"AND THEY RAN
up hill and down
dale, knapping
the chunky stones
to pieces with
hammers, like so
many road
makers gone
daft."
"They say it is to
see how the world
was made."
___
Sir Walter Scott
Index to Quartz
Digging
Cleaning
Worth
Fee Pay Mines
Types
Forms
Inclusions
Geology
Mineralogy
Synthetic
Gemstones
Handedness
Experiments
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.
Faulted and rehealed crystal, Ron Coleman mine, Garland County, AR
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
See the state geology map. 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 orogeny (mountain building process)
Contact the authors of Rockhounding Arkansas Revised July 1998
©Rockhounding Arkansas 1998 http://rockhoundingAR.com