Crocoite

Chromium was first isolated and identified as a metal in 1789 by Vauquelin who was working with a rare mineral, Siberian red lead or crocoite [14654-05-08] PbCrO (3). The name chromium comes from the Greek word chroma color and resulted from the wide variety of brilliant colors... 

The first chromium compound was discovered in the Ural mountains of Russia, during the latter half of the eighteenth century. Crocoite [14654-05-8] a natural lead chromate, found immediate and popular use as a pigment because of its beautihil, permanent orange-red color. However, this mineral was very rare, and just before the end of the same century, chromite was identified as a chrome bearing mineral and became the primary source of chromium [7440 7-3] and its compounds (1) (see Chromiumand chromium alloys). 

Finally, in 1797, the Frenchman L. N. Vauquelin discovered the oxide of a new element in a Siberian mineral, now known as crocoite (PbCr04), and in the following year isolated the metal itself by charcoal reduction. This was subsequently named chromium (Greek xpco ia, chroma, colour) because of the variety of colours found in its compounds. Since their discoveries the metals and their compounds have become vitally important in many industries and, as one of the biologically active transition elements, molybdenum has been the subject of a great deal of attention in recent years, especially in the field of nitrogen fixation (p. 1035). 

Chromium, 122 ppm of the earth s crustal rocks, is comparable in abundance with vanadium (136 ppm) and chlorine (126 ppm), but molybdenum and tungsten (both 1.2 ppm) are much rarer (cf. Ho 1.4 ppm, Tb 1.2 ppm), and the concentration in their ores is low. The only ore of chromium of any commercial importance is chromite, FeCr204, which is produced principally in southern Africa (where 96% of the known reserves are located), the former Soviet Union and the Philippines. Other less plentiful sources are crocoite, PbCr04, and chrome ochre, Cr203, while the gemstones emerald and ruby owe their colours to traces of chromium (pp. 107, 242). 

Chrom-bleii n. lead chromate, -bleispat, m. (Min.) crocoite. -braun, n. chrome brown, -chlorid, n. chromic chloride. chromium(III) chloride, -chlorilr, n, chromous chloride, chromium(II) chloride, -druck, m. (Calico) chrome printing. 

Rotblei-erz, n., -spat, m. red lead ore (crocoite). rotbraim, a. red-brown. 

Occurrence. The ore of Cr of higher commercial importance is chromite (FeCr204). Other minerals are crocoite PbCr04 and chrome ochre Cr203. About 2% Cr in emerald Be3Al2Si6018 is the source of its green colour. Chromium is comparable in abundance in the earth s crustal rocks with V and Cl. 

The first source of chromium was found in the mineral crocoite. Today it is obtained from the mineral chromite (FeCr O ), which is found in Cuba, Zimbabwe, South Africa, Turkey, Russia, and the Philippines. Chromite is an ordinary blackish substance that was ignored for many years. There are different grades and forms of chromium ores and compounds, based on the classification of use of the element. Most oxides of chromium are found mixed with other metals, such as iron, magnesium, or aluminum. 

Historically, chromium ore was known as Siberian red lead, which was used to make bright red paints. The source was soon identified as the mineral crocoite, and analysis indicated that it also contained lead. In 1797 the French chemist Louis-Nicolas Vauquelin (1763—1829) discovered chromium while studying some minerals that were collected in Siberia. To isolate the pure metal from its oxide, he first dissolved the lead out of the mineral with hydrochloric acid (HCl), leaving crystals of chromium oxide, which he then heated. To his surprise he ended up with crystals of pure chromium metal. 

Lead chromate (PbCrO ) is found in nature as yellow crystals in the mineral crocoite. It can be produced by reacting lead chloride and sodium dichromate. It is a popular and safe yellow pigment. 

Chromium occurs in the minerals chromite, Fe0 Cr203 and crocoite, PbCr04. The element is never found free in nature. Its abundance in earth s crust is estimated in the range 0.01% and its concentration in sea water is 0.3 qg/L. The element was discovered by Vaquelin in 1797. 

Formula PbCr04 MW 323.19 Synonyms chrome yellow crocoite... 

Lead chromate occurs in nature as the minerals, crocoite and phoenic-ochroite. It is an important pigment of lead used in oil paints and water col-... 

Lead chromate is found naturally in minerals crocoite and phoenicochroite. It also is readily prepared by adding a soluble chromate such as sodium or potassium chromate to a solution of lead nitrate, lead acetate or other soluble lead(II) salt in neutral or slightly acidic solution ... 

In 1797-98 N.-L. Vauquelin analyzed crocoite and gave a detailed account of its history. All the specimens of this substance which are to be found in the several mineralogical cabinets in Europe, said he, were obtained from this [Beresof] gold mine which indicates that it was... 

Peter Simon Pallas, 1741-1811, German scientist who made extensive scientific journeys to study the natural history of Russia and Siberia. He described the Beresof gold mines and the Siberian red lead (crocoite) in 1770. 

Among the other early investigators of crocoite (Siberian red lead) were Count Apollos Apollosovich Musin-Pushkin0 (1760-1805), Tobias Lowitz (Tovii Egorovich Lovits) (1757-1804), and M. H. Klaproth (82). Count Musin-Pushkin s analyses were made with portable equipment during one of his mineralogical journeys (82). 

In 1799 Citizen Tassaert, a Prussian chemist who had been working for several years at the School of Mines of Paris, discovered chromium in an iron mineral found at the Carrade Villa near Gassin in the department of du Var. He too regarded the mineral as a chromate of iron (89). Since chromium had previously been detected in the red lead of Siberia (crocoite), in the emerald, and in the ruby, the chrome-iron mineral... 

P. S. Pallas describes the red lead of Siberia (crocoite), in which Vauquelin later discovered chromium. This mineral had been analyzed four years earlier by J. G. Lehmann. 

Both lead chromate and lead sulfochromate (the latter is a mixed-phase pigment) can be orthorhombic or monoclinic the monoclinic structure is the more stable [3.132], The greenish yellow orthorhombic modification of lead chromate is metastable at room temperature, and is readily transformed to the monoclinic modification under certain conditions (e.g., concentration, pH, temperature). The latter modification occurs naturally as crocoite. 

Lead Chromate (Crocoite or Chrome Yellow), PbCr04, mw 323.22 yel monocl crysts mp 844° with decomph d 6.12 practically insol in w in ammonia sol in acids alkalies. 

The important valence states of chromium are II, III, and VI. Elemental chromium, chromium(O), does not occur naturally. The divalent state (II or chromous) is relatively unstable and is readily oxidized to the trivalent (III or chromic) state. Chromium compounds are stable in the trivalent state and occur in nature in this state in ores, such as ferrochromite (FeCr204). The hexavalent (VI or chromate) is the second most stable state. However, hexavalent chromium rarely occurs naturally, but is produced from anthropogenic sources (EPA 1984a). Chromium in the hexavalent state occurs naturally in the rare mineral crocoite (PbCr04) (Hurlburt 1971). 

Chromates are a small group of minerals that contain the simple Cr04 molecule. Crocoite is the most common species in this group, a bright orange compound that is sometimes used as a pigment. 

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