Index to Quartz
Digging
Cleaning
Worth
Fee Pay Mines
Types
Forms
Inclusions
Geology
Mineralogy
Synthetic
Gemstones
Handedness
Experiments
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. Isn't chemistry amazing?
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.
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
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
If you're a real glutton for punishment, read our nine-part illustrated series of crystallography articles!
Contact the authors of Rockhounding Arkansas Revised October 1998
©Rockhounding Arkansas 1998 http://rockhoundingAR.com