Cinnabar
[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.
[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.