion years old, from the Ordovician period. The ancient seabed was gradually buried under younger sediments, which eventually covered the future quartz-hosting rocks to about a mile thick. The buried silt and sand was gradually converted from loose sediment to solid sedimentary rock. It remained this way for almost 200 million years.
The slow process of continental drift brought this period to an end with the beginning of the Ouachita Orogeny, the mountain building episode that raised the Ouachita Mountains of central Arkansas. Put simply, South America struck North America from the south, more or less, and the Ouachita Mountains of central Arkansas are the crumpled and fractured remnants of this impact. Books could be written on the details of the process, which occurred between 300 and 250 million years ago. The impact lasted for millions of years, and high mountains and deep basins were formed as the rocks folded. The high mountains eroded and the deep basins filled. The result is a broad area that is hunted for fossil fuels today, as well as some interesting exposed geology and, of course, quartz crystals. The events of the Ouachita Orogeny caused several deep faults and millions of fractures in the distorted rocks that were left behind. Ground water filled these cracks.
The impact of the North American and South American plates also generated heat. Because the water was deep below the surface of the ground and trapped in the rock, most of the water could not boil away as steam. As a result, it reached temperatures of many hundreds of degrees. The super hot and somewhat acidic water dissolved much of the sandstone and shale around it.
Some of the water reached the surface as hydrothermal springs, just as it still does a few miles away in Hot Springs. The part of the water that remained below the surface flowed through cracks and faults in the rock. Some of these cracks were several feet wide. Cracks and other spaces like this are not uncommon in deep rock. In fact, open spaces similar to this contain much of the water we drink from wells. It was quite unusual, however, for the water in these cracks to be so very, very hot.
As the water moved through the cracks and cooled, dissolved silica in the water came back out of solution and formed crystals on the walls of the cracks. You may have a seen a similar process where minerals have built up on pipes or in a tea pot. Quartz (SiO2) molecules have a natural tendency to stack themselves on top of each other in a particular shape due to some fancy bits of physics and chemistry having to do with charges and the shapes of the molecules themselves. In any case, the process continued for millions of years and may continue to this day, although much slower and at lower temperatures.
In any situation where this process of deposition continued long enough and in a very clean and stable environment, the stacking process created what we see as clear quartz crystal. If the environment isn’t quite as stable or clean, or isn’t quite ideal in some other way, we get quartz crystals that have black specks in them, are milky colored, or that contain other inclusions visible to the eye. Because the fault was still active during the growth of the crystals and the ground still experienced violent earthquakes, you can also find crystals that were violently cracked or deformed. Many of these fractured crystals ‘healed,’ through the continued deposition of dissolved silica, and continued to grow to create interesting and thought provoking forms.
Some scientists and miners disagree on whether the fine red clay that surrounds much of the quartz in the area formed at around the same time as the quartz or filtered in with ground water later, but either way, it is fortunate for us that it is there. The clay packed around the quartz has both preserved and protected the crystals more effectively than the most careful packing and padding any living person could ever accomplish.
A laboratory grown quartz crystal is shown in the photo above. You can see the cut quartz crystal 'seed', from which the crystal was grown, suspended in the middle of the sample.
Though most Quartz mining in Arkansas is now oriented towards ornamental and collector specimens, the mines originally served a far more pragmatic purpose. Quartz is piezoelectric, meaning it moves slightly when an electric current is applied to it, and that it will produce a slight electric current when it is bent. If a tiny sliver of quartz crystal is cut at a particular angle relative to the growth of the crystal, and an electric current is passed through it, the sliver of quartz will vibrate like a tuning fork, and create a pulse in the current. The frequency of the pulse relates to how large a sliver is cut. Quartz crystal oscillators are used to produce a precise frequency in an electrical current. When you see the words 'Quartz Watch' on a timepiece, they are literally talking about a little piece of quartz, integrated into the electronic circuitry of the watch. Oscillations of the quartz produce a beat at a known frequency, providing the count by which the watch keeps time. Arkansas quartz crystal oscillators were important in the emergence of the modern electronic era, and quartz oscillators are still used today in a tremendous array of technical and consumer electronics. Arkansas quartz production for electronics purposes peaked in the 1940s. By the 1950s to 1960s, quartz crystals for electronics were being grown in labs, though Arkansas quartz mines provided quartz feed stock for portions of this production for several years. Many of the mines in which we hunt ornamental crystal today were opened during the 1930s and 1940s for oscillator quartz, and contributed to everything from time pieces and telephones to radar and radios.
The word 'crystal' sometimes causes some confusion. The terms 'quartz' and 'crystal' are not synonyms. Arkansas doesn't have crystal mines, it has quartz crystal mines. This distinction is important from a geological perspective, since technically, the clay in which the quartz is found is as much a crystalline material as the quartz itself. The picture above shows crystals of calcite (left), quartz (center), and ruby (right). Though the term quartz and crystal are sometimes used interchangeably, they are not the same thing. Quartz is a kind of crystal, but so are sapphires, diamonds, and garnets. The word crystal simply refers to organized, stacked molecules or atoms of any kind of material. Crystal is a broad term. All minerals are crystalline, and all minerals can form crystals. The quartz crystals mined here in Arkansas are also unrelated to the beautiful 'lead crystal' that we see under various trademarked names, from which chandelier pendants, drinking glasses, bowls, and similar products are made. These products represent a completely different definition of the word 'crystal.' They are special kinds of artisan glass with the ability to scatter light very effectively. These glasses are man made materials, and do not form naturally in the earth. They have nothing to do with Arkansas' quartz crystal mines. Since they are glass, these products are also, somewhat ironically, some of the few inorganic solid materials in the world that are specifically not crystalline.
Visitors can hunt or dig for their own crystals at several different mines in the area. Daily access to mine tailings is granted by private mine owners for a modest fee - usually $10 to $25.
To learn more about Quartz Crystals and the Geology of Arkansas, good places to start are the Arkansas Geological Survey website:
and the excellent Rockhounding Arkansas website produced by Mike Howard:
Thanks for reading! I hope this is useful.
Author's notes and reader questions that I will get around to answering: In the future, do you think crystals will be used in different ways? Do you think it is dangerous for crystal miners to mine for crystals? How did you get interested in crystals? What is your favorite growing crystal? Robert, can you explain a little more in depth how a milky or cloudy quartz crystal is formed? I've always wondered and been curious about this since I was a kid and collected them. Maybe a bit more about the different colours of quartz. Maybe a bit about the history of collecting. A few skeptical remarks about any mystical or homeopathic properties. Maybe a bit about the phantoms or enhydros! Commonly occurs in granite??? <quartz in sedimentary rocks like ours, versus igneous rocks, both in granite and as well formed points in cavities, like in the Rocky Mountains, and why> Is it the lure of beauty as symmetry? Is it the sense of greater sweeps of time, as their form(s) evoke more than we could likely be consciously aware of at any given moment (Did H. G. Wells use Quartz crystals with the 'Time Machine'?). Is it something about the contrast between their clarity and seeming perfection, and the muddier, dull-rock world we more commonly experience visually? Do they hint at the geometry of nature that we are ill equipped to see with naked eyes, more typically seeing curves, tangles, masses, open spaces, etc., except where human stuctures of brick, steel, glass are present? And how do such things as double-terminated varieties form? What makes them pointy in the first place? I want to know more about the Fisher fold, and about the topography of the Ouachita mt range where the crystals are formed and why they are so unique! Also I want to know how the crystals can reheal after they have been broken! Also how they conduct electricity or carry information!