Cinnabar, which is mercuric sulfide, is the most common mercury mineral.
It has a dark red or a bright yellowish-red color and is transparent to
translucent. When rubbed across a streak plate, it leaves a dark red
streak. If pure, cinnabar has a brilliant, shiny, nonmetallic luster. It
is, however, commonly found mixed with impurities, such as clay,
calcite, iron oxide, or bituminous material, and then it looks dull and
earthy. Cinnabar is quite heavy—it has a specific gravity of 8.10. It is
rather soft, and you can scratch it with a copper penny.
Some prospectors use a quick chemical test to identify cinnabar. They
rub a clean, shiny copper coin with a mineral sample that has been
moistened with a drop or two of dilute hydrochloric acid. If the sample
is cinnabar, a light silvery-gray coating appears on the coin.
Cinnabar occurs as small crystals or as fine-grained or compact
crystalline masses. It is found in veins that fill cracks in rocks and
also occurs as crusts and coatings on rocks. It also may be widely
scattered through rocks, such as limestones.
Cinnabar occurs in the Terlingua area of Brewster and Presidio counties
in west Texas. It has been mined there, off and on, since about 1894,
and during this time, mercury worth many millions of dollars has been
produced.
Most of this west Texas cinnabar is found in cracks, pores, and
breccia-filled cavities of Cretaceous limestones and clays. If you will
look at the Texas geologic map (pp. 4-5), you will see that igneous
rocks occur in this district. Many millions of years ago during the
Tertiary Period, when these igneous rocks were still hot magma, some of
them pushed up under the Cretaceous rocks and emitted fluids containing
mercury. The fluids moved upward through cracks and pores in the
Cretaceous rocks where they deposited the mercury as cinnabar and as
other mercury minerals.
[Illustration: Cinnabar crystals (dark) with calcite crystals (white)
from the Terlingua area of Brewster County, Texas.]
Mercury is an unusual element. Instead of occurring as a solid metal at
ordinary room temperatures, as do gold, silver, and lead, it remains a
liquid until it is cooled to 38 degrees below zero Fahrenheit. Because
the silvery little drops of liquid mercury roll about as if they were
alive, this element long has been called _quicksilver_.
Mercury is used in a variety of ways. In some noiseless light-switches,
a glass tube containing a small ball of mercury tilts when the switch is
turned “on.” The mercury then rolls to the end of the tube that contains
electrical contacts and quietly completes the electrical circuit. In
other uses, mercury is added to silver, tin, and other metals to make
fillings for teeth. Some medicines, such as calomel and mercurochrome,
contain mercury. Fulminate of mercury helps to set off dynamite and
other explosives. Mercury is used in many barometers and thermometers,
and farmers use mercury poisons to control insects and fungi.
Mercury also commonly is used to obtain gold from its ores. One method
of accomplishing this is to pass wet gold-bearing gravel or crushed rock
over metal plates that are coated with mercury. The gold particles
quickly mix with the mercury to form an _amalgam_, which later can be
scraped off the plates. The gold is then recovered by heating the
amalgam to drive off the mercury.