n.
A rare lead chromate mineral, PbCrO4, that forms brilliant orange crystals.
[Alteration of French crocoise, from Greek krokoeis, saffron-colored, from krokos, saffron. See crocus.]
crocoite
Dictionary:
croc·o·ite (krŏk'ō-īt', krō'kō-) 
|
also croc·oi·site (krŏk'wə-zīt')
n.
A rare lead chromate mineral, PbCrO4, that forms brilliant orange crystals.
n.
A rare lead chromate mineral, PbCrO4, that forms brilliant orange crystals.
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| Sci-Tech Encyclopedia: Crocoite |
A mineral with the chemical composition PbCrO4. Crocoite occurs in yellow to orange or hyacinth red, monoclinic, prismatic crystals with adamantine to vitreous luster; it is also massive granular. Hardness is 2.5–3 on Mohs scale and specific gravity is 6.0. Streak, or color of the mineral powder, is orangish-yellow. It fuses easily.
Crocoite is a secondary mineral associated with other secondary minerals of lead such as pyromorphite and of zinc such as cerussite. It has been found in mines in California and Colorado. See also Lead.
| Rock & Mineral Guide: crocoite |
PbCrO
Monoclinic -- prismatic
Environment
Oxidized lead-chromium veins, as a secondary mineral.
Crystal descriptionAlmost always in prismatic crystals, up to several inches long and striated parallel to their length. Terminations may be hollow with deep-pitted depressions.
Physical propertiesBrilliant orange. Luster adamantine; hardness 2Ɖ-3; specific gravity 5.9-6.1; fracture uneven; cleavage poor prismatic. Very brittle and fragile; almost transparent to translucent.
CompositionLead chromate (69.1% PbO, 30.9% CrO 3 ).
TestsFuses very easily to black bead. In borax bead gives the green (chromium) color. Darkens in closed tube and flies apart (decrepitates), but upon cooling regains its orange color.
Distinguishing characteristicsDistinguished from wulfenite by the prismatic crystal habit and the borax bead and closed-tube tests (plus its lower specific gravity). Realgar volatilizes completely in white fumes on charcoal.
OccurrenceA very rare but very spectacular mineral that was first found in Russia. Its finest development was reached in Tasmania as crystals several inches (10-20 cm) long. It appears to form under the unusual condition of chromium-bearing solutions altering lead deposits. Small examples have been reported from the Mammoth Mine, Tiger, Arizona, and in California at the Darwin Mine, Inyo Co., and the Eldorado Mine, Indio, Riverside Co., but never in noteworthy specimens. Also reported from Brazil and the Philippines.
Crocoite is among the most showy and colorful of mineral species. The supply of truly elongated and sturdy Tasmanian specimens seems to have been exhausted many years ago; specimens from a recent reopening are much smaller and rather fragile. A Russian source, the first find, had stockier and sounder crystals on a black matrix with green vauquelinite, Pb 2 Cu(CrO 4 )PO 4 (OH).
| Wikipedia: Crocoite |
| Crocoite | |
|---|---|
 Crocoite from Dundas, Tasmania. |
|
| General | |
| Category | Chromate mineral |
| Chemical formula | Lead Chromate PbCrO4 |
| Identification | |
| Color | Orange, red, yellow |
| Crystal habit | Coarsely crystalline to acicular |
| Crystal system | Monoclinic prismatic (2/m) |
| Cleavage | Distince on {110} indistinct on {001} and {100} |
| Fracture | Conchoidal to uneven |
| Tenacity | Sectile |
| Mohs scale hardness | 2.5 - 3 |
| Luster | Adamantine |
| Streak | Yellowish orange |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 5.9 - 6.1 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 2.290(2) nβ = 2.360(2) nγ = 2.660(2) |
| Birefringence | δ = 0.370 |
| Pleochroism | Weak |
| References | [1][2][3] |
Crocoite is a mineral consisting of lead chromate, PbCrO4, and crystallizing in the monoclinic system. It is sometimes used as a paint, being identical in composition with the artificial product chrome yellow; it is the only chromate of any importance found in nature. It was discovered at Berezovsky deposit near Ekaterinburg in the Urals in 1766; and named crocoise by F. S. Beudant in 1832, from the Greek κροκος, saffron, in allusion to its color, a name first altered to crocoisite and afterwards to crocoite. It is found as well-developed crystals, although these are usually poorly terminated. Crystals are of a bright hyacinth-red color, translucent, and have an adamantine to vitreous lustre. On exposure to light much of the translucency and brilliancy is lost. The streak is orange-yellow; Mohs hardness is 2.5-3; and the specific gravity is 6.0. In the Urals the crystals are found in quartz-veins traversing granite or gneiss. Other localities which have yielded good crystallized specimens are Congonhas do Campo near Ouro Preto in Brazil, Luzon in the Philippines, and Mutare in Mashonaland.
Gold is often found associated with this mineral. Crystals far surpassing in beauty any previously known have been found in the Adelaide Mine at Dundas, Tasmania; they are long slender prisms, 3 or 4 in. in length, with a brilliant lustre and color. Crocoite is also the official Tasmanian mineral emblem.
Associated with crocoite at Berezovsk are the closely allied minerals phoenicochroite and vauquelinite. The former is a basic lead chromate, Pb2CrO5, and the latter a lead and copper phosphate-chromate, Pb2CuCrO4PO4OH. Vauquelinite forms brown or green monoclinic crystals, and was named after L. N. Vauquelin, who in 1797 discovered (simultaneously with and independently of M. H. Klaproth) the element chromium in crocoite.
Relative rarity of crocoite is connected with specific conditions required for its formation: an oxidation zone of lead ore bed and presence of ultramafic rocks serving as the source of Cr (in chromite). Oxidation of Cr3+ into CrO42- (from chromite) and decomposition of galena (or other primary Pb minerals) are required for crocoite formation.
See also
References
- This article incorporates text from the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.
- ^ http://rruff.geo.arizona.edu/doclib/hom/crocoite.pdf Handbook of Mineralogy
- ^ http://www.mindat.org/min-1157.html Mindat
- ^ http://webmineral.com/data/Crocoite.shtml Webmineral data
External links
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Rock & Mineral Guide. Peterson Field Guide to Rocks and Minerals, by Frederick H. Pough. Copyright © 1998 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Crocoite". Read more |
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