Kröhnkite

• Kroehnkit

Kröhnkit (also Kroehnkit ) 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 Na2Cu [ SO4 ] 2.2 H2O, so it is a hydrous sodium -copper sulfate.

Kröhnkit developed mostly short to long prismatic crystals or pseudooctahedral, but also occurs in the form of fibrous crusts and dense aggregates. The transparent to translucent crystals of sky-blue to pale blue or greenish blue color and have a glass-like luster. On the dash panel Kröhnkit leaves a white line.

With a Mohs hardness of 2.5 to 3 Kröhnkit one of the soft to medium hard minerals, which can be similar to the reference mineral calcite easily scratched with a copper coin.

Special Features

Before the blowpipe zerknistert Kröhnkit and melts into a green mass, which shatters after cooling. In water, the mineral is readily soluble, the solution is acidic.

Etymology and history

The mineral was first discovered in an open pit copper at Chuquicamata ( Chuquicamata mine ) in the Chilean Atacama Desert, which was sold from there as vitriol. The chemist and former German consul in Chile Berthold Kröhnke but fell to the unusually bright blue color of the crystals. So he took some crystals, analyzed them and gave his analysis results and some descriptions of the mineral in 1875 in a letter to Ignacy Domeyko on. This confirmed the findings of Kröhnke and named the new mineral after its describer, although he whose name alternately Krönke and Kronnke wrote.

Ludwig Darapsky corrected the mineral name in 1889 in his summary of the correspondence in Kröhnkit because he kept this notation for the right one.

Classification

Already in the outdated, but partly still in use 8th edition of the mineral classification by Strunz was one of Kröhnkit to the mineral class of " sulfates ( selenates, tellurates, chromates, molybdates and tungstates ) " and then to the Department of " water -containing sulfates, without foreign anions " where he collaborated with Goldichit the " Kröhnkit - Goldichit group " with the system no. VI/C.16 and the other Member Ferrinatrit formed.

The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the Kröhnkit also in the department of " sulfates ( selenates, etc. ) without additional anions, with H2O" one. However, this is further divided according to the relative size of the cations involved, so that the mineral is found according to its composition in the subsection "With medium -sized and large cations ", where it forms the unnamed group 7.CC.30 the only member.

The mostly commonly used in English-speaking classification of minerals according to Dana assigns the Kröhnkit in the class of " sulfates, chromates and molybdates " and there in the department of " water -containing acids and sulfates " one. Here he is the only member of the unnamed group 29.03.02 within the subdivision " water containing acids and sulfates (A ) 2B ( XO4 ) 2 × x (H2O) " to find.

Education and Locations

Kröhnkit formed secondarily in the oxidation zone of copper deposits, preferably under very dry climatic conditions. Accompanying minerals among other Antlerite, Atacamit, Blödite, Chalkanthit and Natrochalcit may occur.

A rare mineral formation Kröhnkit could be detected only in a few localities, where previously around 20 localities known as apply (as of 2014). Apart from its type locality " Chuquicamata mine " the mineral still occurred in Chile in other mines in the area around the Chuquicamata and Calama and in Mejillones and in several mines at Caracoles evident.

The only known locality in Austria is a slag heap near Walchen (municipality Oeblarn ) in Styria.

Other localities lie among others in Argentina, Australia, China, Greece, Italy, Romania, Hungary, England in the United Kingdom ( UK) as well as California and Virginia in the United States of America (USA).

Crystal structure

Kröhnkit crystallizes in the monoclinic space group P21 / c ( Raumgruppen-Nr. 14) with the lattice parameters a = 5.81 Å; b = 12.66 Å; c = 5.52 Å and 108.3 ° β = and two formula units per unit cell.

The crystal structure of Kröhnkit consists of corner-sharing CuO4 (H2O) octahedra and SO4 tetrahedra, which are lined up parallel to chains. These chains are connected via Na polyhedra and hydrogen bonds.