Science Blog: Quartz

Quartz is an abundant mineral in the Earth's continental crust. It is made up of a continuous framework of SiO₄ silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO₂. There are many different varieties of quartz, several of which are semi-precious gemstones. Throughout the world, varieties of quartz have been, since antiquity, the most commonly used minerals in the making of jewelry and hardstone carvings.

Crystal habit and structure

Quartz belongs to the trigonal crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are often twinned, distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive. Well-formed crystals typically form in a 'bed' that has unconstrained growth into a void, but because the crystals must be attached at the other end to a matrix, only one termination pyramid is present. There are exceptions as doubly terminated crystals do occur. An occurrence in Herkimer County, New York is noted for these Herkimer diamonds with terminations at both ends. A quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward.

"Bloom Quartz"

Occurrence

Quartz is an essential constituent of granite and other felsic igneous rocks. It is very common in sedimentary rocks such as sandstone and shale and is also present in variable amounts as an accessory mineral in most carbonate rocks. It is also a common constituent of schist, gneiss, quartzite and other metamorphic rocks. Because of its resistance to weathering it is very common in stream sediments and in residual soils. Quartz, therefore, occupies the lowest potential to weather in the Goldich dissolution series.

Quartz occurs in hydrothermal veins as gangue along with ore minerals. Large crystals of quartz are found in pegmatites. Well-formed crystals may reach several meters in length and weigh as much as 1,400 pounds (640 kg).

Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing silicon wafers in the semiconductor industry, are expensive and rare. A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States.

Synthetic quartz

Most quartz used in microelectronics is produced synthetically. Large, flawless and untwinned crystals are produced in an autoclave via thehydrothermal process. The process involves treating crushed natural quartz with hot aqueous solution of a base such as sodium hydroxide. The hydroxide serves as a "mineralizer", i.e. it helps dissolve the "nutrient" quartz. High temperatures are required, often around 675 °C. The dissolved quartz then recrystallizes at a seed crystal at slightly lower temperatures. Approximately 200 tons of quartz were produced in the US in 2005; large synthesis facilities exist throughout the world. Synthetic quartz is often evaluated on the basis of its Q factor, a measure of its piezoelectric response and an indicator of the purity of the crystal.

High-temperature glass composed of silicon dioxide with no (or only small amounts of) other components is referred to as "quartz glass" or fused quartz, although it is amorphous in structure, rather than crystalline
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Uses

Quartz is the source of many silicon compounds such as silicones (e.g. high performance polymers), silicon (e.g. microelectronics), and many other compounds of commercial importance. Quartz in the form of sand is reduced by carbothermic reaction as a first step in these energy-intensive processes.

Owing to its high thermal and chemical stability and abundance, quartz is widely used in many large-scale applications related to abrasives, foundry materials, ceramics, and cements.