Bitterroot Gem and Mineral Society

QUARTZ

by Wayne Farley, Bitterroot Gem & Mineral Society

Quartz:  Quartz (SiO2) is the most common mineral of over 4000 different minerals in the Earth’s crust, thus it is logical to pay it great deal of attention.  Quartz crystallizes in the Hexagonal System, Trigonal-Trapezohedral Class.  Only rarely does Quartz outwardly shows its true symmetry by small Trapezohedral x- faces which modify the corners of the prism m-faces where the prism faces intersect the pyramid faces.  On a micron scale, Quartz is made up of a lattice of silica Tetrahedral (SiO4) arranged in right hand or left hand helixes.  Although Quartz is Pseudohexagonal, its unit cell is Rhombohedral.  Quartz has a hardness of 7 on the Mohs scale.  Pure Quartz (Rock Crystal) has a density of 2.69 g/cm³, while other varieties vary from 2.5 to 2.8 with a mean of 2.65.   Quartz is vitreous in luster; usually breaks with a conchoidal fracture, and rarely along cleavage planes (Ref.1).  It is found in nearly every geological environment; and is a component of almost every rock type, and frequently the primary mineral.   Many crystallographers say it is the most varied in terms of varieties, colors and forms; while others say calcite is the most varied.  Quartz forms in several different rare phases other than Normal Quartz (low-quartz), depending on temperature and pressure during formation. 

A Phase Diagram, as indicated below, is the simplest way to show these phases.

Low-Quartz is referred to as Alpha Quartz, or Normal Quartz, and forms crystals in the pseudo-hexagonal crystal system. The normal crystal shape (Habit) is a six-sided prism terminating with six polygons at each end, and is called a Prismatic Habit.  The side prisms are usually horizontally striated.  Alpha Quartz is the type that we usually see and collect, and is the only phase that will be discussed in detail in this report, e.g., varieties, colors and forms (habits).  Alpha crystals are seldom perfect.  They usually have attachment points, are intergrown with other crystals (quartz or others), or form in other habits.

High-Quartz is also called Beta Quartz.  Beta Quartz is unstable at earth’s surface temperature and pressure, and quickly brakes down to other minerals; or converts to octahedral paramorphs of Alpha Quartz after Beta Quartz.  A paramorph retains its same chemical formula.

Tridymite is a high-temperature polymorph of Quartz (SiO2) which paramorphs to Alpha Quartz at normal temperature, and usually occurs as minute tabular white or colorless pseudo-hexagonal triclinic crystals in cavities in acidic volcanic rocks.

Christobolite is another high-temperature polymorph of Quartz (SiO2) which paramorphs to Alpha Quartz at normal temperature.  It occurs as white octahedra in acidic volcanic rocks and in converted diatomaceous deposits in the Monterey Formation of California and similar areas.

Coesite and Stishovite are high-pressure microscopic polymorphs of SiO2 formed in nature under unique physical conditions; and are usually found only in meteor impactites; and in Eclogite nodules in Kimberlite.  History of Quartz:  The names and uses for the varieties of Quartz (flint, jasper, chalcedony, and rock-crystal) have changed over time since its first use by man, perhaps more than one million years ago.   Prehistoric man first used chips of flint for various cutting tools; then later shaped a variety of Quartz Family materials for making arrowheads, ax-heads, knives, etc..  Quartz (Agate and Rock Crystal) was first used by man for jewelry by Sumerians and Egyptians over 3000 years ago.  The early Greeks thought that rock crystal was water from deep in caves that had frozen so solid that it would not thaw. The first man to write about agates, a treatise on “Stones”, was Greek Philosopher Theophrastus, 372-287 BC, who was a great writer and popularizer of science (Ref. 5).  Early Greeks also believed that drinking wine from an amethyst cup would prevent intoxication.  The word “Agate” derives from the historic village of Achates, Sicily where agates were found in great quantities.  Later in history, the Roman Soldiers used Agate Cameo rings as signature rings, so that when they were killed in battle, their bodies could be identified.  For a complete history of ancient and modern lapidary use, see (Ref. 14).  Today, several states have crystalline Quartz as their state gemstone or mineral: Star Blue Quartz in Alabama, Quartz Crystals in Arkansas, Quartz Gemstone in Georgia, and Rose Quartz in S. Dakota.

Varieties of Quartz:  The tremendous variety of Quartz Family Rocks and Minerals are too numerous to completely cover in this report.  Therefore, in this report, I will concentrate on Macrocrystalline Varieties, and supply you with reference material for your own research on other varieties.  A complete bibliography of Crystalline Quartz reports and books (up to 1993) was produced by Sinkankas, 1993, with over 100 listed references (Ref. 2).  “The Nomenclature of Silica”, a 1927 13 page report by Gilbert Hart, which is on the web (Ref. 4), covers all varieties of Quartz.  A good beginner’s book on Quartz is “Quartz Family Minerals”, Dake, et al, 1938 (Ref. 5).  Used copies are available on the web from Amazon.   A couple of modern books on Quartz are those by O’Donoghue, 1987 (Ref. 6); and Dibble, 2002 (Ref.7).  The best reference on Cryptocrystalline or Microcrystalline Quarts is the University of Nebraska web-site (Ref 3), titled “Agate Lexicon and Glossary of Agate, Jasper, Opal and other Amorphous, Cryptocrystalline, or Spherulitic Forms of Gem Silicon Dioxide”.  As of September 10, 2007, there are 3,072 records in the Agate Lexicon.

Major Varieties of Quartz Families:
One of many lists of Quartz Varieties , from web-site “The Quartz Page” (Ref. 8), is as follows:
 “Quartz occurs in a great number of varieties that differ in form and color. It occurs as massive aggregates, dense nodules, or crystals in druses. Quartz is colorless if pure, but may assume any color due to inclusions of other minerals or built-in trace elements.
The more common of the varieties have been given their own names - on these web pages about 25 different varieties are distinguished. The different forms of quartz are usually classified in 2 big groups:

• Macrocrystalline varieties are those that form crystals, like amethyst, or have a macroscopical crystalline structure. When people talk about "quartz", they usually think of macrocrystalline quartz. “

Rock Crystal	Citrine	Prase	Rose Quartz
Smoky Quartz	Pink Quartz	Ametrine	Ferruginous Quartz
Amethyst	Prasiolite	Milky Quartz	Tiger's Eye
Aventurine	Cat's Eye	Blue Quartz	Hawk's Eye

• Cryptocrystalline or microcrystalline varieties that do not show any visible crystals and have a dense structure, like agate. Cryptocrystalline varieties are sometimes grouped together under the term chalcedony.

Chalcedony	Agate	Carnelian	Heliotrope
Flint	Plasma	Chrysoprase	Chert
Jasper	Sard	Onyx

 Macrocrystalline Quartz:
Crystal Classed by Color:

1. Rock Crystal:  Rock Crystal is the name for clear, colorless Quartz.  In a sense, rock crystal is the ideal image of Quartz.  Well known sources outside the U. S. are Minas Gerais in Brazil, Urals in Russia, Madagascar, Peru, and Nepal.  Famous U. S. sources are the Ouachita Mountains in. Arkansas and the Herkimer area in NY.  Montana sources are Crystal Park in Beaverhead County, and Granite Ridge in Mineral County. 

2. Smoky Quartz:  Smoky Quartz is brown to gray in color.  Black Smoky Quartz is called Morion.  Smoky Quartz is usually found in intrusive igneous and certain high grade metamorphic rocks, like granite and ortho-gneiss, as these contain traces of radioactive elements whose radiation cause the coloration.  The color is produced by missing electrons (trapped holes) on oxygen atoms adjoining aluminum atoms that have replaced silicon atoms in the tetrahedral sites in the structure Ref. 11).  Smoky Quartz will lose its color if heated, and get it restored if reradiated.  There are numerous sites around the world where Smoky Quartz can be found, including those listed for Rock Crystals.  In the U. S., very good smoky quartz also comes from the Pikes Peak region in Colorado; the Boulder Batholith near Butte, MT; and the Idaho Batholith in ID and MT.

3. Amethyst:  Amethyst is violet to purple in color, and owes its color to high energy radiation (e.g. gamma rays from radioactive sources) and the presence of iron built into its crystal lattice.   Amethyst is usually synonymous with Brazil-Law-Twining.   Brazil-Law-Twining is Quartz with approximately equal portions of right-hand and left-hand tetrahedral helical oriented along the c-axis. Yellow (Citrine) Quartz found in Brazil with Brazil-Law-Twining will turn to Amethyst when exposed to ionization radiation, e.g., gamma rays or x-rays.  It will revert back to Amethyst if heated (See Citrine). The most famous and commercially important Amethyst locations are in volcanic rocks in Brazil and Uruguay.  Amethyst is also found in igneous rocks, such as Crystal Park in MT; and in metamorphic rocks, such as the northwest shore of Lake Superior near Thunder Bay, Canada.  Most Amethyst will pale if exposed to sunlight.
   
   Prasiolite is an artificially greened Amethyst.  Some Amethyst turns green at a heated temperature of about 932 degrees F.  It gets its name from the leek-green colors it achieves.  The best Amethyst for greening comes mostly from the Montezuma Mine in the state of Minas Gerais, Brazil.

4. Citrine:  Citrine is transparent yellow in color.  Natural citrine is very rare, in fact almost all the material offered on the market under that name is not natural citrine, but heat treated Amethyst; heated to over 800 degrees F, and then slowly cooled.  The color of natural citrine is caused by natural radiation, and traces of iron and aluminum built into its crystal framework.  Citrine occurs in the same environments as Smoky Quartz; mostly intrusive igneous rocks which contain traces of radioactive elements whose radiation cause the coloration.  Good natural Citrines come from Brazil, the Congo, Kazakhstan, Madagascar, Namibia, and Russia.  Natural Citrine in the U. S. is very rare, however, a small amount has been found in the Peterson Mountains along the California/Nevada border.
   
    The unusual color variations of violet and yellow found in Ametrine is due to the presents of iron in different oxidations states within the crystalline structure.  The most well-known natural Ametrine source is the Anahi mine in Bolivia.  Manmade Ametrine has also been made.

5. Sagenite:  Sagenite is one of the four Quartz varieties above with included mineral fibers or laths.  Sagenite is usually named from the mineral included; thus, if Rutile (Titanium Oxide) is present, it is called Rutilated Quartz, or, if Tourmaline (Sodium Iron Aluminum Boro-silicate Hydroxide) is present, it is called Tourmalinated Quartz, etc.  The above mineral inclusions are the most common; however, other included minerals may be Sphene (Titanite), Hornblende, Epidote, Actinolite, Goethite, and many others.  Hyrsl & Niedermayr (Ref. 9) list 130 minerals that have been included in Quartz.  Rose-Quartz may get its color from included fibers composed of an unknown mineral similar to the mineral Dumortierite (Ref. 5), and these inclusions may also account for the six-rayed stars in polished spheres of Rose-Quartz.

Forms of Macrocrystalline Quartz:  The forms & symbols (symbols in parentheses) are:

Positive Rhombohedron (r)               5. Trigonal Bipyramid (s)             2.  Negative Rhombohedron (z)        6. Positive Trapezohedron (x)             3.  Hexagonal Prism (m)                   7. Steep Rhombohedra (M) 4.  Basal Pinacoid (c)                        8. Normal crystals are combinations of r, z, and m.

Habits of Macrocrystalline Quartz:

1. Normal Habit or Prismatic Habit: Parallel sides, Termination – 3 Heptagons & 3 Triangles.
2. Trigonal Habit: Rhombohedron termination of 3 irregular pentagons.  Most Amethyst.
3. Hexagonal or Pseudohexagonal Habit: (Cumberland) Six even triangles on termination.
4. Dauphine Habit: (flute beak) A rhombohedron heptagon face dominates the termination.
5. Tessin Habit: Hexagonal prism gets continuously thinner towards the tip.
6. Transitional Habit: Normal sides & Tessin tip.
7. Binntal Habit: Stubby, barrel-shaped.
8. Muzo Habit: (Lemurian, Lasers) Tapering hexagonal base to triangular top.
9. Cubic Habit: Rhombohedron tip & no side prisms.

Twinning:  Quartz has only 3 types of common twins; 1. Brazil Law Twins, 2. Japan Law Twins, and 3. Dauphiné Law Twins.  There are complex methods to determine if and how a Quartz Crystal is twinned, but a rockhound has to rely solely on crystal morphology.  About half of all Quartz Crystals are twinned (Ref. 12). The four general properties of Quartz Crystals that help identifying twins are 1. Positions of the x and s faces.  2. Surface patterns caused by etching.  3. Oriented indentations on the surface.  Most Amethyst Crystals are Brazil Law Twins.  Japan Law Twins are characterized by their Y configuration, with the V portion always flared exactly at 84.003 degrees.

Optical/Electrical Properties:  Quartz is Birefringence (double refractive).  Complex optical and electrical methods are also used to determine twinning.  Brazil twins are optical twins: Light waves that pass through an untwined right or left Quartz Crystal along its c axis change the orientation of their oscillation plane in either left or right direction.  In a Brazil Twin, the effects of the right and left parts of the twinned crystal will at least partially cancel out each other, and if the left and right quartz portions are equal, the light will pass apparently unaltered.  Dauphiné twins are electrical twins: their optical properties are similar to untwined crystals, but mechanical pressure along the a-axis does not cause an electrostatic polarization of the crystals.  As a result, a commercial process of manufacturing pure, flawless, electronics-grade Quartz was developed, and about 200 metric tons is produced each year.

Unusual Quartz Crystals or Formations:   This section covers those aspects of quartz morphology that are not covered in the descriptions of crystal forms, habits, and twinning, and also covers those related to the overall appearance of crystals or crystal aggregates.

1. Quartz Polymorphs:   The polymorphs of Quartz are: 1.Pseudomorphs, 2. Paramorphs, 3.Parimorphs, and 4. Endomorphs:  A Pseudomorph is a mineral whose outward crystal form is that of another mineral.  It has developed by alteration or substitution, e. g., Quartz after Fluorite.  A Paramorph is a mineral whose internal form has changed, but without change to its chemical composition or external form, e.g., Alpha Quartz after Beta Quartz.  A Perimorph is a mineral that encloses another mineral (encrustation), with the underlying mineral being called an Endomorph. (Ref.15)
2.  Dendritic:  Dendrites are tree or bush like growths within Quartz, most often formed by manganese oxide or manganese oxides.  Some of the other manganese oxides that form dendrites are in the Hollandite Group of manganese oxides. They are Coronadite, Cryptomelene and Hollandite. Two other manganese oxide minerals that form dendrites are Romanechite and Todorokite.
3. Cave Sites:  Cave speleothemes, i.e., stalactites and stalagmites, are frequently found in caves with the speleothemes coated with Quartz Crystals growing at right angles around the structures.  Quartz has been found in Kartcher Caverns, Arizona in three separate varieties, all located along fault zones: 1. stubby, prismatic Quartz Crystals on quartz boxwork, 2. prismatic Quartz Crystals replacing limestone bedrock and 3. Bird-nest like needle crystals on cave walls and ceilings (Ref. 13).
4. Phantoms:  An outline of the shape of a smaller crystal that is visible inside a crystal is called a ghost or a phantom (Ref. 8).
5. Inclusions:  There are basically two ways inclusions can "get into the crystal": 1. the minerals have formed before the Quartz.  The growing Quartz Crystals engulf them and the original form of the included minerals is preserved. These inclusions are called protogenetical.  Included fibers than run through the entire crystal at random orientations are typical examples, 2. Quartz Crystals and the included minerals grow simultaneously. These inclusions are called syngenetical. The shape of the included minerals often deviates from the typical forms and habits that develop during unhindered growth Ref. 8).
6. Skeletals:  In skeleton quartz the edges grew more quickly than the faces, so the edges stand out like the frames of a window.  Crystals that grow very quickly often develop skeletal growth forms (Ref. 8).
7. Enhydros:  Also called Hydrolite or Water-Stone.  Enhydro means "water within". These are crystals that contain pockets of liquid, trapped inside the crystal during its formation.
8.  Rainbows:  Quartz crystals often show cracks running through them, usually caused by mechanical stress, and these sometimes show rainbow-colored reflections (Ref. 8). 
9. Scepters:  Scepter quartz is a Quartz Crystal that has a second generation crystal tip sitting on top of an older first generation crystal.  The second generation tip typically becomes larger than the first generation tip, but might also become smaller.  A scepter can be shifted sideways and does not need to be centered on the first generation tip (Ref. 8).
10. Geodes:  The world’s largest Amethyst Geode removed in one piece was found on 01-30-07 in the Galerias Santinos Mine in Uruguay.  It is 3.26 meters high and weighs 3 tons.
11.  Crystal Size:  The largest single Quartz Crystal which has been substantiated was found at Mancho Felipe, near Itapore, Goiaz, Brazil in 1962; and was 20 feet long, 5 feet across a prism face, and was estimated to have weighed over 44 tons.
12. Fluorescence:  Rock-Crystals are not fluorescent when exposed to ultraviolet (UV) light; thus, fused rock-crystal is used for windows in Black-Lights that are used for finding fluorescent minerals.  Quartz can have mineral inclusions that are fluorescent, and these will show through the transparent Rock-Crystal.  Opaque Quartz may have coatings of fluorescent minerals that will fluoresce when exposed to UV light.  Rock-Crystals are Triboluminescent; and will emit light, from within the crystals, when struck together.

Modern Uses of Quartz:  Crystalline Quartz is used as semiprecious gemstones, e.g., amethyst, citrine, rose quartz, smoky quartz, etc.  The Cryptocrystalline forms are used for jewelry, e.g., agate, jasper, onyx, carnelian, and chalcedony.   Both forms are collected for their natural beauty, or unusual formations, and for metaphysical divination.   Because of its piezoelectric and pyroelectrical properties; quartz is used for pressure gauges, oscillators, resonators, and wave stabilizers.  Because of its ability to rotate the plane of polarization of light, and its transparency in ultraviolet rays; it is used in black-lights, heat-ray lamps, prisms, and spectrographic lenses.  Quartz is used in determining geological morphology, and for thermo-luminescent age dating of rocks.  However, its biggest use, by volume, is in concrete, glass, paints, abrasives, refractories, and a source of silica for electronic chips.

References:

1. http://en.wikipedia.org/wiki/Quartz
2. “Gemology, An Annotated Bibliography”, John Sinkankas, 1993
3. http://snr.unl.edu/Data/agatelexicon.asp
4. http://www.minsocam.org/MSA/collectors_corner/arc/silicanom.htm, 1927
5. “Quartz Family Minerals”, H. C. Dake, et al, 1938
6. “Quartz”, Michael O’Donoghue, 1987
7. “Quartz”, Harold L. Dibble, 2002
8. http://www.quartzpage.de/intro.html
9. “Magic World: Inclusions in Quartz”, J. Hyrsl and G. Niedermayr, 2003
10. “The John Sinkankas Quartz Symposium”, John Sinkankas and Si Frazer, et al, 2003
11. “Friends of Mineralogy, Tucson Gem and Mineral Society, Quartz Symposium”, 2005
12. http://www.minsocam.org/MSA/collectors_corner/arc/qtztwin.htm, 1949
13. “ Cave Minerals of the World” Carol Hill & Paolo Forti, 1997
14. http://www.rockhoundingar.com/quartz/qtzgems01.html
15. http://www.encyclopedia.com/doc/1G1-107757209.html

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