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Cinnabar


A toxic mercury sulfide mineral. The primary ore of mercury, once used as a pigment.



What is Cinnabar?



Cinnabar is a toxic mercury sulfide mineral with a chemical composition of HgS. It has a bright red color and occurs as a fracture-filling mineral in some areas with recent volcanic activity. It is the most important ore of mercury and was used extensively as a pigment for thousands of years in many parts of the world.


Geologic Occurrence of Cinnabar



Cinnabar is a hydrothermal mineral that precipitates from ascending hot waters and vapors as they move through fractured rocks. It forms at shallow depths where temperatures are generally less than 200 degrees celsius. It usually forms in rocks surrounding recent volcanic activity but also forms at Earth's surface near hot springs.

Cinnabar precipitates as coatings on rock surfaces and as fracture fillings. Less often, cinnabar can be deposited in the pore spaces of sediments. It is usually massive in habit and is rarely found as well-formed crystals. Other sulfide minerals are generally found associated with cinnabar. These include pyrite, marcasite, realgar and stibnite. Gangue minerals associated with cinnabar include quartz, dolomite, calcite and barite. Small amounts of liquid mercury are sometimes present with cinnabar.


Properties of Cinnabar



The most striking property of cinnabar is its red color. Its bright color makes it easy to spot in the field and a fascination for those who discovered it. With a Mohs hardness of only 2 to 2.5, it is very easy to grind into a very fine powder. It has a specific gravity of 8.1, which is extremely high for a nonmetallic mineral.

Physical Properties of Cinnabar

Chemical Classification sulfide
Color bright red to brownish red, sometimes gray
Streak red
Luster adamantine to dull
Diaphaneity transparent to opaque
Cleavage perfect, prismatic
Mohs Hardness 2 to 2.5
Specific Gravity 8 to 8.2
Diagnostic Properties specific gravity, color, streak, cleavage, association with volcanic activity
Chemical Composition mercury sulfide, HgS
Crystal System trigonal
Uses the most important ore of mercury, use as a pigment has declined due to toxicity

The luster of cinnabar ranges from dull to adamantine. Specimens with a dull luster are usually massive, contain abundant impurities and do not have the brilliant red color of pure cinnabar. Adamantine specimens are usually the rarely-found crystals.


Metacinnabar



Metacinnabar is a polymorph of cinnabar. It has the same chemical composition (HgS) as cinnabar but a different crystal structure. Cinnabar is trigonal, while metacinnabar is isometric. The two minerals should not be confused with one another because metacinnabar has a metallic gray color, a gray-to-black streak and a metallic-to-submetallic luster.


Uses of Cinnabar



Cinnabar is the most important ore of mercury, and that is the primary reason for mining it today. For thousands of years cinnabar has been heated in a furnace, and the mercury vapors that are produced have been condensed into liquid mercury.

People began using cinnabar for pigments thousands of years ago in Italy, Greece, Spain, China, Turkey, and the Mayan countries of South America. Through time, independently, people in other countries where volcanoes are present discovered cinnabar and realized its utility as a pigment. Cinnabar is one of a very small number of minerals that was independently discovered, processed and utilized by ancient people in many parts of the world.

Chinese red (cinnabar) lacquer box
A carved wooden box with a red lacquer finish from China's Ming Dynasty Period (box c. 1522-1566). Boxes like this were frequently painted with a lacquer containing a cinnabar pigment. Public domain photo by the Los Angeles County Museum of Art.

Cinnabar was mined at the volcano, ground into a very fine powder and then mixed with liquids to produce many types of paint. Sometimes it was applied in powdered form. Powdered cinnabar was used as a cosmetic in many parts of the world for thousands of years. Eventually it was discovered that cinnabar is toxic, and its use in pigments and cosmetics began to decline.


Uses of Mercury



Mercury has many uses, but its toxicity has reduced its use in any application where reasonable substitutes can be found. Large amounts of mercury are currently used in the chemical industry in the production of chlorine and caustic soda during the electrolysis of brine.

Mercury was widely used in temperature- and pressure-measuring instruments such as thermometers and barometers. It was often used in gravity switches because it flowed easily as a liquid and conducted electricity.

mercury switch
A mercury switch. Mercury has the ability to conduct electricy and flow under the influence of gravity. This switch is currently in the "off" position, but if it is moved so that the mercury runs to the right, surrounding the two wires, the circuit will be connected and the switch will be in the "on" position. Photo by Medvedev, used here under a Creative Commons license.

Mercury is currently used in some batteries and light bulbs, but their disposal is often regulated. Because it is toxic, it was once widely used to protect seed corn from fungus and to deworm materials used to make felt. It was used in dental amalgam but is being replaced by polymer resins and other materials.

Mercury has been widely used in mining to separate gold and silver from ores and stream sediments. Large amounts of mercury were spilled during these operations, and today, mercury used in the 1800s is still being recovered from streams.


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massive cinnabar
Massive cinnabar showing its characteristic red color and dull luster. Some contamination by clay. Photograph by H. Zell, used here under a GNU Free Documentation License.




cinnabar crystals on dolomite
Bright red cinnabar crystals on a dolomite matrix. Crystals are about 1.3 centimeters in height, from Hunan, China. Photo by Rob Lavinsky, iRocks.com. Creative Commons license.


cinnabar in sediment porosity
Cinnabar sometimes precipitates from fluids moving through the porosity of a sediment or a sedimentary rock. In those cases it can infill the pore spaces as a weak "cement." Photo © iStockPhoto, only_fabrizio.


metacinnabar
Crystals of metacinnabar on a rock surface. Specimen is from the Mount Diablo mine, Contra Costa County, California. Specimen is about 3.3 x 2.1 x 2.0 centimeters in size. Photo by Rob Lavinsky, iRocks.com. Creative Commons license.


still for producing mercury from cinnabar
Textbook sketch of a still used for the distillation of mercury from cinnabar. Public domain image from Alchimia, Anonymous, 1570.


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