Quartz - most common mineral on the earth's surface
"Greedy politicians are like quartz - they come in many colors and are as common as dirt"
- W. Dan Hausel
Varieties of silica include: (1) quartz (macrocrystalline) and, (2) chalcedony (cryptocrystalline and microcrystalline). In this section, we focus on quartz.
When in veins (or geodes), “quartz” can occur as individual crystals, tiny clusters of crystals (drusy quartz), or as massive quartz with no apparent crystalline form. Quartz can be transparent, translucent or opaque. If vein quartz contains gold, we are impressed. If barren, “bull quartz” (a prospectors term for milky white, massive quartz) we may decorate our rock gardens with quartz. Quartz is the most common mineral on the earth's surface an found as tiny hexagonal crystals or giant crystals in some pegmatites! One large crystal of more than 600 pounds was found in Switzerland.
Some quartz veins have uncommon visible gold filling fractures. In other cases, the precious metal may be hidden in pockets of sulfide minerals (such as pyrite) in a vein. Gold can be visible or unseen in quartz, and it is never evenly distributed in veins.
Gold-bearing veins have ore grades that rarely average as much as 0.5 ounce per ton (opt Au). Today, many gold mines produce ore at average grades of 0.15 to 0.015 opt Au. Low grades veins must have considerable tonnage and other favorable characteristics to make them minable. The famous Homestake mine was considered rich. Yet during its early history, the ore averaged 0.3 opt Au. This gradually decreased to 0.15 opt Au until the mine closed when operations reached depths below 8,000 feet. It just became uneconomic to haul the ore from such great depths.
It is possible to recover high-grade, hand-sized, specimens enriched in gold, but these are restricted to pockets in ore shoots. After searching for gold and other minerals for more than 3 decades, the highest assay I ever got was just over 4 opt Au with more than 11 opt silver at Mineral Hill in the Black Hills (Hausel and Hausel, 2011), and this was from a channel sample dug across the entire width of the vein (because of the nature of the deposit such as a horizontal veins with associated breccia in an alkalic intrusive, this property should be drilled to depth to see what lies below the surface.
If I wanted to be selective and produce an assay with considerable gold, I would use a small, thumbnail-sized, piece of quartz with visible gold – but this would not provide an accurate representation of a vein. Still, many people actually do this in what is called, high-grading. The purpose is to promote properties. But still, there are others who simply invent gold assays.
High silver assays are more common, simply because silver is more common than gold. There are galena-rich and tetrahedrite-rich silver veins in the Kirwin district of the Absaroka Mountains east of Yellowstone. A few of these old mines yielded more than 100 opt silver from channel samples taken across veins by AMAX in the 1970s (Hausel and Hausel, 2011). Some of these would be considered economic at today’s silver prices, if it wasn’t for the US Forest Service piecemeal withdrawal of potentially commercial mineral deposits in the Absaroka Mountains in Montana and Wyoming; effectively closing off 10,000 square miles of public land from the public.
Some veins have visible gold, and some have gold that is not visible to the naked eye let alone with a microscope. Such invisible or ‘noseeum’ gold can be detected by assay: some invisible gold occurs as gold atoms replacing individual atoms within the crystal lattice of a sulfide mineral, such as replacing iron atoms in pyrite or copper and iron in chalcopyrite.
Pyrite, or fool’s gold, sometimes has considerable gold within its crystal lattice. There are reports of pyrite with as much as 1,000 to 2,000 parts per million (ppm) gold (as much as 70 opt) hidden within pyrite. Exploration geologists often assay pyrite because of this relationship, and in some cases, prospectors will crush massive pyrite to a fine greenish-black power and pan the powder for gold. Gold can occur as tiny inclusions in pyrite, or can substitute for iron in the atomic structure: in the latter case, one could only chemically detect gold in assay and no amount of panning would help. To recover such gold, would require roasting and cyanide leaching.
Quartz can occur as massive material and sometimes is found as excellent prismatic crystals. Less common are doubly terminated quartz crystals with pyramids at the terminations, such as those with a misnomer of “Herkimer diamonds” found in silicified dolomite at Little Falls, Herkimer County, New York. Some amethyst found at Thunder Bay, Ontario, was as much as 10 inches in diameter. Yet, much of the best and largest crystalized quartz is found in Hot Springs area of Arkansas.
On Moh’s relative hardness scale, quartz has a hardness of 7 and is thus less hard than diamond (H=10), corundum (H=9) and topaz (H=8). But quartz is harder than your car’s windshield, which has hardness of 5.5 to 6. This is why your windshield gets pitted whenever you travel through Arizona’s dust storms. Quartz will scratch and pit the softer windshield.
Color and Inclusions
Quartz is found in a multitude of colors such as colorless, white, red, orange, yellow, gray, brown, black, lavender, violet, purple, pink, blue and green. Essentially every color of the rainbow has been seen in quartz. When chemically pure, quartz is colorless. But like most gems, small amounts of chemical impurities give it color.
Various valence states of iron yield some color varieties in quartz; the most desired by collectors is purple to red-violet of amethyst, and lemon-yellow of citrine. Iron is responsible for the red, yellow and brown in agate and gives carnelian and jasper brownish-red and orange colors. Iron and/or manganese will produce brown and black in dendritic agate, traces of titanium is responsible for characteristic pink in rose quartz, and nickel gives chrysoprase an apple-green color.
Mineral inclusions in coarsely crystalline quartz may impart color as well as optical effects such as chatoyancy. Inclusions are simply tiny crystals enclosed by a larger crystal. Essentially all natural crystals have mineral inclusions: this is how I was able to show that Wyoming was rewarding employees with not-so-expensive 20 and 25-year pins presented to employees. Such lapel pins, touted as having valuable rubies, actually contain either one, or two, tiny, faceted, ruby. These contain no mineral inclusions (synthetic) and typically cost less than a few dollars each. Such rubies are produced and faceted for <$1.00 in India or Sri Lanka.
Reddish-brown hematite flakes and flakes of green fuchsite mica may yield a glittery metallic appearance (aventurescence) in quartz. Rutilated quartz (quartz with rutile inclusions) may exhibit yellowish-brown to golden-yellow color, but also copper-red and silver-gray color. Although rutile in quartz is relatively common, rutile in highly transparent quartz is uncommon.
Gold has always been noted to have an affinity for quartz to such a degree prospectors almost always dig quartz veins in a search for the precious metal. Striking specimens of gold-in-quartz have been recovered from the Badger mine, Mariposa County, California, the Sixteen-to-One mine in Sierra County, California, the Potato Patch in Arizona, and at several mines in Canada and Australia.
Gas-fluid inclusions can also occur in natural quartz. Analysis of these inclusions provide a wealth of information about pressure, temperature, and chemical environment in which the quartz crystallized. Such inclusions are typically formed of gaseous and liquid carbonic acids. Other fluid inclusions may contain carbon dioxide, water, salt, petroleum fluids. Both gases, and mineral solids are found as inclusions. Doubly-terminate Herkimer quartz crystals have been known to host amorphous hydrocarbons as inclusions.
Quartz has a relatively low specific gravity (2.65). This indicates that even though it is more than twice as heavy as water, it is light enough that it can easily be washed out of a gold pan. As a comparison, gold has a specific gravity of 15 to 19.3, which is why gold is difficult to pan out, unless you’ve drink too much coffee.
Quartz does not have cleavage. In other words, you are not going to break quartz along distinct planes like a diamond or ruby. If you strike quartz with a chisel to try to cleave, it will break into many fragments with conchoidal fracture (like broken glass).
Rock crystal is colorless quartz. Because of its low refractive index (refractive index is the degree that light is bent when passing through solid material) and glassy appearance, quartz is seldom used as gems unless it has attractive color. It is unlike diamond which has high refractive index. Quartz exhibits characteristics similar to synthetic glass and has few properties that would make it attractive for adornment other than color. Some colorless, raw, quartz crystals are periodically used in earrings and necklaces due to the hexagonal prisms that are visually interesting.
Amethyst is colored quartz and ranges from mauve to deep violet. The coloring agent for amethyst is small amounts of iron (Fe3+) distributed in the crystal layers. Thus in some cut amethyst, clear bands of quartz are seen alternating with colored bands. Amethyst is one of the more popular gemstones, and is even mentioned in the Bible as one of 12 sacred stones.
Citrine is pale- to dark-yellow, brownish-yellow, or honey-yellow quartz with russet tint named after its resemblance to citrus lemon. It has been mistaken for topaz in the past and erroneously given names such as ‘topaz quartz’. Even so, citrine can be distinguished in both faceted and natural form from topaz, by refractive index and specific gravity. In hand specimen, crystal habit and cleavage can also be used to distinguish topaz from quartz. The specific gravity for citrine (and feldspar gems) is the lowest for yellow, transparent gemstones, whereas topaz is notably higher. The luster of quartz is slightly inferior to topaz and topaz may show signs of incipient cleavage that is non-existent in citrine. The coloring agent for citrine is iron. Most citrine used as gemstones are transparent to translucent.
Smoky quartz varies from black to brown to smoky yellow and grades to citrine. The dark color of smoky quartz is thought to be caused by radioactive damage during exposure to radiation. Upon heating, smoky quartz will turn colorless, and return back to a smoky appearance with exposure to radiation. The best-known locality for smoky quartz is the Swiss Alps, where veins have yielded many tons of beautiful crystals. Other notable localities are Russia, Brazil, Madagascar, and Scotland. In the US, smoky quartz is reported in the Pikes Peak region of Colorado, and at various localities in Maine and New Hampshire.
Rose quartz often occurs as coarse-crystalline anhedral (formless) quartz that varies from pale pink to deep rose-red, which fades with exposure to sunlight. Rose quartz is seldom transparent and instead is turbid. Its color is thought to be due to trace titanium. In some rose quartz, microscopic needles of rutile are oriented in three directions at 120o from one another, and at right angles to the c-axis of the crystal. When cut into an oriented cabochon, such rose quartz will show distinct, 6-rayed stars due to the light reflected rutile mineral inclusions.
There are several minor ornamental stones of chatoyant quartz. This quartz exhibits parallel, fibrous, mineral inclusions with wavy reflections when rotated in light. Chatoyancy is caused by the presence of thin, hollow, tubes known as etch tubes. Chatoyancy is also seen in Tiger’s Eye, a variety of cryptocrystalline quartz.