Crocoite
- Red lead ore
- Chromate
- Lehmannit
Krokoit, outdated also known as red lead ore or lead chromate and lead chromate under its chemical name, is a rarely occurring mineral from the mineral class of " sulfates (including selenates, tellurates, chromates, molybdates and tungstates ) ". It crystallizes in the monoclinic crystal system with the chemical composition Pb [ CrO4 ].
Krokoit developed some very rich in forms, but usually long prismatic to acicular crystals to about 15 centimeters in length, which are parallel stretched and streaked the c- axis. The crystal ends are often incomplete and often hollow. Just as often, there are radialestrahlige bunch of randomly intergrown crystals and occasionally coarse masses and crusty coatings.
The color of the transparent to translucent crystals varies from a bright yellow to yellow orange to a brilliant Hyazinthrot with a fat - to diamond-like luster. When exposed to light, the color, however, can fade with time.
With a Mohs hardness of 2.5 to 3 Krokoit lies between the reference minerals gypsum ( 2) and calcite ( 3), that already can be scratched with a copper coin.
Special Features
Before the blowpipe Krokoit melts easily and thereby zerknistert strong. On charcoal it evaporates and forms a lead-containing slag and heated to phosphorus salt or borax bead, these are dyed in response to the contained chromium emerald green.
Krokoit dissolves in hot hydrochloric acid, chlorine is deposited free and PbCl2. Also in potassium hydroxide ( KOH) it dissolves with brown color.
Under UV light, show some Krokoite a dark brown fluorescence.
Etymology and history
A first mention of the mineral is found in 1763 in a trip report of Mikhail Vasilyevich Lomonosov, who also reported on his return from St. Petersburg that worked there as a professor of chemistry Johann Gottlob Lehmann a "Red Lead from Siberia " or " red lead ore from Berezovsky "(today Beryozovsky ( Sverdlovsk ) ) had discovered and examined. This name was also found shortened to "red lead ore " ( red lead ore ) in the records of Abraham Gottlob Werner. In his somewhat more detailed description of Lehmann 1766 and among other things suggest that the mineral is dissolved in hydrochloric acid solution with a beautiful green color and was also found lead in the compound, but no new element. However, Lehmann could not lead to an end when he died in an explosion in his laboratory in 1767 his studies.
The exact determination of the composition of the mineral prepared many analysts unusual difficulties and even Martin Heinrich Klaproth failed because it was not enough material available. Only Louis -Nicolas Vauquelin succeeded in 1797, the analysis of the material and was able to isolate the previously unknown element chromium from the connection.
1832 coined François Sulpice Beudant the name " Crocoïse " after the ancient Greek word κρόκος [ crocs ] for saffron because reminded him the flashy color of the mineral to the color of dried saffron threads. As Synonyms still Plomb chromates, Plomb rouge, red lead ore, chromium lead and chromium are listed Treat lead in his description. In his 1854 published 4th edition of James Dwight Dana Mineral classification called the name coined by Beudant form a bad choice, referring to the 1838 published a modification of the name by Franz von Hess Peter after Crocoisit, according to the usual in the mineralogical nomenclature form. A final adjustment of the name on the still valid today Krokoit took place in 1841 by August Breithaupt, which was taken over in 1868 by Dana in his 5th edition of the mineral systematics.
No longer in use, however, is the traditional by Friedrich Hausmann 1813 Kallochrom name from the Greek κάλλος for beauty and χρώμα for color.
Henry James Brooke and William Hallowes Miller in 1852 described a new mineral, and called it after its discoverer Johann Gottlob Lehmann as Lehmannit. In later studies, however, it turned out that this mineral was identical to the Krokoit already known in the composition. The mineral name Lehmannit was therefore discredited and has since become synonymous with the Krokoit.
As exact type locality is today (2014) the pit " Tsvetnoi " on the mountain Uspenskaya in the gold deposit Beryozovsky ( Berezovsk, Berezovsky ), Sverdlovsk Oblast in the Russian Urals. Type material of the mineral is stored in the Muséum national d' histoire naturelle in Paris.
Classification
Already in the now outdated but still in use 8th edition of the mineral classification by Strunz was one of Krokoit to the Department of " chromates " where he. Together with Chromatit and Tarapacait the " Tarapacait - Krokoit group " with the System No. VI/F.01 formed.
The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the Krokoit also in the department of " chromates ". This division, however, is further subdivided according to the possible presence of other anions, so that the mineral is found according to its composition in the subsection " With additional anions " (without further specification ), where it is the only member of the unnamed group 7.FA.20 forms.
The mostly commonly used in English-speaking classification of minerals according to Dana assigns the Krokoit in the class of " sulfates, chromates and molybdates ", in which, however, the selenates, tellurates, Selenite, Tellurite and sulfites are represented. Here he is the only member of the unnamed group 35.03.01 within the Department of " water clear chromate " and the sub-division of " water clear chromate with A XO4 " to find.
Modifications and varieties
As Jossait designated August Breithaupt in 1858 by a Major-General of Jossa acquired in Berezov mineral found in small yellow-orange to orange- colored crystals on Vauquelinite and was regarded as containing zinc chromate. Meanwhile Jossait considered as mixture of Krokoit and Smithsonite.
Education and Locations
Krokoit forms as a rare secondary mineral in the oxidation zone of chromium-containing lead and galena deposits. Accompanying minerals occur in various lead and chromium minerals, such as, among others, anglesite, cerussite, Descloizit, Dundasit, Embreyit, Phönikochroit, pyromorphite, Vanadinit, Vauquelinite, Wulfenite as well as quartz and limonite.
A rare mineral formation Krokoit has so far (as of 2014) are detected only at a few localities, of which approximately 90 localities as have been established. Apart from its type locality in the mine Tsvetnoi and other mines in the vicinity of the gold deposit of Beryozovsky yet entered the mineral in Russia in the copper deposit of Mednorudyanskoye at Nizne Tagil ( Nizhnii Tagil ) and the hamlet Totschilnyi Kliuch northwest of Resch in Sverdlovsk Oblast and at the close Sukhovyaz Werchni Ufalei in the Chelyabinsk Oblast revealed.
In Germany they found Krokoit among others at Mechernicher Bleiberg and in the pit " Green Bleiberg " in Niedergelpe in North Rhine-Westphalia and in Callnberg in the Saxon district Zwickau.
In Austria you know the mineral so far only from Alpleskopf and Dirstentritt near the North Tyrolean Nassereith.
The previous best and largest crystal specimens were recovered in the mine area around Dundas in Tasmania in Australia. Especially from the " Adelaide Mine" were steps from several centimeters to over 25 cm size to light.
Other localities lie among others in Brazil, Chile, China, France, Italy, Canada, the Democratic Republic of Congo ( Zaire ), Mexico, Namibia, the Philippines, Romania, Zimbabwe, Slovakia, South Africa, Tsdschikistan, the United Kingdom ( UK) and the United States of America (USA).
Crystal structure
Krokoit crystallizes in the monoclinic space group P21 / n ( Raumgruppen-Nr. 14) with the lattice parameters a = 7.13 Å; b = 7.44 Å; c = 6.80 Å and 102.4 ° and β = 4 formula units per unit cell.
Structurally similar Krokoit of monazite, but with [ CrO4 ] 2 tetrahedra and Pb2 ions of seven O2 - ions are surrounded from seven different tetrahedra as its nearest neighbor.
In addition to the monoclinic Krokoit there is also an orthorhombic modification, which can be synthetically generated by chemical precipitation of lead chromate. Rhombic lead chromate has the lattice parameters a = 8.67 ± 0.03 Å, however, b = 5.59 ± 0.01 Å and c = 7.13 ± 0.02 Å, is unstable and is transformed after a short time in the stable monoclinic Krokoit to.
Use
As ore Krokoit has no meaning in spite of its high chromium content of up to 16.1%, even though the mineral has been mined for some time in Tasmania. As a synthetically produced chromate it finds, however, a so-called " chrome yellow " frequent use in paints and emulsion paints.
Even as a gem is Krokoit not be used in spite of its beautiful color and its brilliance for trade, because it is too soft and therefore too sensitive. However, in rare cases, it is for collectors in Gemstone Shape, usually step cut, sanded.