Dictionary:

crocoite

  (krŏk'ō-īt', krō'kō-) also crocoisite (krŏk'wə-zīt')

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.]

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.

Environment

Oxidized lead-chromium veins, as a secondary mineral.

Almost always in prismatic crystals, up to several inches long and striated parallel to their length. Terminations may be hollow with deep-pitted depressions.

Brilliant 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.

Lead chromate (69.1% PbO, 30.9% CrO ₃ ).

Fuses 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.

Distinguished 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.

A 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).

Crocoite specimen from the Red Lead Mine, Tasmania, Australia

Crocoite from Dundas, Tasmania.

Crystal intergrowth of the secondary lead mineral crocoite

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 Berezovsk near Ekaterinburg in the Urals in 1766; and named crocoise by F. S. Beudant in 1832, from the Greek if peoc, saffron, in allusion to its color, a name first altered to crocoisite and afterwards to crocoite. It is found as well-developed crystals, but there are most often poorly terminated, and are of a bright hyacinth-red color, which are 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 Umtali 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.

See Also

• Bellite

References

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

External links

• crocoite.com
• List of Tasmanian state emblems

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