Carbonate

Carbonate

Overview
In chemistry, a carbonate is a salt of carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

, characterized by the presence of the carbonate ion, . The name may also mean an ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 of carbonic acid, an organic compound containing the carbonate group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

C(=O)(O–)2.

The term is also used as a verb, to describe carbonation
Carbonation
Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...

: the process of raising the concentrations of carbonate and bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 ions in water to produce carbonated water
Carbonated water
Carbonated water is water into which carbon dioxide gas under pressure has been dissolved, a process that causes the water to become effervescent....

 and other carbonated beverages — either by the addition of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 gas under pressure, or by dissolving carbonate or bicarbonate salts into the water.

In geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 and mineralogy
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...

, the term "carbonate" can refer both to carbonate minerals
Carbonate minerals
Carbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3**Otavite CdCO3**Rhodochrosite MnCO3**Siderite FeCO3**Smithsonite ZnCO3...

 and carbonate rock
Carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...

 (which is made of chiefly carbonate minerals), and both are dominated by the carbonate ion, CO.
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Encyclopedia
In chemistry, a carbonate is a salt of carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

, characterized by the presence of the carbonate ion, . The name may also mean an ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 of carbonic acid, an organic compound containing the carbonate group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

C(=O)(O–)2.

The term is also used as a verb, to describe carbonation
Carbonation
Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...

: the process of raising the concentrations of carbonate and bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 ions in water to produce carbonated water
Carbonated water
Carbonated water is water into which carbon dioxide gas under pressure has been dissolved, a process that causes the water to become effervescent....

 and other carbonated beverages — either by the addition of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 gas under pressure, or by dissolving carbonate or bicarbonate salts into the water.

In geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 and mineralogy
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...

, the term "carbonate" can refer both to carbonate minerals
Carbonate minerals
Carbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3**Otavite CdCO3**Rhodochrosite MnCO3**Siderite FeCO3**Smithsonite ZnCO3...

 and carbonate rock
Carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...

 (which is made of chiefly carbonate minerals), and both are dominated by the carbonate ion, CO. Carbonate minerals are extremely varied and ubiquitous in chemically-precipitated sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

. The most common are calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 or calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

, CaCO3, the chief constituent of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 (as well as the main component of mollusc shells and coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

 skeletons); dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....

, a calcium-magnesium carbonate CaMg(CO3)2; and siderite
Siderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...

, or iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 (II) carbonate, FeCO3, an important iron ore. Sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...

 ("soda" or "natron") and potassium carbonate
Potassium carbonate
Potassium carbonate is a white salt, soluble in water , which forms a strongly alkaline solution. It can be made as the product of potassium hydroxide's absorbent reaction with carbon dioxide. It is deliquescent, often appearing a damp or wet solid...

 ("potash") have been used since antiquity for cleaning and preservation, as well as for the manufacture of glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

. Carbonates are widely used in industry, e.g. in iron smelting, as a raw material for Portland cement
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 and lime
Lime (mineral)
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...

 manufacture, in the composition of ceramic glaze
Ceramic glaze
Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...

s, and more.

Structure and bonding


The carbonate ion is the simplest oxocarbon anion
Oxocarbon anion
In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula CxOyn− for some integers x, y, and n....

. It consists of one carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atom surrounded by three oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 atoms, in a trigonal planar
Trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...

 arrangement, with D3h molecular symmetry. It has a molecular mass of 60.01 daltons and carries a negative two formal charge
Formal charge
In chemistry, a formal charge is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity....

. It is the conjugate base
Conjugate acid
Within the Brønsted–Lowry acid-base theory , a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton in the forward chemical...

 of the hydrogen carbonate (bicarbonate)
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 ion, HCO3, which is the conjugate base of H2CO3, carbonic acid.

The Lewis structure
Lewis structure
Lewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule. A Lewis structure can be drawn for any covalently bonded molecule, as well as coordination compounds...

 of the carbonate ion has two (long) single bonds to negative oxygen atoms, and one short double bond to a neutral oxygen
This structure is incompatible with the observed symmetry of the ion, which implies that the three bonds are equally long and that the three oxygen atoms are equivalent. As in the case of the isoelectronic nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...

 ion, the symmetry can be achieved by a resonance
Resonance (chemistry)
In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula...

 between three structures:
This resonance can be summarized by a model with fractional bonds and delocalized charges:


Chemical properties


Metal carbonates generally decompose on heating, liberating carbon dioxide from the long term carbon cycle to the short term carbon cycle and leaving behind an oxide of the metal. This process is called calcination
Calcination
Calcination is a thermal treatment process applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials...

, after calx, the Latin name of quicklime or calcium oxide
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

, CaO, which is obtained by roasting limestone in a lime kiln.

A carbonate salt forms when a positively charged ion, M+, attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

:
2 M+ + CO32− → M2CO3

M2+ + CO32− → MCO3

2 M3+ + 3 CO32− → M2(CO3)3


Most carbonate salts are insoluble
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

 in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 at standard temperature and pressure, with solubility constants of less than 1×10−8. Exceptions include lithium
Lithium carbonate
Lithium carbonate is a chemical compound of lithium, carbon, and oxygen with the formula Li2CO3. This colorless salt is widely used in the processing of metal oxides and has received attention for its use in psychiatry. It is found in nature as the rare mineral zabuyelite.-Properties:Like almost...

, sodium
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...

, potassium
Potassium carbonate
Potassium carbonate is a white salt, soluble in water , which forms a strongly alkaline solution. It can be made as the product of potassium hydroxide's absorbent reaction with carbon dioxide. It is deliquescent, often appearing a damp or wet solid...

 and ammonium
Ammonium carbonate
Ammonium carbonate is a commercial salt with the chemical formula 2CO3. It is used when crushed as a smelling salt. It can be crushed when needed in order to revive someone who has fainted...

 carbonates, as well as many uranium carbonate
Uranium carbonate
Uranyl carbonate, UO2, is a carbonate of uranium that forms the backbone of several uranyl mineral species such as Andersonite, McKelveyite and Wyartite and most importantly Rutherfordine...

s.

In aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...

, carbonate, bicarbonate, carbon dioxide, and carbonic acid exist together in a dynamic equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...

. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 ion is prevalent. In more acid conditions, aqueous carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, CO2(aq), is the main form, which, with water, H2O, is in equilibrium with carbonic acid - the equilibrium lies strongly towards carbon dioxide. Thus sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...

 is basic, sodium bicarbonate
Sodium bicarbonate
Sodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...

 is weakly basic, while carbon dioxide itself is a weak acid. Note that although the carbonate salts of most metals are insoluble in water, the same is not true of the bicarbonate salts. This equilibrium between carbonate, bicarbonate, carbon dioxide and carbonic acid in water can, under changing temperature or pressure conditions, and in the presence of metal ions with insoluble carbonates, result in formation of insoluble compounds. This is responsible for the buildup of scale inside pipes caused by hard water
Hard water
Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...

.

Carbonated water
Carbonated water
Carbonated water is water into which carbon dioxide gas under pressure has been dissolved, a process that causes the water to become effervescent....

 is formed by dissolving CO2 in water under pressure. When the partial pressure of CO2 is reduced, for example when a can of soda is opened, the equilibrium for each of the forms of carbonate (carbonate, bicarbonate, carbon dioxide, and carbonic acid) shifts until the concentration of CO2 in the solution is equal to the solubility of CO2 at that temperature and pressure. In living systems an enzyme, carbonic anhydrase
Carbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...

, speeds the interconversion of CO2 and carbonic acid.

Organic carbonates


In organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 a carbonate can also refer to a functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

 within a larger molecule that contains a carbon atom bound to three oxygen atoms, one of which is double bonded. These compounds are also known as organocarbonates or carbonate esters, and have the general formula ROCOOR′, or RR′CO3. Important organocarbonates include dimethyl carbonate
Dimethyl carbonate
Dimethyl carbonate, often abbreviated DMC, a carbonate ester, is a flammable clear liquid boiling at 90 °C. This compound has found use as a methylating agent and more recently as a VOC exempt solvent in paints and adhesives . There is also interest in using this compound as a fuel oxygenate...

, the cyclic compounds ethylene carbonate
Ethylene carbonate
Ethylene carbonate is an ester of ethylene glycol and carbonic acid. At room temperature ethylene carbonate is a transparent crystalline solid, practically odorless and colorless, and somewhat soluble in water. In the liquid state Ethylene carbonate is an ester of ethylene glycol and carbonic...

 and propylene carbonate
Propylene carbonate
Propylene carbonate is an organic compound, a cyclic carbonate of propylene glycol. This colorless and odorless liquid is useful as a polar, aprotic solvent...

, and the toxic triphosgene
Triphosgene
Triphosgene is a chemical compound that is used as a safer substitute for phosgene, because at room temperature it is a solid crystal, as opposed to phosgene which is a gas. Triphosgene crystals decompose at around 130 °C, although, the decomposition temperature of impure samples can be much lower...

.

Biological significance


It works as a buffer
Buffer solution
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...

 in the blood as follows:
when pH is too low, the concentration of hydrogen ions is too high, so one exhales CO2. This will cause the equation to shift left, essentially decreasing the concentration of H+ ions, causing a more basic pH.

When pH is too high, the concentration of hydrogen ions in the blood is too low, so the kidneys excrete bicarbonate (HCO3). This causes the equation to shift right, essentially increasing the concentration of hydrogen ions, causing a more acidic pH.

There are 3 important reversible reactions that control the above pH balance.

1. H2CO3(aq) H+(aq) + HCO3-(aq)

2. H2CO3(aq) CO2(aq) + H2O(l)

3. CO2(aq) CO2(g)

Exhaled CO2(g) depletes CO2(aq) which in turn consumes H2CO3 causing the aforementioned shift left in the first reaction by Le Chatelier's principle
Le Châtelier's principle
In chemistry, Le Chatelier's principle, also called the Chatelier's principle, can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently...

. By the same principle when the pH is too high, the kidneys excrete bicarbonate (HCO3-) into urine as urea via the Urea Cycle (aka the Krebs-Henseleit Ornithine Cycle). By removing the bicarbonate more H+ is generated from carbonic acid (H2CO3) which come from CO2(g) produced by cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...

.

Presence outside Earth


It is generally thought that the presence of carbonates in rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 is strong evidence for the presence of liquid water. Recent observations of the Planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...

 NGC 6302
NGC 6302
NGC 6302 is a bipolar planetary nebula in the constellation Scorpius. The structure in the nebula is among the most complex ever observed in planetary nebulae...

 shows evidence for carbonates in space, where aqueous alteration similar to that on Earth is unlikely. Other minerals have been proposed which would fit the observations.

Significant carbonate deposits
Carbonates on Mars
Evidence for carbonates on Mars has remained elusive until recently. For example, most remote sensing instruments such as OMEGA and THEMIS that are sensitive to infrared emissivity spectral features of carbonates, had not suggested the presence of carbonate outcrops at 100 m or coarser spatial...

 have not been found on Mars via remote sensing or in situ missions, even though Martian meteorites contain small amounts. Groundwater may have existed at both Gusev
Gusev crater
Gusev is a crater on the planet Mars and is located at . The crater is about 166 kilometers in diameter and formed approximately three to four billion years ago. It was named after Russian astronomer Matvei Gusev in 1976....

 and Meridiani Planum
Meridiani Planum
Meridiani Planum is a plain located 2 degrees south of Mars' equator , in the westernmost portion of Terra Meridiani. It hosts a rare occurrence of gray crystalline hematite...

.

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