Geology and Mineralogy of Quartz Crystals
General Geology
MOST OF THE QUARTZ veins are restricted to a belt about 30 to 40 miles wide that extends a distance of about 170 miles west southwest from Little Rock, Arkansas, to eastern Oklahoma. This area corresponds to the core region of the Ouachita Mountains.
Productive Veins
The most productive quartz veins are present in both Paleozoic sandstones and shales, but those having shale as the host rock typically are massive milky vein deposits with a smaller proportion of clear, well-developed crystals. Deposits in sandstone units may be in the form of veins, sheeted zones, and stock works. Sandstone-hosted deposits usually contain less quartz volumetrically than shale-hosted deposits, but often yield a higher percentage of clear crystals in cavities or pockets. Many crystal-bearing pockets were distorted or crushed by structural adjustments during the Ouachita orogeny (mountain-building episode) after initial quartz deposition. The deformation commonly causes the veins to show complex fabrics.
Quartz formed in the cracks
The quartz veins were formed by the filling of open fissures and display little evidence of significant replacement of wall rock. Milky quartz crystals and associated vein minerals of the Ouachita Mountains were deposited from hot waters during the closing stages of mountain building, ranging from the Late Pennsylvanian (300-286 million years ago into the Permian (286-245 million years ago). The veins attain a maximum width of 60 feet in Arkansas and nearly 100 feet in Oklahoma. They are most numerous along the central core of the Ouachita Mountain region, where they are present in shale, slate, sandstone, and other rock types. Along and near the borders of this region, the veins are usually confined to sandstone beds encased within thick shale units.
Most of the collectible quartz crystal is obtained from deposits in the Blakey and Crystal Mountain Sandstones (both Ordovician), but attractive quartz crystal may occasionally be discovered from any of the Paleozoic units. The more than 25,000 feet of Paleozoic rocks exposed in the Ouachita Mountains have been deformed into complex, gently plunging folds that trend nearly east-west. Steeply dipping fractures closely related to the major folds and faults of the region controlled the location and deposition of most of the quartz.
Geologic environments
The mineral quartz forms in a variety of geologic environments. These include crystallization in magmatic rocks, particularly granites; authigenic crystals in sedimentary carbonate rocks; from hydrothermal fluids in veins filling fractures in various host-rock types; by the dissolving and reordering of silica in metamorphic rocks due to the agents of heat, pressure, or chemically active fluids; and as deposits from hot, warm, or cool water-based solutions in gas cavities, solution and breccia cavities, pockets, and even cave-sized voids in pre-existing rocks. Because quartz forms under so many conditions and is resistant to most of the forces of weathering, it is the second most common mineral in the earth's crust, feldspar being the most common. Let's discuss each one of these types of formation in some detail.
Quartz that forms from the crystallization of magmatic rocks crystallizes from a melt that is rich in silica and water. The crystals usually do not express their own crystal form, but instead fill voids between other earlier-formed minerals. Sometimes they even encase other minerals as inclusions. The grain size is determined by the size of the void being filled and the supply of silica. Certain types of igneous rocks called pegmatites contain gigantic-sized crystals of various minerals, including quartz. However, very large crystals of quartz tend to be whitish to milky in color, due to the presence of minute fluid-filled cavities. These cavities disperse the light and reduce the transparency of the quartz.
Pegmatites
Quartz-bearing pegmatites are often associated with masses of granite and may be seen in many places in New England, Colorado, and Canada. Whitish quartz crystals to 6.5 feet long by 1.5 feet in diameter have come from pegmatites in New Hampshire. A single crystal 8 feet long and 6 feet in diameter was on exhibit in Tucson, Arizona, a few years ago. It was from a pegmatite in Africa and was the typical milky color.
Authigenic crystals form after the deposition of the original sediment, and either before, during, or after the processes of compaction and lithification. Silica is dissolved and then reprecipitated, crystallizing as quartz during this process. Usually the crystals are free-floating in the matrix rock and never reach very large size. Some quartz crystals present in the matrix of dolostone or limestone formed in this manner. They may or may not contain adjacent minerals in the sedimentary rock, such as clay or feldspar. A type of doubly terminated quartz from south Texas called "Pecos diamonds" and reddish doubly terminated quartz from Spain both contain iron oxide inclusions from the original sedimentary host rock.
In Arkansas, the best known quartz is that which formed from hydrothermal fluids in veins that fill fractures in differing types of host rocks. Minable veins are present in either sandstone or shale. Sandstone-hosted quartz veins normally have a higher percentage of rock crystal (water-clear quartz) than quartz veins in shale. Shale-hosted veins are predominantly milky quartz, but tend to occur as larger individual veins than those in sandstone. Milky quartz is the most common variety, making up the great bulk of all veins. Rock crystal is much less common, although in places it is very abundant.
Veins may be very large and complex
Milky veins in shale in the Ouachita Mountains have been reported that measure several hundred feet in outcrop length and 60 to100 feet in thickness. Only the core of such veins, along with isolated pockets scattered throughout the vein, produce any rock crystal. The major commercial deposits of rock crystal are usually sandstone hosted. They tend not to be one single vein, but a complex series of veins that follow the fracture patterns of the rocks that were broken and shattered by the mountain-building processes. Deposition of quartz took place several times, often interrupted by breakage and refracturing of the host rock.
The major deposition of quartz in the Ouachita Mountains of Arkansas and Oklahoma took place during the Late Pennsylvanian to Early Permian Periods, some 300 to 250 million years ago. The rocks that we see the veins now exposed in were buried under a mile or more of cover during the time the quartz was being deposited. There is a common misconception by both hobbyists and miners, that somehow the present topography had an influence over the deposition of quartz veins. Actually, the presence of quartz veins, as a cementing agent in sandstones and as a highly erosion-resistant unit when present as thick veins, exerts an influence over how the topography develops.
Quartz deposits in sandstone units are often present on the crests of ridges where they help cement the sandstone fragments and make the entire unit more erosion resistant. Major faults are commonly filled by quartz veining, which may have been fractured many times during mountain building. The sandstone-hosted veins contain a lot of milky quartz, but usually have a higher percentage of rock crystal present. This is due to the nature of quartz crystallization and the geometry of the actual deposits themselves. When quartz begins crystallize, it must have a nucleation site. If one is already available, such as a fractured quartz grain on a sandstone face, then quartz crystal will start to grow. But since not all the grains will be oriented in the same direction, some of these early formed crystals begin to dissolve and their silica added to those that are oriented properly for the local conditions.
In hydrothermal veins, quartz typically grows as elongate crystals normal (perpendicular) to the wall rock. The crystals are attached at the wall rock and grow inwards from both sides to the center of the fracture. When two fractures in the host rock intersect, an open pocket may be formed because there is more space for the fluids to pass and continue to supply the crystals with silica necessary for continued growth. In some simple undistorted veins, you may actually be able to tell which direction the fluids were flowing by the orientation of the majority of the quartz crystals on the wallrock face. The side of the crystal facing the flowing fluids grows at a more rapid pace than the faces on the downstream or eddy side. The dominant face on the termination usually faced into the current. The size of individual crystals in hydrothermal veins is dependent on a number of factors, including the size of the vein and subsequent pockets and the nature of growth conditions.
Early digging
Up until World War II, the local diggers had a major misconception concerning the extent and nature of the Arkansas veins. In the early 1940s, about a year into the war, the need for quartz for oscillators became critical because the Allies supply from South America was cut off by German U-boats. Exploration work on the Arkansas deposits proved that the veins extended far deeper than the old timers ever thought possible. They had thought that once the first milky zone in the veins were encountered, no rock crystal would be found deeper. We now know that rock crystal may be present at any depth in the right rock type. When the rock type changes, then often the quartz veins pinch out.
Hydrothermal fluids
To many people, hydrothermal quartz brings up the source model of an igneous parent, like granite, rich in silica and water. From the source, the hot water (with its load of dissolved silica) moved through fractures into the surrounding country rock. Many quartz veins, especially those with gold, are in such close proximity to granitic bodies that other sources are rarely considered. Yet, in Arkansas, where we have the greatest concentration of collectable vein quartz in North America, no granitic rocks have been discovered associated with the deposits. In fact, what igneous rocks are in the region are very deficient in silica. So what is the source of the silica to form the veins? Several lines of evidence lead to it being one of metamorphic sweat out of water, silica, and some of the more mobile metals in the metamorphic environment, like antimony, mercury, lead, and zinc. The reason no gold, other than typical traces, has been discovered in Arkansas is that there was no gold of any consequence in the original sediments that were metamorphosed. Miners have been digging quartz in this state for well over 100 years and they have never reported a trace of visible gold. Not like in California, Colorado, and the western states where deposits of quartz associated with granitic rocks often contain gold!
Quartz is present in many metamorphic rocks
At low grades of metamorphism, quartz is only slightly mobile unless the rocks are water-saturated. Then, along with water, silica is relatively mobile. In fact, metamorphism may be viewed as a dewatering process. At lower grades, water and silica are expelled while at higher metamorphic grades water-bearing minerals are dehydrated (like micas). At higher metamorphic grades, quartz not oriented properly to the pressure is dissolved and those grains with the correct orientation grow. Quartz augen or eyes form in this manner. In gneisses, quartz actually separates into bands which are seen as light-colored bands alternating with dark bands of mafic minerals. Much silica along with water is released during reactions that take place at the higher grades of metamorphism. Rarely are collectable crystals reported from metamorphic rocks, but they may be the source of many hydrothermal-appearing veins.
Host rock
Although any type of rock may make a favorable host if holes or voids are present, limestone and dolostone commonly have secondary deposits of quartz crystals which fill the void space. Some highly vuggy lava hosts major deposits of crystal. Warm to cool silica-saturated water may deposit any of several varieties of quartz, ranging from quartz crystal to amethyst, agate, chalcedony, or opal, depending on the conditions. Literally thousands of geodes containing millions of quartz crystals have been collected from the sedimentary rocks near Keokuk, Iowa. These geodes were deposited as quartz linings in solution cavities, but never as large crystals. They make attractive specimens as aggregate clusters, but no gem-grade quartz is present. The major deposits of rock crystal in Herkimer County, New York, occur as infillings in pockets in dolomitic limestone. The so-called herkimer diamond deposits of central New York are one of the more important deposits of rock crystal in the world. The individual crystals have high luster, are doubly terminated, and nearly equidimensional in size. They occur in an extremely dense hard dolostone -- the Little Falls formation, a Cambrian age unit. Small irregular solution pockets are scattered throughout the sedimentary unit, with larger pockets being restricted to a zone termed by miners as the "pocket layer". The pocket layer is overlain by a mud seam which is recognized as a location marker by the miners. The pockets run as large as 4 feet in diameter, but average closer to 3 feet across in this layer, which is some 18 inches thick. Each pocket encountered varies in crystal content, crystal size, and quality. The better pockets may be round in plane view and are domal in cross-section, like a tire that is cut in half. Space between the roof and floor of any individual pocket varies. The floor of the pocket is usually domed upwards. Individuals and clusters of individual crystals that have coalesced together are encased in wet clay in fresh pockets, many as loose "floaters" not attached to the matrix rock. Herkimer crystals may be on the floor, ceiling, and in the clay pack. Crystals from these pockets must be allowed to warm up slowly because when collected they are ground temperature and, if heated too quickly, will break from thermal expansion. Specimens can be ruined in this manner from any collecting site, but herkimer diamonds appear to be particularly susceptible to this type of damage. Several other minerals may be present, including dolomite and calcite crystals. Very interesting infillings of anthraxolite, a type of dead petroleum, may coat the walls and predates the infilling of the cavities by the clay pack. Sometimes smoky quartz crystals are recovered from these deposits, the coloration due to finely divided anthraxolite captured during quartz crystallization. Sometimes larger spots of anthraxolite are seen in herkimer diamonds. Some pockets have no clay, but a solid pack of anthraxolite and are known for very brilliant-lustered high-quality crystals.
Other locations of rock crystal
Other than the locations we've mentioned above, several other localities for rock crystal should been mentioned. Brazil, especially in the states of Minas Geraes, Sao Paulo, and Goyza, contains commercially important deposits of rock crystal. Two types of deposits are important -- primary hydrothermal and alluvial gravels. The primary hydrothermal deposits occur as pockets in veins which fill fractures. The weathering and transportation of crystal from primary vein-type deposits result in well-rounded frosted, but optically clean, alluvial gravels. This quartz gravel was the source for many years of quartz for optical and electronic applications as well as being used to manufacture the seed blanks for growing synthetic quartz crystals. In South America, deposits of rock crystal are mined in both Columbia and Bolivia for specimens. Pockets of both rock crystal and smoky quartz from the Swiss Alps are also notable, having been collected for over 200 years. They are called "alpine cleft" deposits and sometimes yield thousands of pounds of high-lustered prismatic crystals. A famous characteristic crystal form of these deposits is called a "Gwindel". A gwindel is a crystal that displays a rotational twist, either right or left, along its C axis. Since the mid-1930's Madagascar deposits have yielded rock crystal for both the collector and industrial uses. In the 1970's and 1980's the Japanese developed extensive deposits of rock crystal in various countries for electronic applications and as a feedstock for synthetic quartz production.
Mike has a geologist colleague who maintains that his idea of dying and going to heaven is to understand the Ouachita mountain building process
Mineralogy of Quartz Crystals
ENTIRE BOOKS have been written on quartz and its many varieties. Here we'll only discuss the properties of crystalline quartz and end with a list of what we know about Arkansas quartz vein formation in the Ouachita Mountains.
Although quartz has a unique structure, so do many other minerals. That in combination with the chemistry is what makes a mineral a mineral.
We will begin with some basics: To meet the definition of a mineral, quartz must be composed of an orderly arrangement of certain elements, so that we may describe its internal structure and present its chemistry by a representative formula: SiO2.
Here is a hand-sized piece of metallic silicon. This metal combined with oxygen makes quartz crystals: silicon dioxide.
Any mineralogist would agree with me when I say that quartz is the most diverse species in terms of varieties, shapes and forms for a single mineral species. The feldspars or the pyroxenes and amphiboles include a whole host of minerals with similar structural characteristics, but variable chemistry. Quartz certainly has the most COLLECTABLE varieties of any single species.
Silica
We know that quartz is the low-temperature stable form of silicon dioxide or silica. Several other forms of silica exist at higher temperatures and pressures. Quartz forms over a temperature range, the upper limit of which is 867°C at one atmosphere of pressure. Think of the earth being a giant pressure cooker, different things happen at high temperatures and pressures than what we see on the surface around us.
For your own data base of trivial knowledge, the temperatures of an industrial blast furnace where iron is processed is about 400°F (200°C) at the top of the furnace, and near the bottom it is about 3,000°F (1,650°C) or higher. So for making crystals, we see the conditions are much hotter than a pizza oven, but less than a steel mill. For information about how man-made crystals are grown in industrial autoclaves, read Synthetic Crystals.
Alpha and beta quartz
Two forms of quartz exist, alpha-quartz (the kinds of crystals we all have in our collections) which is stable from the low end of the temperature range up to 573° C and beta-quartz, the high temperature stable form from 573° to 867° C. The actual temperature that alpha-quartz can form at depends on the pressure in the system. The higher the pressure, the higher above 573° C that it may crystallize from the fluid system. Our crystals won't melt unless you chunk them into a foundry, but they can fracture from thermal shock. (See notes about cleaning quartz.)
Now most of these numbers won't mean anything to you unless you try to figure how the world was made. But some people do, so here's a little more. Crystalline quartz may be described as alpha-quartz (low quartz) or beta-quartz (high quartz). Alpha-quartz forms at temperatures lower than 573° C at one atmosphere pressure, where beta-quartz forms at the temperatures above 573° C and lower than 867° C. at the same pressure. If the pressure increases, so may the temperature of formation of both alpha- and beta-quartz. For example, at about 2 miles in depth, alpha-quartz may form at as high as around 600° C and beta-quartz at over 1000°C. These conditions may exist in our present world today at the margins of the continental plates in subduction zones or at a depth of 2 miles below where you happen be reading this article.
Beta-quartz is relatively uncommon, most occurrences being confined to rhyolite lava flows where the mineral "froze" in the rapidly cooled rock. Examples of beta-quartz from rhyolitic lava flows appear as small equidimensional crystals floating in the fine-grained (and rapidly cooled or quenched) matrix. At room temperature, beta-quartz is meta-stable, that is it will, given geologic time and some energy, invert or change its internal structure to that of alpha-quartz.
All the quartz from Arkansas is alpha-quartz, so from here on we'll simply call it quartz. Studies on rock crystal from Arkansas indicate a range of temperatures of formation, from as low as around 200° C to about 265° C. Note that this is well above the boiling point of water at atmospheric pressure so there was some confining pressure due to depth of burial. Perhaps as much as 2 miles of sediment and rock overlay the formations which contain the bulk of the quartz deposits when the veins were forming. These formations have been exposed on the surface by over 200 million years of erosion.
Quartz forms in a variety of geologic environments. To learn about most modes of rock crystal formation, see the discussions in the Ask Mikey section.
Physical Properties of Quartz
Quartz has several unique physical properties:
Cleavage
Although quartz has the most cleavage directions of any mineral (7), these are rarely seen in nature. In the laboratory, cleavage can be induced by either electrical or thermal shock in oriented plates cut from natural quartz crystal.
Fracture
Fracture is simply the manner in which a mineral breaks when cleavage is not well developed. Quartz has a well developed fracture which mineralogists call conchoidal, meaning shell-like. The mineral fractures equally well in any direction. If you look at the broken edge of a piece of glass, you will see conchoidal fracture. This property was recognized by early man as very useful one. With some practice, anyone can learn how to control conchoidal fracturing. Once prehistoric man mastered the chipping of quartz and learned the technique of making projectile points using chipping (controlled conchoidal fracturing), he gained a degree of independence. He could simply carry some basic materials with him and as he needed them, he could stop and make some more tools for hunting. However, flint and chert, both microcrystalline varieties of quartz, are more readily available and easier to chip than rock crystal. Careful working by early Oriental artisans involved the fracturing of large blocks of rock crystal to attain a roughly rounded shape before grinding in a trough with water and sand to smooth the piece into a sphere.
Hardness
Due to its internal structure, quartz is equal hardness in all directions. At 7, it also is the hardest of all the common minerals on the Moh's hardness scale. This hardness explains why it is the most common detrital (a product of disintegration and/or wearing away of a rock) mineral in sediments. Since it has no cleavage and is pretty hard in all directions, it does not get abraded very rapidly during transport.
Remember the Moh's Scale of Hardness? Here's the jingle:
The girl could flirt and flirt quickly though Connie didn't.
Talc Gypsum Calcite Fluorite Apatite Feldspar Quartz Topaz Corundum Diamond
Doesn't easily dissolve
Quartz is insoluble in most fluids. Note that I said in most fluids, like normal ground water. However, in carbonate-rich water and in very salty water with a lot of chlorine and sodium, quartz is somewhat soluble, especially if the water has a little heat also. Quartz from the Ouachita Mountains formed from hot water, expelled from some depth during and shortly after the mountain building processes were active.
Cool stone, dude
Quartz is a good conductor of heat. Ancient peoples were well aware of this property. Objects and spheres carved from quartz always feel cool when touched or held, even in the heat of the day.
Piezoelectric property
The piezioelectric effect was first observed in the laboratory. Several minerals, including tourmaline and sphalerite, exhibit this effect. When you alternately apply and release pressure on a quartz crystal, during the pressure changes on the structure a small amount of electricity is released. So by applying cyclic pressure, a current may be generated. Conversely, when a small amount of electricity is applied to a crystal, the internal structure vibrates. This is the principle involved in the manufacture of new highly accurate generation of quartz watches and quartz tuners on stereo systems.
During World War II, very pure, untwinned pieces of quartz were in high demand for radio oscillators. The term crystal in CB radios was first used in the electronics industry for quartz crystal wafers, although now substitutes have replaced quartz. By cutting the wafer at a certain angle to its C crystallographic axis, we can control the frequency of the vibration. The original crystals in CB radios were cut from wafers of quartz, each having a specific frequency. This determined the frequency of the band for broadcasting and receiving. Very handy!
See experiments with quartz crystals
Triboluminescence
Luminescence is defined as the emission of visible radiation due to some external cause other than heat. Triboluminescence is light that is produced by pressure, friction, or mechanical shock. It may be readily demonstrated with two hand-sized milky quartz crystals in a darkened room. Simply take the prism edge of one crystal and rub it back and forth on the prism face of the other crystal. You may simply rub two prism faces together, but you get more light using the former method. This makes a good classroom demonstration!
Catholuminescence
Cathodeluminescence is a distinctive visible color that is emitted by
bombarding a small piece of quartz with cathode rays. This must be done in a vacuum to best see the visible color. Trace elements influence the cathodeluminescent color of the mineral.
Star of the C axis
Asterism is the last property I will mention. Asterism is not present in all quartz specimens. To see this property exhibited the specimen is best cut into a sphere or at least a high domed cabochon. In alpha-quartz that forms at higher temperatures there may be other chemical compounds that are "dissolved" in the structure. As the mineral cools, the dissolved material exsolves out of the quartz structure into discrete mineral particles. In the case of asteriated quartz, the dissolved material is thought to be very small amounts of TiO2. When it exsolves, it becomes oriented along the three principal A Crystallographic directions. These lie in a plane at right angle to the C axis and each of the 3 A axes are at 120 degrees to each other. When light shines on a sphere or is reflected back through a sphere of quartz that exhibits asterism, there is the appearance of a sharp 6-rayed star when the sphere is properly oriented. You will need to rotate the sphere around until you see the star, then you are viewing down the C axis. Asterism is present in many minerals, particularly gemstones of the Hexagonal system, like ruby or sapphire.
Crystal structure
The basic building blocks of a quartz crystal are silica tetrahedra. In quartz these tetrahedra are linked corner to corner to build up the crystal. During this linking or bonding the overall structure may twist to the left or right as we view the crystal vertically along the C axis. Because a quartz crystal's structure twists either left or right, we term this property enantiomorphism, a fancy term for right- or left-handedness. The term simply means that their respective structures are mirror images of each other. With close examination of the external form of a quartz crystal and a knowledge of what growth faces are present, one may determine which form is present.
A silica tetrahedra consists of a single silicon atom linked to 4 equally spaced oxygen atoms. The tetrahedra are linked together in a ring-like manner in layers. The tetrahedra alternate in the structure - one with the point up, the next with the point down. These linked rings spiral around the C crystallographic axis in either a clockwise or a counter clockwise manner. This was discovered long before the advent of X-ray diffraction analysis of the structure by 19th century investigators observing the rotary power of various crystalline materials on light. John and Marie Curie were two early investigators in this field. Anyway, because the crystal structure rotates we see two crystal forms described by crystallographers as right- and left-handed crystals.
SO....We can deduce
Knowing about the physical properties of quartz can tell us something about the mineral's formation in veins. Having seen many veins in the field, I can use the physical properties and the field evidence to make the following statements about the conditions that existed at the time of quartz growth in the Ouachita Mountains:
Growth took place at some significant depth (1 - 2 miles).
The quartz grew from hot water solutions (>200 degrees C.).
The water was rich in dissolved silica and was salty.
During growth, earth movement and vein adjustment were both active.
There were certain sedimentary beds that were more favored for vein formation, due to open fractures. Sandstone beds were favored because they were 1) more fractured and 2) provided better nucleation sites for quartz to begin growth.
There were several periods of crystal growth in the veins over time.
Temperature generally decreased during the period of crystal growth.
Quartz veins may be either simple or complex in form, depending on the local geologic history. Quartz veins are more numerous in the tightly folded portions of the sedimentary beds than other areas. Veins containing rock crystal may extend for significant depth if a favorable host rock is present.
References:
Dana's System of Mineralogy, Volume III - Silica Minerals, by Palache, Berman, and Frondel, John Wiley and Sons, University of Chicago.
Handbook of Mineralogy, Volume II - Silica, Silicates (part 2), 1995, by Anthony, Bideaux, Bladh, and Nichols, Mineral Data Publishing, Tucson, AZ
The properties of silica, an introduction to the properties of substances in the solid non-conducting state, by R. B. Sosman, American Chemical Society Monograph Series Number 37, 1927, The Chemical Catalog Company, Inc., New York, J. J. Little and Ives Co., printers.
Electronic and Optical Materials by J. A. Ober in Industrial Rocks and Minerals, 6th Edition, D. D. Carr, Sr. Ed., 1994, SMME, Braun-Brumfield, Inc., Ann Arbor, MI