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Crystallization in a fluid-rich environment, where ion migration is enhanced,
results in the formation of crystals several cm, or even a few meters, in length. The
resulting rocks, called pegmatites, are composed of these unusually large crystals.
Most pegmatites are granitic in composition and consist of unusually large
crystals of quartz, feldspar, and muscovite. Feldspar is used in the production of
ceramics, and muscovite is used for electrical insulation and glitter. Further, pegmatites
often contain some of the least abundant elements. Minerals containing the elements
lithium, cesium, uranium, and the rare earths are occasionally found. Moreover, some
pegmatites contain semiprecious gems such as beryl, topaz, and tourmaline. Most
pegmatites are located within large igneous masses or as dikes or veins that cut into the
host rock that surrounds the magma chamber.
Not all late-stage magmas produce pegmatites, nor do all have a granitic
composition. Rather, some magmas become enriched in iron or occasionally copper.
Among the best known and most important ore deposits are those generated from hot,
ion-rich fluids called hydrothermal (hot-water) solutions. The majority of hydrothermal
deposits originate from hot, metal-rich fluids that are remnants of late-stage magmatic
processes. During solidification, liquids plus various metallic ions accumulate near the
top of the magma chamber. Because of their mobility, these ion-rich solutions can
migrate great distances through the surrounding rock before they are eventually
deposited, usually as sulfides of various metals. Some of these fluids move along
openings such as fractures or bedding planes, where they cool and precipitate metallic
ions to produce vein deposits. Many of the most productive deposits of gold, silver, and
mercury occur as hydrothermal vein deposits.
Another important type of accumulation generated by hydrothermal activity is
called a disseminated deposit. Rather than being concentrated in narrow veins and
dikes, these ores are distributed as minute masses throughout the entire rock mass.
Much of the world’s copper is extracted from disseminated deposits, including those at
Chuquicamata, Chile, and the huge Bingham Canyon copper mine in Utah, the USA.
Another economically important mineral with an igneous origin is diamond.
Although best known as gems, diamonds are used extensively as abrasives. Diamonds
are thought to originate at depths of nearly 200 km, where the confining pressure is
great enough to generate this high-pressure form of carbon. Once crystallized, the
diamonds are carried upward through pipe-shaped conduits that increase in diameter
toward the surface. In diamond-bearing pipes, nearly the entire pipe contains diamond
crystals that are disseminated throughout an ultramafic rock called kimberlite. The most
productive kimberlite pipes are those in South Africa.

Task 2. Copy out all the names of minerals mentioned in the text, transcribe
and translate them. Answer the question: which of them are economically
important and why?

Task 3. Fill in the summary of the text with the required words and word
combinations:

Magmatic, igneous, vein deposits, metal-rich, hydrothermal solutions, disseminated
deposit
Some of the most important accumulations of metals, such as gold, silver, lead,
and copper, are produced by … processes. The best-known and most important ore
deposits are generated from … . Hydrothermal deposits are thought to originate from
hot, … fluids that are remnants of late-stage … processes. These ion-rich solutions
move along fractures or bedding planes, cool, and precipitate the metallic ions to
produce … . In a … (e.g., much of the world’s copper deposits), the ores from

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