Quartz | Common Minerals - University of Minnesota

Quartz, with its diverse range of varieties, has significantly impacted human civilization, creating a complex web of terminologies that can be overwhelming. The term "quartz" is believed to stem from German or Saxon origins, though its exact meaning remains elusive.

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In the early days of humankind, various microcrystalline forms of quartz played a pivotal role in advancing societal development. The many forms of chert were among the first materials that early humans used beyond wood and bones to fashion tools. By striking chert, a conchoidal fracture formed sharp edges, allowing our ancestors to create effective tools through systematic knapping. Remarkably, some of these ancient tools remain sharp even after millions of years, demonstrating quartz's inherent stability. Eventually, humans also began to utilize obsidian, a volcanic glass, but during the Neolithic period, chert remained the primary material for stone tool crafting.

Today, quartz is just as prevalent in our lives. Glass, one of its most recognized products, exemplifies its versatility, but its applications extend far beyond that. Quartz is utilized in metallurgy as flux, as an abrasive in refractories, and as a filler. High-temperature crucibles are made of fused silica, while pure quartz sands are integral in the production of glass and glass ceramics. Additionally, quartz-rich materials like quartzite and quartz sandstone serve as essential building materials, and certain colored quartz varieties are prized as ornamental and semi-precious stones. Notably, a significant portion of mined quartz is employed in construction—used as aggregate in concrete and sand for mortar and cement—making quartz a foundational element of modern infrastructure.

Due to its robust physical properties, ground quartz also finds a place in stone cutting, sandblasting, and even scouring soaps. Its chemical stability means that crushed quartzite fragments are ideal for use as ballast in railway lines and highway shoulders. Pure, fine quartz sands serve as vital filtration media in water purification systems, effectively removing impurities without reacting with water. Interestingly, many marine plankton species, such as diatoms, create their shells from quartz. Upon their death, these tiny organisms sink and form layers of porous diatom shells on the ocean floor, known as diatomaceous earth. This material has unique applications in filtering high-quality wines and is a common abrasive in toothpaste. While polishing one’s teeth, individuals are actually using these quartz-derived shells, which are harder than the minerals that compose our teeth.

Another captivating aspect of pure quartz crystals is their piezoelectric property, meaning they generate an electric voltage when subjected to pressure. This feature enables the application of quartz crystals in measuring pressure and regulating electrical impulses, thereby paving the way for their incorporation in radio technologies and timekeeping devices.

Quartz veins have historically served as critical hosts for precious metals like gold, attracting many mining endeavors. However, from a historical viewpoint, one of quartz's most vital applications has been its role in fire-starting. The combination of striking chert against iron produces enduring sparks, a reliable fire-starting method used since prehistoric times, well before the invention of matches and even and the flintlock firearm mechanisms used in colonial times.

The hardness and variety of quartz make its chert forms ideal for use as gemstones. Varieties such as jasper, tiger's eye, amethyst, and citrine are commonly fashioned into jewelry. The term "onyx" can refer to both a banded quartz variety and a banded calcite variety, so caution is essential in understanding the difference. While both are utilized decoratively, their applications differ significantly—calcite onyx is softer and more readily carved, while quartz onyx is favored for jewelry due to its superior durability.

Opal, a widely recognized gemstone, is often mistakenly identified as a form of finely crystalline quartz (chert). Although opal consists primarily of SiO2, its structure contains water [SiO2-(H2O)n], distinguishing it from true minerals. Technically, opal is classified as a mineraloid—an amorphous solid lacking a defined crystal structure. While some minerals are also composed solely of silicon dioxide (SiO2), many are stable only under high-pressure or high-temperature conditions, rarely found at the Earth's surface. Although opal is neither a variety of quartz nor a true mineral, it is often regarded as both within the jewelry trade.

Volume 30, pages 219-229

GEOLOGY OF QUARTZ CRYSTAL DEPOSITS

RICHARD E. STOIBER, CARL TOLMAN, AND ROBERT D. BUTLER

INTRODUCTION

Before the conflict, quartz crystal was a relatively minor nonmetallic resource. However, the demand for sizeable clear single quartz crystals, utilized in quartz oscillator plates for radio frequency control during wartime, underscored the strategic significance of this mineral. The availability of high-quality quartz crystal is limited, and scant literature exists regarding the geology of these deposits, although significant research transpired during wartime. This paper aims to discuss the geological and mineralogical aspects of quartz crystal deposits and to highlight the interplay between quartz crystal geology and supply issues in Brazil.

SIZE, QUALITY STANDARDS, AND THEIR IMPACT ON SUPPLY

The precise size and quality requirements for economically usable quartz crystal remain somewhat ambiguous, influenced by manufacturing methods, fluctuating economic factors like price, and user preferences. Crystals ideally weigh around 100 grams (approximately 4 oz.) to 200 grams (about 42 lbs.); the shape becomes crucial in pieces weighing less than 200 grams. While weights above and below these parameters are also utilized in the oscillator plate industry, larger crystals tend to be challenging to process and more costly. In contrast, smaller crystals require additional labor for use but are less effective in fabricating the standardized square plates. The industry has adapted to accommodate unfaced crystals of over 200 grams, while clarity and the absence of visual defects remain pivotal in radio-grade crystal classification. Generally, each stone must exhibit at least 50% clarity under suitable lighting. If over half the volume shows blemishes, the crystal typically is deemed economically unviable. Even the National Bureau of Standards specifies slightly more stringent criteria.

These standards have evolved since the pre-war period. Typically, stones weighing less than 2 lbs. found limited acceptance, as crystals were expected to have several natural faces. Various minor imperfections that may now be permissible previously rendered crystals unsuitable. Of the oscillator plate manufacturers who reported in February, only one of the twenty-eight claimed to use crystals averaging under 1 lb. in weight, utilizing those averaging ⅓ of a pound. Nineteen of the twenty-eight companies needed one pound as a minimum size. The proportion of smaller-sized stones began to rise during the early war years, with significant consumption in the 200 to 500-gram range as well as larger crystals weighing more than 500 grams. By the war's end, it was estimated that 25% of crystals consumed were within the 200 to 500 grams range, while 75% consisted of heavier crystals.

SUPPLY OF QUARTZ CRYSTAL

The U.S. relies almost exclusively on Brazil for quartz crystal supplies. Notable deposits exist in the USSR and smaller ones have been explored elsewhere. Although quartz from U.S. states like Arkansas, Virginia, North Carolina, and California, as well as small quantities imported from Guatemala and Colombia, has been utilized, these sources contribute minimally to the overall U.S. supply situation. For instance, the volume of Brazilian quartz imported for U.S. consumption rose from 67,052 lbs. in 1942 to 126,521 lbs. in 1943, and imports increased significantly post-war to hundreds of tons. Ensuring quartz crystal availability for essential wartime production required collaboration between private industries and governments in both the U.S. and Brazil.

GEOLOGY OF QUARTZ DEPOSITS

The geological landscape of quartz deposits worldwide is still largely being understood; most available data has not yet been adequately correlated due to the pressure of wartime efforts. The following discussion outlines observations regarding the deposits' geology and mineralogy, recognizing that additional studies will clarify this area.

Quartz Deposits in Australia

In Australia, productive quartz deposits are located in northern New South Wales, specifically at the Kingsgate mining camp, situated twenty miles from Glen Innis. Diverse crystal sizes have been sourced for Australian oscillator plate manufacturers, yet the production levels have not fulfilled domestic requirements. These mines previously focused on molybdenite and bismuth extraction in earlier conflicts, but with the current demand for quartz crystals, several have reopened. A sizable mineralized region showcases quartz within a granite area, hosting several quartz pipes of variable sizes, capable of extending to considerable depths.

Quartz in Guatemala

Guatemala hosts quartz deposits mainly within the Department of Baja Verapaz, characterized by small gash veins embedded in gneisses and schists. The veins primarily yield fine-grained quartz at the periphery, while well-formed crystals arise from clay deposits within the central regions, with recent production mostly falling within the 200-500 grams weight range.

Quartz in Colombia

Near the town of Muzo in Boyaca, Colombia, a significant quartz deposit has emerged comprising one main vein alongside numerous subordinate veins. Trapped within a fracture-dominated structure, milky fractured quartz constitutes the primary vein, which measures approximately 32 feet in thickness.

Quartz in Madagascar

Historically, Madagascar served as an importing country for quartz intended for oscillator plate manufacturing in the U.S. Despite its reputation for lower quality, Madagascar quartz has proven competitive against alternatives, particularly from Brazil.

US Quartz Sources

Searching for radio-grade quartz in the U.S. during wartime led to efforts backed by governmental and private sectors. The regions yielding usable crystals notably include Arkansas and California, alongside western North Carolina and southwestern Virginia.

GEOGRAPHICAL DISTRIBUTION OF QUARTZ

DEPOSITS IN BRAZIL

Brazil is home to principal quartz deposits centered around states like Minas Gerais, Goiaz, and Baia. Other states play a minor role, producing considerable deposits only in Espirito Santo and Para. The estimated production distribution among these regions is Minas Gerais at 35%, Goiaz at 30%, Baia at 20%, Espirito Santo at 5%, and others contributing 10%.

Mining Operations in Minas Gerais

A significant amount of Minas Gerais' production stems from an area extending from Belo Horizonte north to the Serra de Minas and Serra do Cabral. Although this region houses burgeoning deposits, various mined areas persistently yield lower crystal quantities.

Mining in Goiaz

Goiaz encapsulates diverse production sites with production centers near Pium and Cristalina, contributing to both mining and shipping of quartz crystals.

PRODUCTION OF QUARTZ CRYSTALS

In summary, quartz crystal production is nearly entirely confined to Brazil. The established pricing structures and classifications stem from Brazilian practices. Consequently, commercial operations are intricately related to the geological characteristics of quartz deposits.

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